William Weber

The temperature dependence of amorphization in a high-entropy pyrochlore, (Yb_0.2Tm_0.2Lu_0.2Ho_0.2Er_0.2)₂Ti₂O₇, under irradiation with 600 keV Xe ions has been studied using in situ transmission electron microscopy (TEM). The critical amorphization dose increases with temperature, and the critical temperature for amorphization is 800 K. At room temperature, the critical amorphization dose is larger than that previously determined for this pyrochlore under bulk-like 4 MeV Au ion irradiation but is similar to the critical doses determined in two other high-entropy titanate pyrochlores under 800 keV Kr ion irradiation using in situ TEM, which is consistent with reported behavior in simple rare-earth titanate pyrochlores.

Graphical abstract

Multi-principal component transition metal (TM) diborides represent a class of high-entropy ceramics (HECs) that have received considerable interest in recent years owing to their promising properties for extreme environment applications that include thermal/ environmental barriers, hypersonic vehicles, turbine engines, and next-generation nuclear reactors. While the addition of chemical disorder through the random distribution of TM elements on the cation sublattice has offered opportunities to tailor elastic stiffness and hardness, the effects of irradiation-induced structural damage on the physical properties of these complex materials have remained largely unexplored. To this end, changes in the hardness and elastic moduli of a high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 and three of its quaternary subsets following irradiation with 10 MeV gold (Au) ions to fluences of up to 6 × 1015 Au cm−2 are investigated at the micrometer and sub-micrometer length-scales via the dispersion of laser-generated surface acoustic waves (SAW) and nanoindentation, respectively. The nanoindentation measurements show that the TM diborides exhibit an initial increase in hardness following irradiation with energetic Au ions, with a subsequent decrease in hardness following further irradiation. One quaternary composition, (Hf1/3Ta1/3Ti1/3)B2, exhibits a notable exception to the trend and continues to exhibit an increase in hardness with ion irradiation fluence. Although differences in the absolute values of the effective elastic moduli obtained from the measured SAW dispersion and nanoindentation are observed (and attributed to microstructural variations at the measurement length-scale), both techniques yield similar trends in the form of an initial reduction and subsequent saturation in the elastic modulus with increasing ion irradiation fluence. The quaternary TM diboride (Hf1/3Ta1/3Ti1/3)B2 again exhibits a departure from this trend. The high-entropy TM diboride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)B2 exhibits the greatest recovery in hardness and modulus when irradiated to high ion fluences following initial changes at low fluence, indicating superior resistance to radiation-induced damage over its quaternary counterparts. Opportunities for designing HECs with superior hardness and modulus for enhanced radiation resistance (compared to their single constituent counterparts) by tailoring chemical disorder and bond character in the lattice are discussed.

Effects of electronic to nuclear energy losses (S _e/S _n) ratio on damage evolution in defective KTaO₃ have been investigated by irradiating pre-damaged single crystal KTaO₃ with intermediate energy O ions (6 MeV, 8 MeV and 12 MeV) at 300 K. By exploring these processes in pre-damaged KTaO₃ containing a fractional disorder level of 0.35, the results demonstrate the occurrence of a precursory stage of damage production before the onset of damage annealing process in defective KTaO₃ that decreases with O ion energy. The observed ionization-induced annealing process by ion channeling analysis has been further mirrored by high resolution transmission electron microscopy analysis. In addition, the reduction of disorder level is accompanied by the broadening of the disorder profiles to greater depth with increasing ion fluence, and enhanced migration is observed with decreasing O ion energy. Since S _e (∼3.0 keV nm⁻¹) is nearly constant for all 3 ion energies across the pre-damaged depth, the difference in behavior is due to the so-called ‘velocity effect’: the lower ion velocity below the Bragg peak yields a confined spread of the electron cascade and hence an increased energy deposition density. The inelastic thermal spike calculation has further confirmed the existence of a velocity effect, not previously reported in KTaO₃ or very scarcely reported in other materials for which the existence of ionization-induced annealing has been reported. In other words, understanding of ionization-induced annealing has been advanced by pointing out that ion velocity effect governs the healing of pre-existing defects, which may have significant implication for the creation of new functionalities in KTaO₃ through atomic-level control of microstructural modifications, but may not be limited to KTaO₃.

The high-entropy concept was applied to synthesize a set of rare-earth perovskites REBO3 (RE = La, Pr, Nd, Sm, Eu, Gd) with the B-site occupied by Sc, Al, Cr, Ni, and Fe in equimolar ratios. All samples crystallize in the orthorhombic Pnma space group. Using an extended set of characterization measurements, the effects of multi-component material design and rare-earth selection on the electronic properties are explored. Transport measurements show semiconducting behavior. PrBO3, SmBO3, and LaBO3 show low-temperature magnetic ordering, with the ordering temperature shifting with the moment on the A-site.

Compositionally complex materials have demonstrated extraordinary promise for structural robustness in extreme environments. Of these, the most commonly thought of are high entropy alloys, where chemical complexity grants uncommon combinations of hardness, ductility, and thermal resilience. In contrast to these metal–metal bonded systems, the addition of ionic and covalent bonding has led to the discovery of high entropy ceramics (HECs). These materials also possess outstanding structural, thermal, and chemical robustness but with a far greater variety of functional properties which enable access to continuously controllable magnetic, electronic, and optical phenomena. In this experimentally focused perspective, we outline the potential for HECs in functional applications under extreme environments, where intrinsic stability may provide a new path toward inherently hardened device design. Current works on high entropy carbides, actinide bearing ceramics, and high entropy oxides are reviewed in the areas of radiation, high temperature, and corrosion tolerance where the role of local disorder is shown to create pathways toward self-healing and structural robustness. In this context, new strategies for creating future electronic, magnetic, and optical devices to be operated in harsh environments are outlined.

It is widely accepted that the interaction of swift heavy ions with many complex oxides is predominantly governed by the electronic energy loss that gives rise to nanoscale amorphous ion tracks along the penetration direction.

The selective amorphization of SiGe in Si/SiGe nanostructures via a 1 MeV Si+ implant was investigated, resulting in single-crystal Si nanowires (NWs) and quantum dots (QDs) encapsulated in amorphous SiGe fins and pillars, respectively. The Si NWs and QDs are formed during high-temperature dry oxidation of single-crystal Si/SiGe heterostructure fins and pillars, during which Ge diffuses along the nanostructure sidewalls and encapsulates the Si layers. The fins and pillars were then subjected to a 3 × 1015 ions/cm2 1 MeV Si+ implant, resulting in the amorphization of SiGe, while leaving the encapsulated Si crystalline for larger, 65-nm wide NWs and QDs. Interestingly, the 26-nm diameter Si QDs amorphize, while the 28-nm wide NWs remain crystalline during the same high energy ion implant. This result suggests that the Si/SiGe pillars have a lower threshold for Si-induced amorphization compared to their Si/SiGe fin counterparts. However, Monte Carlo simulations of ion implantation into the Si/SiGe nanostructures reveal similar predicted levels of displacements per cm3. Molecular dynamics simulations suggest that the total stress magnitude in Si QDs encapsulated in crystalline SiGe is higher than the total stress magnitude in Si NWs, which may lead to greater crystalline instability in the QDs during ion implant. The potential lower amorphization threshold of QDs compared to NWs is of special importance to applications that require robust QD devices in a variety of radiation environments.

High entropy alloys (HEAs) are promising materials for various applications including nuclear reactor environments. Thus, understanding their behavior under irradiation and exposure to different environments is important. Here, two sets of near-equiatomic CoCrCuFeNi thin films grown on either SiO₂/Si or Si substrates were irradiated at room temperature with 11.5 MeV Au ions, providing similar behavior to exposure to inert versus corrosion environments. The film grown on SiO₂ had relatively minimal change up to peak damage levels above 500 dpa, while the film grown on Si began intermixing at the substrate–film interface at peak doses of 0.1 dpa before transforming into a multi-silicide film at higher doses, all at room temperature with minimal thermal diffusion. The primary mechanism is radiation-enhanced diffusion via the inverse Kirkendall and solute drag effects. The results highlight how composition and environmental exposure affect the stability of HEAs under radiation and give insights into controlling these behaviors.

Results recently reported on the effect of thermochemical treatments on the (He-Cd) laser-excited emission spectra of strontium titanate (STO) are re-analyzed here and compared with results obtained under ion-beam irradiation. Contributing bands centered at 2.4 eV and 2.8 eV, which appear under laser excitation, present intensities dependent upon previous thermal treatments in oxidizing (O₂) or reducing atmosphere (H₂). As a key result, the emission band centered at 2.8 eV is clearly enhanced in samples exposed to a reducing atmosphere. From a comparison with the ionoluminescence data, it is concluded that the laser-excited experiments can be rationalized within a framework developed from ion-beam excitation studies. In particular, the band at 2.8 eV, sometimes attributed to oxygen vacancies, behaves as expected for optical transitions from conduction-band (CB) states to the ground state level of the self-trapped exciton center. The band at 2.0 eV reported in ion-beam irradiated STO, and attributed to oxygen vacancies, is not observed in laser-excited crystals. As a consequence of our analysis, a consistent scheme of electronic energy levels and optical transitions can now be reliably offered for strontium titanate.

Graphical abstract

LiTaO₃ crystals irradiated with 3 MeV and 1.162 GeV Au ions were studied by single crystal x-ray diffraction and Raman scattering measurements. The maximum lattice strains after 3 MeV Au ion irradiation to a fluence of 1.2 × 10¹³ cm⁻² were 1.2% and 0.6% along the c- and a-/b-axes, respectively. Two effects were observed in 1.162 GeV Au ion irradiated samples: (i) the (0006) and (1120) Bragg peaks were split into doublets, which suggested a subtle structural change due to slight modification of chemical composition; and (ii) the pre-damaged 1.2% lattice strain along the c-axis was relaxed to 0.9% after subsequent irradiation with 1.162 GeV Au ions, while relaxation along the a- or b-axis was not obvious. A distinct change in the Raman spectrum of the 〈0001〉 oriented LiTaO₃ crystals was observed after 1.162 GeV Au ion irradiation, but no obvious change was observed in the 〈1120〉 oriented samples or in 3 MeV Au ion irradiated samples. Strain and structural changes in crystalline LiTaO₃, with or without pre-existing defects, upon ion irradiation are delineated in its responding to inelastic ionization and elastic nuclear collisions.

Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been a subject of intense discussion. Interest in oxygen vacancies is not purely academic. Precise control of oxygen vacancies has potential technological benefits in optoelectronic devices. In this review paper, we focus our attention on the generation of oxygen vacancies by irradiation with high energy particles. Irradiation constitutes an efficient and reliable strategy to introduce, monitor, and characterize oxygen vacancies. Unfortunately, this technique has been underexploited despite its demonstrated advantages. This review revisits the main experimental results that have been obtained for oxygen vacancy centers (a) under high energy electron irradiation (100 keV–1 MeV) in LiNbO3, and (b) during irradiation with high-energy heavy (1–20 MeV) ions in SrTiO3. In both cases, the experiments have used real-time and in situ optical detection. Moreover, the present paper discusses the obtained results in relation to present knowledge from both the experimental and theoretical perspectives. Our view is that a consistent picture is now emerging on the structure and relevant optical features (absorption and emission spectra) of these centers. One key aspect of the topic pertains to the generation of self-trapped electrons as small polarons by irradiation of the crystal lattice and their stabilization by oxygen vacancies. What has been learned by observing the interplay between polarons and vacancies has inspired new models for color centers in dielectric crystals, models which represent an advancement from the early models of color centers in alkali halides and simple oxides. The topic discussed in this review is particularly useful to better understand the complex effects of different types of radiation on the defect structure of those materials, therefore providing relevant clues for nuclear engineering applications.

Oxygen vacancies are known to play a central role in the optoelectronic properties of oxide perovskites. A detailed description of the exact mechanisms by which oxygen vacancies govern such properties, however, is still quite incomplete. The unambiguous identification of oxygen vacancies has been a subject of intense discussion. Interest in oxygen vacancies is not purely academic. Precise control of oxygen vacancies has potential technological benefits in optoelectronic devices. In this review paper, we focus our attention on the generation of oxygen vacancies by irradiation with high energy particles. Irradiation constitutes an efficient and reliable strategy to introduce, monitor, and characterize oxygen vacancies. Unfortunately, this technique has been underexploited despite its demonstrated advantages. This review revisits the main experimental results that have been obtained for oxygen vacancy centers (a) under high energy electron irradiation (100 keV–1 MeV) in LiNbO3, and (b) during irradiation with high-energy heavy (1–20 MeV) ions in SrTiO3. In both cases, the experiments have used real-time and in situ optical detection. Moreover, the present paper discusses the obtained results in relation to present knowledge from both the experimental and theoretical perspectives. Our view is that a consistent picture is now emerging on the structure and relevant optical features (absorption and emission spectra) of these centers. One key aspect of the topic pertains to the generation of self-trapped electrons as small polarons by irradiation of the crystal lattice and their stabilization by oxygen vacancies. What has been learned by observing the interplay between polarons and vacancies has inspired new models for color centers in dielectric crystals, models which represent an advancement from the early models of color centers in alkali halides and simple oxides. The topic discussed in this review is particularly useful to better understand the complex effects of different types of radiation on the defect structure of those materials, therefore providing relevant clues for nuclear engineering applications.

Quantifying chemical compositions around nanovoids is a fundamental task for research and development of various materials. Atom probe tomography (APT) and scanning transmission electron microscopy (STEM) are currently the most suitable tools because of their ability to probe materials at the nanoscale. Both techniques have limitations, particularly APT, because of insufficient understanding of void imaging. Here, we employ a correlative APT and STEM approach to investigate the APT imaging process and reveal that voids can lead to either an increase or a decrease in local atomic densities in the APT reconstruction. Simulated APT experiments demonstrate the local density variations near voids are controlled by the unique ring structures as voids open and the different evaporation fields of the surrounding atoms. We provide a general approach for quantifying chemical segregations near voids within an APT dataset, in which the composition can be directly determined with a higher accuracy than STEM-based techniques.

We investigate the energy dissipation and track formation due to ion irradiation in SrTiO3 and KTaO3. We use molecular dynamics simulations combined with the inelastic thermal spike model to simulate 21 MeV Ni ion irradiation in pristine and predamaged samples. The results are validated against experimental findings, showing that the level of initial disorder affects the electron-phonon interactions and the energy dissipation and deposition to the atoms. It is predicted that the ion track size increases linearly for low disorder levels, while its size saturates for high levels of disorder.

We investigate the energy dissipation and track formation due to ion irradiation in SrTiO3 and KTaO3. We use molecular dynamics simulations combined with the inelastic thermal spike model to simulate 21 MeV Ni ion irradiation in pristine and predamaged samples. The results are validated against experimental findings, showing that the level of initial disorder affects the electron-phonon interactions and the energy dissipation and deposition to the atoms. It is predicted that the ion track size increases linearly for low disorder levels, while its size saturates for high levels of disorder.

A nanoscale hierarchical dual‐phase structure is reported to form in a nanocrystalline NiFeCoCrCu high‐entropy‐alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single‐phase face‐centered‐cubic (FCC) structure partially transforms into alternating nanometer layers of a body‐centered‐cubic (BCC) structure. The orientation relationship follows the Nishiyama–Wasser‐man relationship, that is, (011)_BCC || ( 1¯1¯1)_FCC and [100]_BCC || [ 11¯0]_FCC. Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual‐phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.

Single crystals of 4H-SiC irradiated with 900 keV Si and 21 MeV Ni ions separately and sequentially were studied by Rutherford backscattering spectrometry in channeling geometry, single crystal X-ray diffraction, and Raman scattering. SiC irradiated with 900 keV Si ions to a fluence of 6.3 × 1014 ions/cm2 experiences 7.3% strain over the depth of 650 nm. Strain relaxation from ionization-induced annealing was directly observed due to subsequent irradiation with 21 MeV Ni ions to a fluence of 2 × 1014 ions/cm2. These competitive processes suggest the use of ion irradiation to create a specific strain state in 4H-SiC, particularly in films.

An up-to-date review on recent results for self-trapping of free electrons and holes, as well as excitons, in strontium titanate (STO), which gives rise to small polarons and self-trapped excitons (STEs) is presented. Special attention is paid to the role of carrier and exciton self-trapping on the luminescence emissions under a variety of excitation sources with special emphasis on experiments with laser pulses and energetic ion-beams. In spite of the extensive research effort, a definitive identification of such localized states, as well as a suitable understanding of their operative light emission mechanisms, has remained lacking or controversial. However, promising advances have been recently achieved and are the objective of the present review. In particular, significant theoretical advances in the understanding of electron and hole self-trapping are discussed. Also, relevant experimental advances in the kinetics of light emission associated with electron-hole recombination have been obtained through time-resolved experiments using picosecond (ps) laser pulses. The luminescence emission mechanisms and the light decay processes from the self-trapped excitons are also reviewed. Recent results suggest that the blue emission at 2.8 eV, often associated with oxygen vacancies, is related to a transition from unbound conduction levels to the ground singlet state of the STE. The stabilization of small electron polarons by oxygen vacancies and its connection with luminescence emission are discussed in detail. Through ion-beam irradiation experiments, it has recently been established that the electrons associated with the vacancy constitute electron polaron states (Ti3+) trapped in the close vicinity of the empty oxygen sites. These experimental results have allowed for the optical identification of the oxygen vacancy center through a red luminescence emission centered at 2.0 eV. Ab-initio calculations have provided strong support for those experimental findings. Finally, the use of Cr-doped STO has offered a way to monitor the interplay between the chromium centers and oxygen vacancies as trapping sites for the electron and hole partners resulting from the electronic excitation.

Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett−Robinson−Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-dpa by providing more physically realistic descriptions of primary defect creation in materials and may become additional standard measures for radiation damage quantification.

Ab initio molecular dynamics simulations of low energy recoil events in wurtzite AlN have been performed to determine threshold displacement energies, defect production and evolution mechanisms, role of partial charge transfer during the process, and the influence of irradiation-induced defects on the properties of AlN. The results show that the threshold displacement energies, Ed, along the direction parallel to the basal planes are smaller than those perpendicular to the basal planes. The minimum Ed values are determined to be 19 eV and 55 eV for N and Al atom, respectively, which occur along the [1¯1¯20] direction. In general, the threshold displacement energies for N are smaller than those for Al atom, indicating the N defects would be dominant under irradiation. The defect production mechanisms have been analyzed. It is found that charge transfer and redistribution for both the primary knock-on atom and the subsequent recoil atoms play a significant role in defect production and evolution. Similar to the trend in oxide materials, there is a nearly linear relationship between Ed and the total amount of charge transfer at the potential energy peak in AlN, which provides guidance on the development of charge-transfer interatomic potentials for classic molecular dynamics simulations. Finally, the response behavior of AlN to low energy irradiation is qualitatively investigated. The existence of irradiation-induced defects significantly modifies the electronic structure, and thus affects the magnetic, electronic and optical properties of AlN. These findings further enrich the understanding of defects in the wide bandgap semiconductor of AlN.

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra.

We use the two-temperature model in molecular dynamic simulations of 150 keV Ni ion cascades in nickel and nickel-based alloys to investigate the effect of the energy exchange between the atomic and the electronic systems during the primary stages of radiation damage. We find that the electron-phonon interactions result in a smaller amount of defects and affect the cluster formation, resulting in smaller clusters. These results indicate that ignoring the local heating due to the electrons results in the overestimation of the amount of damage and the size of the defect clusters. A comparison of the average defect production to the Norgett-Robinson-Torrens (NRT) prediction over a range of energies is provided.

In the present study, we have revealed that (NiCoFeCr)100−xPdx (x= 1, 3, 5, 20 atom%) high-entropy alloys (HEAs) have both local- and long-range lattice distortions by utilizing X-ray total scattering, X-ray diffraction, and extended X-ray absorption fine structure methods. The local lattice distortion determined by the lattice constant difference between the local and average structures was found to be proportional to the Pd content. A small amount of Pd-doping (1 atom%) yields long-range lattice distortion, which is demonstrated by a larger (200) lattice plane spacing than the expected value from an average structure, however, the degree of long-range lattice distortion is not sensitive to the Pd concentration. The structural stability of these distorted HEAs under high-pressure was also examined. The experimental results indicate that doping with a small amount of Pd significantly enhances the stability of the fcc phase by increasing the fcc-to-hcp transformation pressure from ~13.0 GPa in NiCoFeCr to 20–26 GPa in the Pd-doped HEAs and NiCoFeCrPd maintains its fcc lattice up to 74 GPa, the maximum pressure that the current experiments have reached.

Single-phase concentrated solid-solution alloys (SP-CSAs) have recently gained unprecedented attention due to their promising properties. To understand effects of alloying elements on irradiation-induced defect production, recombination and evolution, an integrated study of ion irradiation, ion beam analysis and atomistic simulations are carried out on a unique set of model crystals with increasing chemical complexity, from pure Ni to Ni₈₀Fe₂₀, Ni₅₀Fe₅₀, and Ni₈₀Cr₂₀ binaries, and to a more complex Ni₄₀Fe₄₀Cr₂₀ alloy. Both experimental and simulation results suggest that the binary and ternary alloys exhibit higher radiation resistance than elemental Ni. The modeling work predicts that Ni₄₀Fe₄₀Cr₂₀ has the best radiation tolerance, with the number of surviving Frenkel pairs being factors of 2.0 and 1.4 lower than pure Ni and the 80:20 binary alloys, respectively. While the reduced defect mobility in SP-CSAs is identified as a general mechanism leading to slower growth of large defect clusters, the effect of specific alloying elements on suppression of damage accumulation is clearly demonstrated. This work suggests that concentrated solid-solution provides an effective way to enhance radiation tolerance by creating elemental alternation at the atomic level. The demonstrated chemical effects on defect dynamics may inspire new design principles of radiation-tolerant structural alloys for advanced energy systems.

A pressure-induced phase transition from the fcc to a hexagonal close-packed (hcp) structure was found in NiCoCrFe solid solution alloy starting at 13.5 GPa. The phase transition is very sluggish and the transition did not complete at ∼40 GPa. The hcp structure is quenchable to ambient pressure. Only a very small amount (<5%) of hcp phase was found in the isostructural NiCoCr ternary alloy up to the pressure of 45 GPa and no obvious hcp phase was found in NiCoCrFePd system till to 74 GPa. Ab initio Gibbs free energy calculations indicated the energy differences between the fcc and the hcp phases for the three alloys are very small, but they are sensitive to temperature. The critical transition pressure in NiCoCrFe varies from ∼1 GPa at room temperature to ∼6 GPa at 500 K.

We use the inelastic thermal spike model for insulators and molecular dynamic simulations to investigate the effects of pre-existing damage on the energy dissipation and structural alterations in KTaO3 under irradiation with 21 MeV Ni ions. Our results reveal a synergy between the pre-existing defects and the electronic energy loss, indicating that the defects play an important role on the energy deposition in the system. Our findings highlight the need for better understanding on the role of defects in electronic energy dissipation and the coupling of the electronic and atomic subsystems.

We use the inelastic thermal spike model for insulators and molecular dynamic simulations to investigate the effects of pre-existing damage on the energy dissipation and structural alterations in KTaO3 under irradiation with 21 MeV Ni ions. Our results reveal a synergy between the pre-existing defects and the electronic energy loss, indicating that the defects play an important role on the energy deposition in the system. Our findings highlight the need for better understanding on the role of defects in electronic energy dissipation and the coupling of the electronic and atomic subsystems.

Site preferences occur under epitaxial strain, resulting in orders of magnitude differences in vacancy concentrations on different oxygen sites.

Site preferences occur under epitaxial strain, resulting in orders of magnitude differences in vacancy concentrations on different oxygen sites.

Site preferences occur under epitaxial strain, resulting in orders of magnitude differences in vacancy concentrations on different oxygen sites.

Defect energetics in structural materials has long been recognized to be affected by specific alloy compositions. Local structural distortion greatly affects the physical properties and performance of alloys. To reveal the atomic-level lattice distortion, the local structures of Ni and Fe in Ni1-xFex (x = 0.10, 0.20, 0.35 and 0.50) solid solution alloys were measured with extended X-ray absorption fine structure (EXAFS) technique. The EXAFS measurements have revealed that the bond length of Fe with surrounding atoms is 0.01–0.02 Å larger than that of Ni with its neighbors in the alloys. Both the lattice constant and the interatomic distance of the nearest neighbors increase with the addition of Fe content in the solid solutions. The local bonding environments in Ni1-xFex alloys were also calculated from ab initio and compared with the experimental results.

Defect energetics in structural materials has long been recognized to be affected by specific alloy compositions. Local structural distortion greatly affects the physical properties and performance of alloys. To reveal the atomic-level lattice distortion, the local structures of Ni and Fe in Ni1-xFex (x = 0.10, 0.20, 0.35 and 0.50) solid solution alloys were measured with extended X-ray absorption fine structure (EXAFS) technique. The EXAFS measurements have revealed that the bond length of Fe with surrounding atoms is 0.01–0.02 Å larger than that of Ni with its neighbors in the alloys. Both the lattice constant and the interatomic distance of the nearest neighbors increase with the addition of Fe content in the solid solutions. The local bonding environments in Ni1-xFex alloys were also calculated from ab initio and compared with the experimental results.

A systematic study of the ion beam heating effect was performed in a temperature range of −170 to 900 °C using a 10 MeV Au3+ ion beam and a Yttria stabilized Zirconia (YSZ) sample at a flux of 5.5 × 1012 cm−2 s−1. Different geometric configurations of beam, sample, thermocouple positioning, and sample holder were compared to understand the heat/charge transport mechanisms responsible for the observed temperature increase. The beam heating exhibited a strong dependence on the background (initial) sample temperature with the largest temperature increases occurring at cryogenic temperatures and decreasing with increasing temperature. Comparison with numerical calculations suggests that the observed heating effect is, in reality, a predominantly electronic effect and the true temperature rise is small. A simple model was developed to explain this electronic effect in terms of an electrostatic potential that forms during ion irradiation. Such an artificial beam heating effect is potentially problematic in thermostated ion irradiation and ion beam analysis apparatus, as the operation of temperature feedback systems can be significantly distorted by this effect.

A grand challenge in material science is to understand the correlation between intrinsic properties and defect dynamics. Radiation tolerant materials are in great demand for safe operation and advancement of nuclear and aerospace systems. Unlike traditional approaches that rely on microstructural and nanoscale features to mitigate radiation damage, this study demonstrates enhancement of radiation tolerance with the suppression of void formation by two orders magnitude at elevated temperatures in equiatomic single-phase concentrated solid solution alloys, and more importantly, reveals its controlling mechanism through a detailed analysis of the depth distribution of defect clusters and an atomistic computer simulation. The enhanced swelling resistance is attributed to the tailored interstitial defect cluster motion in the alloys from a long-range one-dimensional mode to a short-range three-dimensional mode, which leads to enhanced point defect recombination. The results suggest design criteria for next generation radiation tolerant structural alloys.

We report on unexpected dramatic radial variations in ion tracks formed by irradiation with energetic ions (2.3 GeV ²⁰⁸Pb) at a constant electronic energy-loss (~42 keV/nm) in pyrochlore-structured Gd₂TiZrO₇. Though previous studies have shown track formation and average track diameter measurements in the Gd₂Ti_xZr_(1−x)O₇ system, the present work clearly reveals the importance of the recrystallization process in ion track formation in this system, which leads to more morphological complexities in tracks than currently accepted behavior. The ion track profile is usually considered to be diametrically uniform for a constant value of electronic energy-loss. This study reveals the diameter variations to be as large as ~40% within an extremely short incremental track length of ~20 nm. Our molecular dynamics simulations show that these fluctuations in diameter of amorphous core and overall track diameter are attributed to the partial substitution of Ti atoms by Zr atoms, which have a large difference in ionic radii, on the B-site in pyrochlore lattice. This random distribution of Ti and Zr atoms leads to a local competition between amorphous phase formation (favored by Ti atoms) and defect-fluorite phase formation (favored by Zr atoms) during the recrystallization process and finally introduces large radial variations in track morphology.

Low‐energy recoil events in Ti₃SiC₂ are studied using ab initio molecular dynamics simulations. We find that the threshold displacement energies are orientation dependent because of anisotropic structural and/or bonding characteristic. For Ti and Si in the Ti–Si layer with weak bonds that have mixed covalent, ionic, and metallic characteristic, the threshold displacement energies for recoils perpendicular to the basal planes are larger than those parallel to the basal planes, which is an obvious layered‐structure‐related behavior. The calculated minimum threshold displacement energies are 7 eV for the C recoil along the direction, 26 eV for the Si recoil along the direction, 24 eV for the Ti in the Ti–C layer along the direction and 23 eV for the Ti in the Ti–Si layer along the direction. These results will advance the understanding of the cascade processes of Ti₃SiC₂ under irradiation and are expected to yield new perspective on the MAX phase family that includes more than 100 compounds.

Low‐energy recoil events in Ti₃SiC₂ are studied using ab initio molecular dynamics simulations. We find that the threshold displacement energies are orientation dependent because of anisotropic structural and/or bonding characteristic. For Ti and Si in the Ti–Si layer with weak bonds that have mixed covalent, ionic, and metallic characteristic, the threshold displacement energies for recoils perpendicular to the basal planes are larger than those parallel to the basal planes, which is an obvious layered‐structure‐related behavior. The calculated minimum threshold displacement energies are 7 eV for the C recoil along the direction, 26 eV for the Si recoil along the direction, 24 eV for the Ti in the Ti–C layer along the direction and 23 eV for the Ti in the Ti–Si layer along the direction. These results will advance the understanding of the cascade processes of Ti₃SiC₂ under irradiation and are expected to yield new perspective on the MAX phase family that includes more than 100 compounds.

Low‐energy recoil events in Ti₃SiC₂ are studied using ab initio molecular dynamics simulations. We find that the threshold displacement energies are orientation dependent because of anisotropic structural and/or bonding characteristic. For Ti and Si in the Ti–Si layer with weak bonds that have mixed covalent, ionic, and metallic characteristic, the threshold displacement energies for recoils perpendicular to the basal planes are larger than those parallel to the basal planes, which is an obvious layered‐structure‐related behavior. The calculated minimum threshold displacement energies are 7 eV for the C recoil along the direction, 26 eV for the Si recoil along the direction, 24 eV for the Ti in the Ti–C layer along the direction and 23 eV for the Ti in the Ti–Si layer along the direction. These results will advance the understanding of the cascade processes of Ti₃SiC₂ under irradiation and are expected to yield new perspective on the MAX phase family that includes more than 100 compounds.

Equiatomic alloys (e.g. high entropy alloys) have recently attracted considerable interest due to their exceptional properties, which might be closely related to their extreme disorder induced by the chemical complexity. In order to understand the effects of chemical complexity on their fundamental physical properties, a family of (eight) Ni-based, face-center-cubic (FCC), equiatomic alloys, extending from elemental Ni to quinary high entropy alloys, has been synthesized and their electrical, thermal and magnetic properties are systematically investigated in the range of 4–300 K by combining experiments withab initioKorring-Kohn-Rostoker coherent-potential-approximation (KKR-CPA) calculations. The scattering of electrons is significantly increased due to the chemical (especially magnetic) disorder. It has weak correlation with the number of elements but strongly depends on the type of elements. Thermal conductivities of the alloys are largely lower than pure metals, primarily because the high electrical resistivity suppresses the electronic thermal conductivity. The temperature dependence of the electrical and thermal transport properties is further discussed and the magnetization of five alloys containing three or more elements is measured in magnetic fields up to 4 T.

We used a combination of ion cascades and the unified thermal spike model to study the electronic effects from 800 keV Kr and Xe ion irradiation in zircon. We compared the damage production for four cases: (a) due to ion cascades alone, (b) due to ion cascades with the electronic energy loss activated as a friction term, (c) due to the thermal spike from the combined electronic and nuclear energy losses, and (d) due to ion cascades with electronic stopping and the electron-phonon interactions superimposed. We found that taking the electronic energy loss out as a friction term results in reduced damage, while the electronic electron-phonon interactions have additive impact on the final damage created per ion.

Effects of He irradiation on polycrystalline yttria‐stabilized zirconia (YSZ) are studied with the focus on irradiation‐induced damage buildup, He behavior, and volume swelling. The evolution of irradiation‐induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three‐step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single‐crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single‐crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.

The response of titanate pyrochlores (A₂Ti₂O₇, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O₂-like molecules are formed and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd₂Ti₂O₇ pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

The structure and ion-conducting properties of the defect-fluorite ring structure formed around amorphous ion-tracks by swift heavy ion irradiation of Gd₂Ti₂O₇ pyrochlore are investigated. High angle annular dark field imaging complemented with ion-track molecular dynamics simulations show that the atoms in the ring structure are disordered and have relatively larger cation-cation interspacing than in the bulk pyrochlore, illustrating the presence of tensile strain in the ring region. Density functional theory calculations show that the non-equilibrium defect-fluorite structure can be stabilized by tensile strain. The pyrochlore to defect-fluorite structure transformation in the ring region is predicted to be induced by recrystallization during a melt-quench process and stabilized by tensile strain. Static pair-potential calculations show that planar tensile strain lowers oxygen vacancy migration barriers in pyrochlores, in agreement with recent studies on fluorite and perovskite materials. In view of these results, it is suggested that strain engineering could be simultaneously used to stabilize the defect-fluorite structure and gain control over its high ion-conducting properties.

A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiation-tolerant structural alloys for energy applications.

A long-standing objective in materials research is to effectively heal fabrication defects or to remove pre-existing or environmentally induced damage in materials. Silicon carbide (SiC) is a fascinating wide-band gap semiconductor for high-temperature, high-power and high-frequency applications. Its high corrosion and radiation resistance makes it a key refractory/structural material with great potential for extremely harsh radiation environments. Here we show that the energy transferred to the electron system of SiC by energetic ions via inelastic ionization can effectively anneal pre-existing defects and restore the structural order. The threshold determined for this recovery process reveals that it can be activated by 750 and 850 keV Si and C self-ions, respectively. The results conveyed here can contribute to SiC-based device fabrication by providing a room-temperature approach to repair atomic lattice structures, and to SiC performance prediction as either a functional material for device applications or a structural material for high-radiation environments.

Cross sections of five pre-damaged systems with different initial defect concentration along the [001] direction, after irradiation with 21 MeV Ni ions. Sr is shown in blue, Ti in green, and O in purple.

The irradiation of lithium niobate with swift heavy ions results in the creation of amorphous nano-sized channels along the incident ion path. These nano-channels are on the order of a hundred microns in length and could be useful for photonic applications. However, there are two major challenges in these nano-channels characterization: (i) it is difficult to investigate the structural characteristics of these nano-channels due to their very long length and (ii) the analytical electron microscopic analysis of individual ion track is complicated due to electron beam sensitive nature of lithium niobate. Here, we report the first high resolution microscopic characterization of these amorphous nano-channels, widely known as ion-tracks, by direct imaging them at different depths in the material, and subsequently correlating the key characteristics with electronic energy loss of ions. Energetic Kr ions (84Kr22 with 1.98 GeV energy) are used to irradiate single crystal lithium niobate with a fluence of 2 × 1010 ions/cm2, which results in the formation of individual ion tracks with a penetration depth of ∼180 μm. Along the ion path, electron energy loss of the ions, which is responsible for creating the ion tracks, increases with depth under these conditions in LiNbO3, resulting in increases in track diameter of a factor of ∼2 with depth. This diameter increase with electronic energy loss is consistent with predictions of the inelastic thermal spike model. We also show a new method to measure the band gap in individual ion track by using electron energy-loss spectroscopy.

The irradiation of lithium niobate with swift heavy ions results in the creation of amorphous nano-sized channels along the incident ion path. These nano-channels are on the order of a hundred microns in length and could be useful for photonic applications. However, there are two major challenges in these nano-channels characterization: (i) it is difficult to investigate the structural characteristics of these nano-channels due to their very long length and (ii) the analytical electron microscopic analysis of individual ion track is complicated due to electron beam sensitive nature of lithium niobate. Here, we report the first high resolution microscopic characterization of these amorphous nano-channels, widely known as ion-tracks, by direct imaging them at different depths in the material, and subsequently correlating the key characteristics with electronic energy loss of ions. Energetic Kr ions (84Kr22 with 1.98 GeV energy) are used to irradiate single crystal lithium niobate with a fluence of 2 × 1010 ions/cm2, which results in the formation of individual ion tracks with a penetration depth of ∼180 μm. Along the ion path, electron energy loss of the ions, which is responsible for creating the ion tracks, increases with depth under these conditions in LiNbO3, resulting in increases in track diameter of a factor of ∼2 with depth. This diameter increase with electronic energy loss is consistent with predictions of the inelastic thermal spike model. We also show a new method to measure the band gap in individual ion track by using electron energy-loss spectroscopy.

Combined x-ray diffraction and first-principles studies of various epitaxial rutile-type metal dioxide films on Al2O3(0001) substrates reveal an unexpected rectangle-on-parallelogram heteroepitaxy. Unique matching of particular lattice spacings and crystal angles between the oxygen sublattices of Al2O3(0001) and the film(100) result in coexisted crystal rotation and lattice twinning inside the film. We demonstrate that, besides symmetry and lattice mismatch, angular mismatch along a specific crystal direction is also an important factor determining epitaxy. A generalized theorem has been proposed to explain epitaxial behaviors for tetragonal metal dioxides on Al2O3(0001).

Sr/Ti composition dependent intrinsic defect complexes are predicted; providing guidelines for optimizing the functionality of SrTiO₃in experiments.

Sr/Ti composition dependent intrinsic defect complexes are predicted; providing guidelines for optimizing the functionality of SrTiO₃in experiments.

Sr/Ti composition dependent intrinsic defect complexes are predicted; providing guidelines for optimizing the functionality of SrTiO₃in experiments.

Ab initio molecular dynamics simulations are performed to investigate the effects of a boron nitride (BN) substrate on Stone-Wales (SW) defect formation and recovery in graphene. It is found that SW defects can be created by an off-plane recoil atom that interacts with the BN substrate. A mechanism with complete bond breakage for formation of SW defects in suspended graphene is also revealed for recoils at large displacement angles. In addition, further irradiation can result in recovery of the SW defects through a bond rotation mechanism in both graphene and graphene/BN, and the substrate has little effect on the recovery process. This study indicates that the BN substrate enhances the irradiation resistance of graphene.

Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to measure the depth profiles of implanted Au ions in SiC for energies from 700 keV to 15 MeV. The measured ion distributions by both RBS and SIMS are considerably deeper than the SRIM predictions, but agree well with predictions based on our derived stopping powers.

Accurate information on electronic stopping power is fundamental for broad advances in materials science, electronic industry, space exploration, and sustainable energy technologies. In the case of slow heavy ions in light targets, current codes and models provide significantly inconsistent predictions, among which the Stopping and Range of Ions in Matter (SRIM) code is the most commonly used one. Experimental evidence, however, has demonstrated considerable errors in the predicted ion and damage profiles based on SRIM stopping powers. In this work, electronic stopping powers for Cl, Br, I, and Au ions are experimentally determined in two important functional materials, SiC and SiO2, based on a single ion technique, and new electronic stopping power values are derived over the energy regime from 0 to 15 MeV, where large deviations from the SRIM predictions are observed. As an experimental validation, Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) are utilized to measure the depth profiles of implanted Au ions in SiC for energies from 700 keV to 15 MeV. The measured ion distributions by both RBS and SIMS are considerably deeper than the SRIM predictions, but agree well with predictions based on our derived stopping powers.

Zirconia is viewed as a material of exceptional resistance to amorphization by radiation damage, and consequently proposed as a candidate to immobilize nuclear waste and serve as an inert nuclear fuel matrix. Here, we perform molecular dynamics simulations of radiation damage in zirconia in the range of 0.1–0.5 MeV energies with account of electronic energy losses. We find that the lack of amorphizability co-exists with a large number of point defects and their clusters. These, importantly, are largely isolated from each other and therefore represent a dilute damage that does not result in the loss of long-range structural coherence and amorphization. We document the nature of these defects in detail, including their sizes, distribution, and morphology, and discuss practical implications of using zirconia in intense radiation environments.

Understanding scintillation physics and nonproportionality is essential to accelerate materials discovery that has been restricted due to the difficulties inherent to large crystal growth and complex nature of gamma-solid interaction. Taking advantage of less restrictive growth and deposition techniques for smaller crystal sizes or thin films and better fundamental understanding of ion-solid interactions, a unique ion approach is demonstrated to effectively screen candidate scintillators with relatively small size and evaluate their nonlinear scintillation response. Response of CaF2:Eu and YAlO3:Ce scintillators to single ions of H+, He+, and O3+ are measured by the corresponding pulse height over a continuous energy range using a time-of-flight–scintillator–photoelectric multiplier tube apparatus. Nonlinear response of the scintillators under ionizing ion irradiation is quantitatively evaluated by considering the energy partitioning process. In a differential energy deposition region with negligible displacement damage, the low, medium and high excitation energy deposition density (Dexci) can be produced by energetic H+, He+ and O3+ ions, respectively, and significantly different impacts on the response characteristics of these two benchmark scintillators are observed. For CaF2:Eu, the scintillation efficiency under ion irradiation monotonically decreases with increasing excitation-energy density. In contrast, the response efficiency of YAlO3:Ce scintillation initially increases with excitation-energy density at low excitation-energy densities, goes through a maximum, and then decreases with further increasing excitation-energy density. The fundamental mechanism causing these different response behaviours in the scintillators is based on the competition between the scintillation response and the nonradiative quenching process under different excitation densities, which is also the main origin of the nonlinear response of the scintillators to irradiation.

Understanding scintillation physics and nonproportionality is essential to accelerate materials discovery that has been restricted due to the difficulties inherent to large crystal growth and complex nature of gamma-solid interaction. Taking advantage of less restrictive growth and deposition techniques for smaller crystal sizes or thin films and better fundamental understanding of ion-solid interactions, a unique ion approach is demonstrated to effectively screen candidate scintillators with relatively small size and evaluate their nonlinear scintillation response. Response of CaF2:Eu and YAlO3:Ce scintillators to single ions of H+, He+, and O3+ are measured by the corresponding pulse height over a continuous energy range using a time-of-flight–scintillator–photoelectric multiplier tube apparatus. Nonlinear response of the scintillators under ionizing ion irradiation is quantitatively evaluated by considering the energy partitioning process. In a differential energy deposition region with negligible displacement damage, the low, medium and high excitation energy deposition density (Dexci) can be produced by energetic H+, He+ and O3+ ions, respectively, and significantly different impacts on the response characteristics of these two benchmark scintillators are observed. For CaF2:Eu, the scintillation efficiency under ion irradiation monotonically decreases with increasing excitation-energy density. In contrast, the response efficiency of YAlO3:Ce scintillation initially increases with excitation-energy density at low excitation-energy densities, goes through a maximum, and then decreases with further increasing excitation-energy density. The fundamental mechanism causing these different response behaviours in the scintillators is based on the competition between the scintillation response and the nonradiative quenching process under different excitation densities, which is also the main origin of the nonlinear response of the scintillators to irradiation.

Understanding scintillation physics and nonproportionality is essential to accelerate materials discovery that has been restricted due to the difficulties inherent to large crystal growth and complex nature of gamma-solid interaction. Taking advantage of less restrictive growth and deposition techniques for smaller crystal sizes or thin films and better fundamental understanding of ion-solid interactions, a unique ion approach is demonstrated to effectively screen candidate scintillators with relatively small size and evaluate their nonlinear scintillation response. Response of CaF2:Eu and YAlO3:Ce scintillators to single ions of H+, He+, and O3+ are measured by the corresponding pulse height over a continuous energy range using a time-of-flight–scintillator–photoelectric multiplier tube apparatus. Nonlinear response of the scintillators under ionizing ion irradiation is quantitatively evaluated by considering the energy partitioning process. In a differential energy deposition region with negligible displacement damage, the low, medium and high excitation energy deposition density (Dexci) can be produced by energetic H+, He+ and O3+ ions, respectively, and significantly different impacts on the response characteristics of these two benchmark scintillators are observed. For CaF2:Eu, the scintillation efficiency under ion irradiation monotonically decreases with increasing excitation-energy density. In contrast, the response efficiency of YAlO3:Ce scintillation initially increases with excitation-energy density at low excitation-energy densities, goes through a maximum, and then decreases with further increasing excitation-energy density. The fundamental mechanism causing these different response behaviours in the scintillators is based on the competition between the scintillation response and the nonradiative quenching process under different excitation densities, which is also the main origin of the nonlinear response of the scintillators to irradiation.

The unraveling disorder-driven grain growth mechanism may be utilized to control grain sizes and tailor the functionality of nanocrystalline materials.

Thin films of nanocrystalline ceria deposited onto a silicon substrate have been irradiated with 3 MeV Au⁺ ions to a total dose of 34 displacements per atom to examine the film/substrate interfacial response upon displacement damage. Under irradiation, a band of contrast is observed to form that grows under further irradiation. Scanning and high‐resolution transmission electron microscopy imaging and analysis suggest that this band of contrast is a cerium silicate phase with an approximate Ce:Si:O composition ratio of 1:1:3 in an amorphous nature. The slightly nonstoichiometric composition arises due to the loss of mobile oxygen within the cerium silicate phase under the current irradiation condition. This nonequilibrium phase is formed as a direct result of ion‐beam‐induced chemical mixing caused by ballistic collisions between the incoming ion and the lattice atoms. This may hold promise in ion beam engineering of cerium silicates for microelectronic applications e.g., the fabrication of blue LEDs.

Thin films of nanocrystalline ceria deposited onto a silicon substrate have been irradiated with 3 MeV Au⁺ ions to a total dose of 34 displacements per atom to examine the film/substrate interfacial response upon displacement damage. Under irradiation, a band of contrast is observed to form that grows under further irradiation. Scanning and high‐resolution transmission electron microscopy imaging and analysis suggest that this band of contrast is a cerium silicate phase with an approximate Ce:Si:O composition ratio of 1:1:3 in an amorphous nature. The slightly nonstoichiometric composition arises due to the loss of mobile oxygen within the cerium silicate phase under the current irradiation condition. This nonequilibrium phase is formed as a direct result of ion‐beam‐induced chemical mixing caused by ballistic collisions between the incoming ion and the lattice atoms. This may hold promise in ion beam engineering of cerium silicates for microelectronic applications e.g., the fabrication of blue LEDs.

Thin films of nanocrystalline ceria deposited onto a silicon substrate have been irradiated with 3 MeV Au⁺ ions to a total dose of 34 displacements per atom to examine the film/substrate interfacial response upon displacement damage. Under irradiation, a band of contrast is observed to form that grows under further irradiation. Scanning and high‐resolution transmission electron microscopy imaging and analysis suggest that this band of contrast is a cerium silicate phase with an approximate Ce:Si:O composition ratio of 1:1:3 in an amorphous nature. The slightly nonstoichiometric composition arises due to the loss of mobile oxygen within the cerium silicate phase under the current irradiation condition. This nonequilibrium phase is formed as a direct result of ion‐beam‐induced chemical mixing caused by ballistic collisions between the incoming ion and the lattice atoms. This may hold promise in ion beam engineering of cerium silicates for microelectronic applications e.g., the fabrication of blue LEDs.

Thin films of nanocrystalline ceria on a Si substrate have been irradiated with 3 MeV Au⁺ ions to fluences of up to 1x10¹⁶ ions cm^-2, at temperatures ranging between 160 to 400 K. During the irradiation, a band of contrast is observed to form at the thin film/substrate interface. Analysis by scanning transmission electron microscopy in conjunction with energy dispersive and electron energy loss spectroscopy techniques revealed that this band of contrast was a cerium silicate amorphous phase, with an approximate Ce:Si:O ratio of 1:1:3.

We have recently found that the radiation tolerance of SiC is highly enhanced by introducing nanolayers of stacking faults and twins [Y. Zhang et al., Phys. Chem. Chem. Phys. 14, 13429 (2012)]. To reveal the origin of this radiation resistance, we used in situ transmission electron microscopy to examine structural changes induced by electron beam irradiation in 3C-SiC containing nanolayers of (111) planar defects. We found that preferential amorphization, when it does occur, takes place at grain boundaries and at (1¯11) and (11¯1) planar defects. Radiation-induced point defects, such as interstitials and vacancies, migrate two-dimensionally between the (111) planar defects, which probably enhances the damage recovery.

Density functional theory (DFT) with a tailored Hartree-Fock hybrid functional, which can overcome the band gap problem arising in conventional DFT and gives a valence band width comparable with experiment, is applied to determine formation energies and electronic structures of intrinsic defects in cubic silicon carbide (3C-SiC). Systematic comparison of defect formation energies obtained with the tailored hybrid functional and a conventional DFT functional clearly demonstrates that conventional DFT results are not satisfactory. The understanding on intrinsic defects, which were previously investigated mainly with conventional DFT functionals, is largely revised with regard to formation energies, electronic structures and transition levels. It is found that conventional DFT functionals basically lead to (i) underestimation of the formation energy when the defect charge is more negative and (ii) overestimation when the defect charge is more positive. The underestimation is mainly attributed to the well-known band gap problem. The overestimation is attributed to shrinkage of the valence bands, although in some cases such band shrinkage may lead to underestimation depending on how the defect alters the valence band structure. Both the band gap problem and the valence band shrinkage are often observed in semiconductors, including SiC, with conventional DFT functionals, and thus need to be carefully dealt with to achieve reliable computational results.

Radiation damage by ions is conventionally believed to be produced either by displacement cascades or electronic energy deposition acting separately. There is, however, a range of ion energies where both processes are significant and can contribute to irradiation damage. The combination of two computational methods, namely binary collision approximation and molecular dynamics, the latter with input from the inelastic thermal spike model, makes it possible to examine the simultaneous contribution of both energy deposition mechanisms on the structural damage in the irradiated structure. We study the effect in amorphous SiO₂ irradiated by Au ions with energies ranging between 0.6 and 76.5 MeV. We find that in the intermediate energy regime, the local heating due to electronic excitations gives a significant contribution to the displacement cascade damage.

Single crystals of 6H-SiC have been irradiated with a variety of ions over a wide range of fluences and temperatures. The temperature and dose dependence of damage accumulation has been investigated using in-situ Rutherford Backscattering Spectrometry in channeling geometry. At low temperatures, the accumulation of structural disorder exhibits a sigmoidal dependence on dose. At room temperature and higher, simultaneous recovery processes during irradiation significantly reduce the damage accumulation rates by up to a factor of five. Isochronal and isothermal annealing studies have been used to study the damage recovery behavior. For low defect concentrations introduced by 550 keV Si⁺ irradiation at 160 K, complete recovery is observed at 300 K. However, defects introduced by He⁺ irradiation on the Si sublattice are more difficult to anneal at room temperature, which suggests trapping of the implanted helium may inhibit defect recombination. Below room temperature, the thermal recovery of defects on the Si sublattice has an activation energy on the order of 0.3 ± 0.1 eV. Defect recovery above 570 K has an activation energy on the order of 1.5 ± 0.3 eV.

Ion irradiations of the rare-earth orthosilicate, Ca₂Nd₈(SiO₄)₆O₂, have been carried out using both alpha particles (emitted from a ²³⁸PuO₂ source) and 3 MeV argon ions. The unit cell exhibits anisotropic expansion under irradiation, consistent with expectations based on the polyhedral connectivity within the structure. A least-squares analysis of the interatomic distances suggests that the unit-cell expansions are primarily due to changes in oxygen-oxygen distances and cation separations between neighboring polyhedra rather than to bonds within polyhedra. The irradiation-induced change in unit-cell volume is proportional to 1 – exp (BD), where B is an annealing rate constant and D is the dose, in agreement with a model for the accumulation of isolated point defects in the structure. The volume expansion saturates at 2.56% and 1.40% for the alpha and argon irradiations, respectively. Analysis of the results suggests that a significant fraction of the defects produced in the argon-ion displacement cascades are lost to in-cascade recombination. Differential scanning calorimetry of powder irradiated with 3 MeV argon ions to 20 ions/nm² reveals an exothermic recovery peak at 350 °C with an activation energy of 1.3 ± 0.1 eV and average stored energy release of 28.2 J/g. There is no evidence for amorphization of this material under alpha or argon irradiation.

In a previous computer-simulation experiment, the accumulation of damage in silicon carbide (SiC) from the overlap of 10 keV Si displacement cascades at 200 K was investigated, and the damage states produced following each cascade were archived for further analysis. In the current study, interstitial clustering, system energy, and volume changes are investigated as the damage states evolve due to cascade overlap. An amorphous state is achieved at a damage energy density of 27.5 eV/atom (0.28 displacements per atom). At low-dose levels, most defects are produced as isolated Frenkel pairs, with a small number of defect clusters involving only four to six atoms; however, after the overlap of five cascades (0.0125 displacements per atom), the size and number of interstitial clusters increases with increasing dose. The average energy per atom increases linearly with increasing short-range (or chemical) disorder. The volume change exhibits two regimes of linear dependence on system energy and increases more rapidly with dose than either the energy or the disorder, which indicates a significant contribution to swelling of isolated interstitials and antisite defects. The saturation volume change for the cascade-amorphized state in these simulations is 8.2%, which is in reasonable agreement with the experimental value of 10.8% in neutron-irradiated SiC.

Molecular dynamics (MD) were employed in atomic-level simulations of fundamental damage production processes due to multiple ion–solid collision events in cubic SiC. Isolated collision cascades produce single interstitials, vacancies, antisite defects, and small defect clusters. As the number of cascades (or equivalent dose) increases, the concentration of defects increases, and the collision cascades begin to overlap. The coalescence of defects and clusters with increasing dose is an important mechanism leading to amorphization in SiC and is consistent with the homogeneous amorphization process observed experimentally in SiC. The driving force for the crystalline– amorphous (c–a) transition is the accumulation of both interstitials and antisite defects. High-resolution transmission electron microscopy (HRTEM) images of the defect accumulation process and loss of long-range order in the MD simulation cell are consistent with experimental HRTEM images and disorder measurements. Thus, the MD simulations provide atomic-level insights into the interpretation of experimentally observed features associated with multiple ion–solid collision events in SiC.

We have performed molecular dynamics simulation of displacement events on silicon and carbon sublattices in silicon carbide for displacement doses ranging from 0.005 to 0.5 displacements per atom. Our results indicate that the displacement threshold energy is about 21 eV for C and 35 eV for Si, and amorphization can occur by accumulation of displacement damage regardless of whether Si or C is displaced. In addition, we have simulated defect production in high-energy cascades as a function of the primary knock-on atom energy and observed features that are different from the case of damage accumulation in Si. These systematic studies shed light on the phenomenon of non-ionizing energy loss that is relevant to understanding space radiation effects in semiconductor devices.

Irradiation experiments have been performed 60° off the surface normal for 6H-SiC single crystals at various temperatures (185-870 K) using 550 keV C⁺ ions over a fluence range from 1×10¹⁸ to 5×10⁹ ions/m². Atomic disorder on the Si sublattice, as determined by in-situ RBS/channeling analysis, ranged from dilute defects to complete amorphization. The critical amorphization dose of ∼0.23 dpa (on the Si sublattice) at 185 K has been determined. Asymmetric shapes in angular yield profiles across the crystallographic axis <0001> emerged above 1.5×10¹⁹ C⁻/m² (∼0.05 dpa in the near-surface region), which might be associated with the lattice disturbance in the crystal structure. A gradual decrease in half-angular width was observed with the increase of ion fluence in the experiment. The minimum yield exhibits a rather linear relationship with ion dose at the surface. Post-irradiation annealing at the irradiation temperature did not result in measurable recovery for fluences ranging from 4 × 10¹⁸ to 2×10¹⁹ C⁺/m² at 300, 470 and 670 K. Results also show that low fluence (<8×10¹⁸ C⁺/m²) irradiation at 185 K followed by thermal annealing results in similar defect concentrations to irradiation at that same temperature to the same ion fluence. Thus, at low fluences, the accumulated defects are in thermal equilibrium with the structure.

The corrosion resistance of a series of zirconium-substituted gadolinium pyrochlore, Gd₂(Ti_1-x Zr_x)₂O₇, where x = 0.0, 0.25, 0.50, 0.75, and 1.00, were evaluated using single-pass flow-through (SPFT) apparatus at 90°C and pH = 2. The zirconate end-member, Gd₂Zr₂O₇, has a defect fluorite structure, which distinguishes it from the face-centered cubic structure of the true pyrochlore specimens. In addition to the chemical variation, the samples include annealed, un-annealed, and ion-bombarded monoliths. In the case of the titanate end-member, Gd₂Ti₂O₇, the annealed specimen exhibited the least reactivity, followed by the un-annealed and ion-bombarded samples (2.39×10^-3, 1.57×10^-2, and 1.12×10^-1 g m^-2 d^-1, respectively). With increasing zirconium content, the samples displayed less sensitivity to processing or surface modification with the zirconate end-member exhibiting no difference in reactivity between annealed, un-annealed, and ion-bombarded specimens (rate = 4.0×10^-3 g m^-2 d^-1). In all cases, the dissolution rate decreased with increasing zirconium content to the Gd₂(Ti_0.25Zr_0.75)₂O₇ composition (1.33x10^-4 g m^-2 d^-1), but the zirconate end-member yielded rates nearly equal to that of the titanate end-member. These results demonstrate that to achieve the greatest radiation and corrosion resistance in this series, the Gd₂(Ti_0.25Zr_0.75)₂O₇ composition should be considered.

Although GaN is an important semiconductor material, its amorphous structures are not well understood. Currently, theoretical atomistic structural models which contradict each other, are proposed for the chemical short-range order of amorphous GaN: one characterizes amorphous GaN networks as highly chemically ordered, consisting of heteronuclear Ga-N atomic bonds; and the other predicts the existence of a large number of homonuclear bonds within the first coordination shell. In the present study, we examine amorphous structures of GaN via radial distribution functions obtained by electron diffraction techniques. The experimental results demonstrate that amorphous GaN networks consist of heterononuclear Ga-N bonds, as well as homonuclear Ga-Ga and N-N bonds.

With the increasing demand for the development of nuclear power comes the responsibility to address the issue of waste, including the technical challenges of immobilizing high-level nuclear wastes in stable solid forms for interim storage or disposition in geologic repositories. The immobilization of high-level nuclear wastes has been an active area of research and development for over 50 years. Borosilicate glasses and complex ceramic composites have been developed to meet many technical challenges and current needs, although regulatory issues, which vary widely from country to country, have yet to be resolved. Cooperative international programs to develop advanced proliferation-resistant nuclear technologies to close the nuclear fuel cycle and increase the efficiency of nuclear energy production might create new separation waste streams that could demand new concepts and materials for nuclear waste immobilization. This article reviews the current state-of-the-art understanding regarding the materials science of glasses and ceramics for the immobilization of highlevel nuclear waste and excess nuclear materials and discusses approaches to address new waste streams.

We have studied the effect of the yttria content on the paramagnetic centers in electron-irradiated yttria-stabilized zirconia (ZrO2: Y3+) or YSZ. Single crystals with 9.5 mol % or 18 mol % Y2O3 were irradiated with electrons of 1.0, 1.5, 2.0, and 2.5 MeV. The paramagnetic center production was studied by X-band electron paramagnetic resonance (EPR) spectroscopy. The same paramagnetic centers were identified for both chemical compositions, namely two electron centers, i.e., (i) F+-type centers (involving singly ionized oxygen vacancies), and (ii) so-called T centers (Zr3+ in a trigonal symmetry site), as well as hole-centers. A strong effect is observed on the production of hole-centers that is strongly enhanced when doubling the yttria content. However, no striking effect is found on the electron centers (except the enhancement of an extra line associated with the F+-type centers). It is concluded that hole-centers are produced by inelastic interactions, whereas F+-type centers are produced by elastic collisions with no effect of the yttria content on the defect production rate. In the latter case, the threshold displacement energy (Ed) of oxygen is estimated from the electron-energy dependence of the F+-type center production rate, with no significant effect of the yttria content on Ed. An Ed value larger than 120 eV is found. This is supported by classical molecular dynamics (MD) simulations with a Buckingham-type potential that show Ed values for Y and O are likely to be in excess of 200 eV. Due to the difficulty in displacing O or Y atoms, the radiation-induced defects may alternatively be a result of Zr atom displacements for Ed = 80 ± 1 eV with subsequent defect rearrangement.

Zircon (ZrSiO₄) is proposed as a waste form for excess weapons-grade plutonium. Zircon is an extremely durable ceramic that is often found as an accessory mineral in Precambrian terranes with ages up to 4 billion years. The chemical durability of zircon in groundwater far exceeds that of other waste forms, as modeled leach rates may be as low as 10^-11g/m²d. At least 10 wt% Pu can substitute for Zr in zircon. Self-radiation damage from alpha decay leads to a crystalline-to-amorphous transformation that is modeled as a function of time and temperature for deep borehole conditions. Based on the results of this assessment, zircon could meet all necessary durability and criticality criteria required for a Pu waste form. The types of data used in this analysis are generally not available for other crystalline ceramics or glasses.

The investigation of plutonium in glasses (amorphous ceramics lacking long-range order), in crystalline ceramics, and in composite materials composed of multiple crystalline or glass and crystalline phases, relieson multidisciplinary studies of physics, chemistry, and materials science. It involves the study of the plutonium atoms in materials with only short-range periodicity, as in glasses, to materials with long-range periodicity, as in crystals. The materials studied over the past 30 years include simple binary crystals, used to investigate the electronic structure of plutonium, to complex glasses and ceramics selected not only for the safety and durability that they provide for the immobilization of nuclear waste and plutonium, but also for the high flexibility they offer in composition. The lack of long-range order at the atomic level in glasses permits the inclusion of abroad range of waste elements, but it renders more difficult the interpretation of data from many commonly used experimental techniques. Regardless of the challenge, much of the research conducted in this field over the past few decades has been motivated by the use of plutonium as a surrogate for all nuclear-waste actinides or on its own in immobilization studies, in order to develop a durable glass or ceramic matrix that can resist leaching and mobilization of the plutonium on a geologic time scale.

Radiation effects from alpha-decay events in crystalline oxides, which are proposed for the immobilization of actinides, often lead to amorphization, macroscopic swelling and order-of-magnitude increases in dissolution rates for all of the phases currently under consideration. However, the results of systematic experimental studies using short-lived actinides and ion-beam irradiations, studies of radiation effects in U- and Th-bearing minerals, and the development of new models of the damage process over the past 20 years have led to a substantial increase in the understanding of the processes of damage accumulation in apatite, zircon, perovskite, zirconolite, and pyrochlore/fluorite structures. This fundamental scientific understanding now provides a basis for predicting the performance of nuclear waste forms in a radiation field. One of the recent successes of these studies has been the discovery of a class of radiation-resistant pyrochlore/fluorite structures that can serve as highly durable, radiation-resistant host phases for the immobilization of actinides.

A key challenge in the permanent disposal of high-level waste (HLW), plutonium residues/scraps, and excess weapons plutonium in glass waste forms is the development of predictive models of long-term performance that are based on a sound scientific understanding of relevant phenomena. Radiation effects from β-decay and α-decay can impact the performance of glasses for HLW and Pu disposition through the interactions of the α-particles, β-particles, recoil nuclei, and γ-rays with the atoms in the glass. Recently, a scientific panel convened under the auspices of the DOE Council on Materials Science to assess the current state of understanding, identify important scientific issues, and recommend directions for research in the area of radiation effects in glasses for HLW and Pu disposition. The overall finding of the panel was that there is a critical lack of systematic understanding on radiation effects in glasses at the atomic, microscopic, and macroscopic levels. The current state of understanding on radiation effects in glass waste forms and critical scientific issues are presented.

Thin films nano-crystalline zirconia of ~ 300 nm thick were deposited on Si substrate, and the samples were irradiated with 2 MeV Au⁺ ions at temperatures of 160 and 400 K, up to fluences of 35 displacements per atom. The films were then studied using glancing incidence x-ray diffraction, Rutherford backscattering, secondary ion mass spectroscopy and transmission electron microscopy. During the irradiation, cavities were observed to form at the zirconia/silicon interface. The morphology of the cavities was found to be related to the damage state of the underlying Si substrate. Elongated cavities were observed when the substrate is heavily damaged but not amorphized; however, when the substrate is rendered amorphous, the cavities become spherical. As the ion dose increases, the cavities then act as efficient gettering sites for the Au. The concentration of oxygen within the cavities determines the order in which the cavities getter. Following complete filling of the cavities, the interface acts as the secondary gettering site for the Au. The Au precipitates are determined to be elemental in nature due to the high binding free energy for the formation of Au-silicides.

Chemical disorder in ion-irradiated SiC and GaN has been examined by means of transmission electron microscopy. Radial distribution functions obtained by a quantitative analysis of electron diffraction intensities revealed that homonuclear bonds, which do not exist in the crystalline state, are formed in both ion-irradiated specimens. The origin of the homonuclear bonds is quite different between SiC and GaN. The constitute elements mix on the atomic-scale in amorphous SiC, while phase separation induced by irradiation is attributed to the formation of self-bonded Ga atomic pairs in amorphous/nanocrystalline GaN.

Zircon, ZrSiO₄, is a well-characterized, naturally occurring phase that is extremely durable. Zircon has been synthesized with Pu-concentrations up to 10 wt. % and radiation-damage effects studied to saturation doses of nearly 0.8 displacements per atom. We propose that zircon be used as a waste form for the disposal of the more than 100 metric tons of plutonium that will result from the dismantling of nuclear weapons. There are already several demonstrated processing technologies, of which hot pressing offers the most potential. This highly durable material, even under hydrothermal conditions, with its high waste loading and smaller volume allows deep, permanent disposal of the weapons plutonium in geologic environments in which the borosilicate waste-form glass would not be stable.

Irradiation-induced amorphization in nanocrystalline and single-crystal 3C-SiC has been studied using 1 MeV Si⁺ ions under identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in situ ion channeling and ex situ x-ray diffraction methods. The results show that, compared with single-crystal 3C-SiC, full amorphization of small 3C-SiC grains (˜3.8 nm in size) at room temperature occurs at a slightly lower dose. Grain size decreases with increasing dose until a fully amorphized state is attained. The amorphization dose increases at 400 K relative to room temperature. However, at 400 K, the amorphization dose for 2.0 nm grains is about a factor of 4 and 8 smaller than for 3.0 nm grains and bulk single-crystal 3C-SiC, respectively. The behavior is attributed to the preferential amorphization at the interface.

Irradiation-induced amorphization in nanocrystalline and single-crystal 3C-SiC has been studied using 1 MeV Si⁺ ions under identical irradiation conditions at room temperature and 400 K. The disordering behavior has been characterized using in situ ion channeling and ex situ x-ray diffraction methods. The results show that, compared with single-crystal 3C-SiC, full amorphization of small 3C-SiC grains (˜3.8 nm in size) at room temperature occurs at a slightly lower dose. Grain size decreases with increasing dose until a fully amorphized state is attained. The amorphization dose increases at 400 K relative to room temperature. However, at 400 K, the amorphization dose for 2.0 nm grains is about a factor of 4 and 8 smaller than for 3.0 nm grains and bulk single-crystal 3C-SiC, respectively. The behavior is attributed to the preferential amorphization at the interface.

The electronic properties of wurtzite/zinc‐blende (WZ/ZB) heterojunction GaN are investigated using first‐principles methods. A small component of ZB stacking formed along the growth direction in the WZ GaN nanowires does not show a significant effect on the electronic property, whereas a charge separation of electrons and holes occurs along the directions perpendicular to the growth direction in the ZB stacking. The later case provides an efficient way to separate the charge through controlling crystal structure. These results have significant implications for most state of the art excitonic solar cells and the tuning region in tunable laser diodes.

Codoping of p-type GaN nanowires with Mg and oxygen was investigated using first-principles calculations. The Mg becomes a deep acceptor in GaN nanowires with high ionization energy due to the quantum confinement. The ionization energy of Mg doped GaN nanowires containing passivated Mg–O complex decreases with increasing the diameter, and reduces to 300 meV as the diameter of the GaN nanowire is larger than 2.01 nm, which indicates that Mg–O codoping is suitable for achieving p-type GaN nanowires with larger diameters. The codoping method to reduce the ionization energy can be effectively used in other semiconductor nanostructures.

Damage and microstructure evolution in gallium nitride (GaN) under Au⁺ ion irradiation has been investigated using complementary electron microscopy, secondary ion mass spectrometry and ion-beam analysis techniques. Epitaxially-grown GaN layers (2 µm thick) have been irradiated by 2.0 MeV Au ions to 1.0 × 10¹⁵ and 1.4 × 10¹⁵ cm⁻² at 155 K and to 7.3 × 10¹⁵ cm⁻² at 200 K. The irradiation-induced damage has been analysed by Rutherford backscattering spectroscopy in a channelling direction (RBS/C). For a better determination of the ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) are combined to analyse the ion channelling spectra. With increasing irradiation dose, separated amorphous layers develop from the sample surface and near the damage peak region. Formation of large nitrogen bubbles with sizes up to 70 nm is observed in the buried amorphous layer, while the surface layer contains small bubbles with a diameter of a few nanometres due to significant nitrogen loss from the surface. Volume expansion from 3% to 25% in the irradiated region is suggested by cross-sectional transmission electron microscope and RBS/C measurement. The anomalous shape of the Au distributions under three irradiations indicates out-diffusion of Au towards the sample surface. The results from the complementary techniques suggest that nitrogen is retained in the damaged GaN where the crystallinity is preserved. Once the amorphous state is reached in the surface region, GaN starts to decompose and nitrogen escapes from the surface. Furthermore, experimental results show considerable errors in both the disorder profile and the ion range predicted by the Stopping and Range of Ions in Matter code, indicating a significant overestimation of electronic stopping powers of Au ions in GaN.

Atomic configurations and formation energies of native defects in an unsaturated GaN nanowire grown along the [001] direction and with (100) lateral facets are studied using large-scale ab initio calculation. Cation and anion vacancies, antisites, and interstitials in the neutral charge state are all considered. The configurations of these defects in the core region and outermost surface region of the nanowire are different. The atomic configurations of the defects in the core region are same as those in the bulk GaN, and the formation energy is large. The defects at the surface show different atomic configurations with low formation energy. Starting from a Ga vacancy at the edge of the side plane of the nanowire, a N–N split interstitial is formed after relaxation. As a N site is replaced by a Ga atom in the suboutermost layer, the Ga atom will be expelled out of the outermost layers and leaves a vacancy at the original N site. The Ga interstitial at the outmost surface will diffuse out by interstitialcy mechanism. For all the tested cases N–N split interstitials are easily formed with low formation energy in the nanowires, indicating N2 molecular will appear in the GaN nanowire, which agrees well with experimental findings.

Previous computer simulations of multiple 10 keV Si cascades in 3C–SiC demonstrated that many damage-state properties exhibit relatively smooth, but noticeably different, dose dependencies. A more recent analysis of these damage-state properties, which includes additional data at low and intermediate doses, reveals more complex relationships between system energy, swelling, energy per defect, relative disorder, elastic modulus, and elastic constant, C₁₁. These relationships provide evidence for the onset of both defect clustering and solid-state amorphization, which appear to be driven by local energy and elastic instabilities from the accumulation of defects. The results provide guidance on experimental approaches to reveal the onset of these processes.

The mechanical behavior of twinned silicon carbide (SiC) nanowires under combined tension-torsion and compression-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading. The torsion rate has no effect on the elastic properties of the twinned SiC nanowires. The collapse of the twinned nanowires takes place in a twin stacking fault of the nanowires.

The dynamics of track development due to the passage of relativistic heavy ions through solids is a long-standing issue relevant to nuclear materials, age dating of minerals, space exploration, and nanoscale fabrication of novel devices. We have integrated experimental and simulation approaches to investigate nanoscale phase transitions under the extreme conditions created within single tracks of relativistic ions in Gd₂O₃(TiO₂)_xand Gd₂Zr_2–xTi_xO₇. Track size and internal structure depend on energy density deposition, irradiation temperature, and material composition. Based on the inelastic thermal spike model, molecular dynamics simulations follow the time evolution of individual tracks and reveal the phase transition pathways to the concentric track structures observed experimentally. Individual ion tracks have nanoscale core-shell structures that provide a unique record of the phase transition pathways under extreme conditions.

Slow heavy ions inevitably produce a significant concentration of defects and lattice disorder in solids during their slowing-down process via ion-solid interactions. For irradiation effects research and many industrial applications, atomic defect production, ion range, and doping concentration are commonly estimated by the stopping and range of ions in matter (SRIM) code. In this study, ion-induced damage and projectile ranges of low energy Au ions in SiC are determined using complementary ion beam and microscopy techniques. Considerable errors in both disorder profile and ion range predicted by the SRIM code indicate an overestimation of the electronic stopping power, by a factor of 2 in most cases, in the energy region up to 25 keV/nucleon. Such large discrepancies are also observed for slow heavy ions, including Pt, Au, and Pb ions, in other compound materials, such as GaN, AlN, and SrTiO3. Due to the importance of these materials for advanced device and nuclear applications, better electronic stopping cross section predictions, based on a reciprocity principle developed by Sigmund, is suggested with fitting parameters for possible improvement.

Depth profiles of local volume expansions are precisely measured in 6H-SiC after irradiation at 150 K with 2 MeV Pt ions and following annealing at 770 K using transmission electron microscopy equipped with electron energy loss spectroscopy. It is found that the depth profile of local volume expansion from the as-implanted sample matches well with the depth profile of irradiation-induced local disorder measured by Rutherford backscattering spectrometry. Further, the local volume expansion increases linearly with local dose up to ∼10%. By systematically comparing the depth profiles of local volume expansion and local relative disorder, it is revealed that the atomic volume of amorphous SiC continues to increase until it saturates at ∼14% due to the increased chemical short-range disorder. This is believed to be one of the reasons for significant scatter in values of volume expansion previously reported for the irradiation-induced amorphous state of SiC.

A single-crystal 3C-SiC film on a Si/SiO2/Si (separation by implantation of oxygen ) substrate was irradiated in different areas at 156 K with Au2+ ions to low fluences. The disorder profiles as a function of dose on both the Si and C sublattices have been determined in situ using a combination of 0.94 MeV D+ Rutherford backscattering spectrometry and nuclear reaction analysis in channeling geometry along the ⟨100⟩, ⟨110⟩, and ⟨111⟩ axes. The results indicate that for the same damage state, the level of disorder on the Si sublattice in 3C-SiC follows a decreasing order along the ⟨111⟩, ⟨100⟩, and ⟨110⟩ axes, while that on the C sublattice shows comparable values. Similar levels of Si and C disorder are observed along the ⟨111⟩ axis over the applied dose range. However, the level of C disorder is higher than that of Si disorder along all axes at low doses. The amount of disorder recovery during thermal annealing depends on the sublattice (Si or C) and crystallographic orientation. Room-temperature recovery occurs for both sublattices in 3C-SiC irradiated to a dose of 0.047 dpa or lower. Significant recovery is observed along all directions during thermal annealing at 600 K. The results are discussed and compared to those for 6H-SiC and 4H-SiC under similar irradiation conditions.

This report presents the results of classical molecular dynamics simulations of the diffuse premelting transition, melting, and defect production by 1 keV U recoils in UO2 using five different rigid ion potentials. The experimentally observed premelting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after ∼15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from ∼28 to ∼83 eV and the number of Frenkel pairs is different by a factor of 3 depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration, and binding energies in UO2 from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions.

This report presents the results of classical molecular dynamics simulations of the diffuse premelting transition, melting, and defect production by 1 keV U recoils in UO2 using five different rigid ion potentials. The experimentally observed premelting transition occurred for all five cases. For all the potentials studied, dynamic defect annealing is highly effective and is accompanied by replacement events on the anion sublattice. The primary damage state after ∼15 ps consists of isolated Frenkel pairs and interstitial and vacancy clusters of various sizes. The average displacement energy varies from ∼28 to ∼83 eV and the number of Frenkel pairs is different by a factor of 3 depending on the choice of potential. The size and spatial distribution of vacancy and interstitial clusters is drastically different for the potentials studied. The results provide statistics of defect production. They point to a pressing need to determine defect formation, migration, and binding energies in UO2 from first principles and to develop reliable potentials based on this data for simulating microstructural evolution in nuclear fuel under operating conditions.

A first-principles method was used to investigate the structural and energetic properties for A₂Ti₂O₇ (A = Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the 〈A–O_48f〉 interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the 〈Ti–O_48f〉 interactions are relatively stronger than the 〈A–O_48f〉 interactions. It was found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.

A first-principles method was used to investigate the structural and energetic properties for A₂Ti₂O₇ (A = Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the 〈A–O_48f〉 interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the 〈Ti–O_48f〉 interactions are relatively stronger than the 〈A–O_48f〉 interactions. It was found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.

A first-principles method was used to investigate the structural and energetic properties for A₂Ti₂O₇ (A = Lu, Er, Y, Gd, Sm, Nd, La), including the formation energies of the cation antisite-pair, the anion Frenkel pair that defines anion-disorder, and the coupled cation antisite-pair/anion-Frenkel. It is proposed that the 〈A–O_48f〉 interaction may have more significant influence on the radiation resistance behavior of titanate pyrochlores, although the 〈Ti–O_48f〉 interactions are relatively stronger than the 〈A–O_48f〉 interactions. It was found that the defect formation energies are not simple functions of the A-site cation radii. The formation energy of the cation antisite-pair increases continuously as the A-site cation varies from Lu to Gd, and then decreases continuously with the variation of the A-site cation from Gd to La, in excellent agreement with the radiation-resistance trend of the titanate pyrochlores. The band gaps in these pyrochlores were also measured, and the band gap widths changed continuously with cation radius.

Atomistic structures of high-energy ion irradiated GaN were examined using transmission electron microscopy (TEM). Single crystalline GaN substrates were irradiated at cryogenic temperatures with 2 MeV Au2+ ions to a fluence of 7.35×1015 Au/cm2. Cross-sectional TEM observations revealed that damaged layers consisting of amorphous and nanocrystalline phases are formed at the surface and buried depth of the as-irradiated GaN substrate. Atomic radial distribution functions of the amorphous/polynanocrystalline regions showed that not only heteronuclear Ga–N bonds but also homonuclear Ga–Ga bonds exist within the first coordination shell. It was found that the ratio of heteronuclear-to-homonuclear bonds, i.e., the degree of chemical disorder, is different between the surface and buried damaged layers. The alternation of chemical disorder was attributed to the difference in the defect formation processes between these layers.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

The defect creation at low energy events was studied using density functional theory molecular dynamics simulations in silicon carbide nanotubes, and the displacement threshold energies determined exhibit a dependence on sizes, which decrease with decreasing diameter of the nanotubes. The Stone–Wales (SW) defect, which is a common defect configurations induced through irradiation in nanotubes, has also been investigated, and the formation energies of the SW defects increase with increasing diameter of the nanotubes. The mean threshold energies were found to be 23 and 18 eV for Si and C in armchair (5,5) nanotubes.

The thermal evolution of the microstructure created by irradiation of a GaN single crystal with 2 MeV Au2+ ions at 150 K is characterized following annealing at 973 K using transmission electron microscopy. In the as-irradiated sample characterized at 300 K, Ga nanocrystals with the diamond structure, which is an unstable configuration for Ga, are directly observed together with nitrogen bubbles in the irradiation-induced amorphous layer. A simple model is proposed to explain Ga nanocrystal formation. Upon thermal annealing, the thickness of the amorphous layer decreases by ∼13.1% and nanobeam electron diffraction analysis indicates no evidence for residual Ga nanocrystals, but instead reveals a mixture of hexagonal and cubic GaN phases in the annealed sample. Nitrogen molecules, captured in the as-irradiated bubbles, appear to disassociate and react with Ga nanocrystals during the thermal annealing to form crystalline GaN. In addition, electron energy loss spectroscopy measurements reveal an volume change of 18.9% for the as-irradiated amorphous layer relative to the virgin single crystal GaN. This relative swelling of the damaged layer reduces to 7.7% after thermal annealing. Partial recrystallization and structural relaxation of the GaN amorphous state are believed responsible for the volume change.

Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies Ed along five specific directions and to determine the defect configurations created during low energy events. The Ed shows a significant dependence on direction. The minimum Ed is determined to be 39 eV along the ⟨1¯010⟩ direction for a gallium atom and 17.0 eV along the ⟨1¯010⟩ direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average Ed values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N–N dumbbell configuration), but various configurations for Ga defects are formed in GaN.

Large-scale ab initio molecular dynamics method has been used to determine the threshold displacement energies Ed along five specific directions and to determine the defect configurations created during low energy events. The Ed shows a significant dependence on direction. The minimum Ed is determined to be 39 eV along the ⟨1¯010⟩ direction for a gallium atom and 17.0 eV along the ⟨1¯010⟩ direction for a nitrogen atom, which are in reasonable agreement with the experimental measurements. The average Ed values determined are 73.2 and 32.4 eV for gallium and nitrogen atoms, respectively. The N defects created at low energy events along different crystallographic directions have a similar configuration (a N–N dumbbell configuration), but various configurations for Ga defects are formed in GaN.

Ab initio total energy calculations have been performed to investigate the properties of intrinsic defects in GaN. It is found that the nitrogen defects are more stable than the Ga defects under nitrogen-rich conditions, and the results are generally consistent with those obtained by recent first-principles calculations. For the four types of nitrogen interstitials investigated, relaxation of all configurations leads to a N–N⟨112¯0⟩ split configuration. The most stable configuration for Ga interstitials is the Ga octahedral interstitial, but the energy difference between the octahedral and tetrahedral configurations is small (&lt;0.35 eV) and depends on the basis set employed. While the ⟨N–N⟩ bond distance in the N–N split interstitial is very close to that of a free N2 molecule, the Mulliken charge analysis indicates that the N atoms are partially charged, which is in contrast with previous theoretical suggestions. Based on the calculated results, the relative stabilities of various defects in GaN are determined.

Ab initio total energy calculations have been performed to investigate the properties of intrinsic defects in GaN. It is found that the nitrogen defects are more stable than the Ga defects under nitrogen-rich conditions, and the results are generally consistent with those obtained by recent first-principles calculations. For the four types of nitrogen interstitials investigated, relaxation of all configurations leads to a N–N⟨112¯0⟩ split configuration. The most stable configuration for Ga interstitials is the Ga octahedral interstitial, but the energy difference between the octahedral and tetrahedral configurations is small (&lt;0.35 eV) and depends on the basis set employed. While the ⟨N–N⟩ bond distance in the N–N split interstitial is very close to that of a free N2 molecule, the Mulliken charge analysis indicates that the N atoms are partially charged, which is in contrast with previous theoretical suggestions. Based on the calculated results, the relative stabilities of various defects in GaN are determined.

Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion beam irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. It was confirmed that the structural changes observed by the in situ TEM study agree qualitatively with those of the bulk material. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.

We have observed efficient damage recovery in large-scale molecular dynamics simulations of 30 keV Zr recoils in pure zirconia and yttria-stabilized zirconia, which is in stark contrast to radiation damage accumulation in zircon. Dynamic annealing is highly effective in zirconia during the first 5 ps of damage evolution, especially in the presence of oxygen structural vacancies. This results in near-complete recovery of damage. Damage recovery on the cation sublattice is assisted by the anion sublattice recovery, which explains the remarkable radiation tolerance of stabilized zirconia. Ceramics engineered to heal themselves in this fashion hold great promise for use in high-radiation environments or for safely encapsulating high-level radioactive waste over geological time scales.

First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that ⟨Gd-O48f⟩ bond is less covalent than ⟨Sm-O48f⟩ and ⟨Er-O48f⟩ bonds, while ⟨Ti-O48f⟩ bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A=Sm, Gd, Er) pyrochlores, ⟨Ti-O48f⟩ bonds may play more significant role in determining their radiation resistance to amorphization.

First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that ⟨Gd-O48f⟩ bond is less covalent than ⟨Sm-O48f⟩ and ⟨Er-O48f⟩ bonds, while ⟨Ti-O48f⟩ bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A=Sm, Gd, Er) pyrochlores, ⟨Ti-O48f⟩ bonds may play more significant role in determining their radiation resistance to amorphization.

First-principles calculations have been carried out to study the electronic properties of A2Ti2O7 (A=Sm, Gd, Er) pyrochlores. It was found that f electrons have negligible effect on the structural and energetic properties, but have significant effect on the electronic properties. Density of state analysis shows that A-site 4f electrons do take part in the chemical bonding. Also, we found that ⟨Gd-O48f⟩ bond is less covalent than ⟨Sm-O48f⟩ and ⟨Er-O48f⟩ bonds, while ⟨Ti-O48f⟩ bond in Gd2Ti2O7 is more covalent. It was proposed that for A2Ti2O7 (A=Sm, Gd, Er) pyrochlores, ⟨Ti-O48f⟩ bonds may play more significant role in determining their radiation resistance to amorphization.

Electron-beam-induced effects in preamorphized Sr₂Nd₈(SiO₄)₆O₂ were investigated in situ using transmission electron microscopy with 200-keV electrons at temperatures ranging from 380 to 780 K. Within the electron-irradiated area, epitaxial recrystallization was observed from the amorphous/crystalline interface toward the surface, with the rate of recrystallization increasing as temperature increased from 380 to 580 K. Structural contrast features (i.e., O deficient amorphous material), as well as recrystallization, were observed outside of the irradiation area at temperatures from 680 to 780 K. Ionization-induced processes and local nonstoichiometry induced by oxygen migration and desorption are possible mechanisms for the electron-beam- induced recrystallization and for the formation of the structural contrast features, respectively.

The tensile mechanical behavior of single crystalline gallium nitride (GaN) nanotubes under combined tension-torsion is investigated using molecular dynamics simulations with an empirical potential. The simulation results show that a small torsion rate (&lt;0.010degps−1) does not affect the tensile behavior of GaN nanotube, i.e., the nanotubes show brittle properties at low temperatures; whereas at high temperatures, they behave as ductile materials. However, the failure stress decreases with increasing rate of torsion above 0.010degps−1, and the nanotube fails in a different manner. The torsion rate has no effect on the elastic properties of GaN nanotubes.

Molecular dynamics simulations with Tersoff potentials were used to study the response of single crystalline SiC nanotubes under tensile, compressive, torsional, combined tension-torsional, and combined compression-torsional strains. The simulation results reveal that the nanotubes deform through bond-stretching and breaking and exhibit brittle properties under uniaxial tensile strain, except for the thinnest nanotube at high temperatures, which fails in a ductile manner. Under uniaxial compressive strain, the SiC nanotubes buckle with two modes, i.e., shell buckling and column buckling, depending on the length of the nanotubes. Under torsional strain, the nanotubes buckle either collapse in the middle region into a dumbbell-like structure for thinner wall thicknesses or fail by bond breakage for the largest wall thickness. Both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading.

Due to events of the past two decades, there has been new and increased usage of radiation-detection technologies for applications in homeland security, nonproliferation, and national defense. As a result, there has been renewed realization of the materials limitations of these technologies and greater demand for the development of next-generation radiation-detection materials. This review describes the current state of radiation-detection material science, with particular emphasis on national security needs and the goal of identifying the challenges and opportunities that this area represents for the materials-science community. Radiation-detector materials physics is reviewed, which sets the stage for performance metrics that determine the relative merit of existing and new materials. Semiconductors and scintillators represent the two primary classes of radiation detector materials that are of interest. The state-of-the-art and limitations for each of these materials classes are presented, along with possible avenues of research. Novel materials that could overcome the need for single crystals will also be discussed. Finally, new methods of material discovery and development are put forward, the goal being to provide more predictive guidance and faster screening of candidate materials and thus, ultimately, the faster development of superior radiation-detection materials.

Well-aligned ripple structures on the surface of a single crystal of 3C-SiC were created by focused ion beam bombardment, and the resulting morphology and topography were characterized using in situ focused ion beam/scanning electron microscopy, as well as ex situ atomic force microscopy. The ripple structure formed as a result of ion sputtering beyond a critical incident angle (∼50°), and its characteristic wavelength varied from 158to296nm with changes in the incident angle and ion beam flux. The geometry, ordering, and homogeneity of the ripples can be well controlled by varying the ion beam incident angle and beam current, as required for the fabrication of nanostructures that use SiC for optical and electronic applications.

Ab initio molecular dynamics simulations are employed to study the atomistic mechanisms and pathways of high-pressure phase transformation in GaN and SiC. Our simulations bring a fundamental level of understanding of the wurtzite to rocksalt phase transformation that undergoes inhomogeneous displacements via a tetragonal atomic configuration, and suggest that the transition path may be independent of the presence of d electrons on the cation in GaN. The discrepancies between experimental and theoretical studies of transition paths are discussed.

Electronic band structures of single-walled silicon carbide nanotubes are studied under uniaxial strain using first principles calculations. The band structure can be tuned by mechanical strain in a wide energy range. The band gap decreases with uniaxial tensile strain, but initially increases with uniaxial compressive strain and then decreases with further increases in compressive strain. These results may provide a way to tune the electronic structures of silicon carbide nanotubes, which may have promising applications in building nanodevices.