Tiffany Kaspar

Oxide heterointerfaces are extremely common in both natural and artificial composite structures, including corroded structural materials. Often, key properties such as segregation and atomic transport are dictated by the structure of these interfaces. However, despite this critical link, very few heterointerfaces have been studied in any detail at the atomic scale. Here, one important oxide heterointerface is examined, between spinel and corundum, using the chemical system FeCr₂O₄/Cr₂O₃ as a representative and technologically important case. Using atomistic simulation techniques, it is found that the structure, particularly the local chemistry, of the interface depends on the crystal chemistry at the interface. This atomic and chemical structure further impacts important properties such as defect segregation and mass transport. It is found that defects can nucleate at some regions of these interfaces and migrate back and forth across the corundum layer, suggesting high atomic mobility that may be important for the evolution of spinel/corundum composite structures in extreme conditions.

Thin film deposition is a fundamental technology for the discovery, optimization, and manufacturing of functional materials. Deposition by molecular beam epitaxy (MBE) typically employs reflection high-energy electron diffraction (RHEED) as a real-time in situ probe of the growing film. However, the state-of-the-art for RHEED analysis during deposition requires human observation. Here, we present an approach using machine learning (ML) methods to monitor, analyze, and interpret RHEED images on-the-fly during thin film deposition. In the analysis workflow, RHEED pattern images are collected at one frame per second and featurized using a pretrained deep convolutional neural network. The feature vectors are then statistically analyzed to identify changepoints; these changepoints can be related to changes in the deposition mode from initial film nucleation to a transition regime, smooth film deposition, and in some cases, an additional transition to a rough, islanded deposition regime. The feature vectors are additionally analyzed via graph analysis and community classification. The graph is quantified as a stabilization plot, and we show that inflection points in the stabilization plot correspond to changes in the growth regime. The full RHEED analysis workflow is termed RHAAPsody and includes data transfer and output to a visual dashboard. We demonstrate the functionality of RHAAPsody by analyzing the precaptured RHEED images from epitaxial depositions of anatase TiO2 on SrTiO3(001) and show that the analysis workflow can be executed in less than 1 s. Our approach shows promise as one component of ML-enabled real-time feedback control of the MBE deposition process.

The functionality of nuclear structural materials, sensors, and microelectronics in harsh environments such as radiation relies on understanding defect generation and evolution processes in oxide layers. The initial radiation response of epitaxial thin films of Fe₃O₄(111), Cr₂O₃(0001), and Fe₃O₄(111)/Cr₂O₃(0001) heterostructures deposited on Al₂O₃(0001) by oxygen‐assisted molecular beam epitaxy and irradiated with 200 keV He⁺ is characterized. X‐ray diffraction and X‐ray absorption near edge spectroscopy showed that the Cr₂O₃ layers underwent significant lattice expansion and disordering under irradiation, whereas the Fe₃O₄ layers do not exhibit noticeable changes. In contrast, positron annihilation spectroscopy revealed an evolution of cation vacancy point defects in the Fe₃O₄ layers into larger vacancy clusters with increasing irradiation, while the cation vacancies in Cr₂O₃ remained primarily as single vacancies and small clusters. The results suggest that the Fe₃O₄ lattice can utilize the free volume of the larger vacancy clusters to relax but the small vacancies in the Cr₂O₃ lattice do not facilitate relaxation. Comparing defect concentrations in the single layer films versus the heterostructure suggests that point defects may cross the interface from Fe₃O₄ into Cr₂O₃. Together, these results enhance the understanding of the initial defect evolution mechanisms in oxide layers in harsh irradiation environments.

Irradiation induced non-equilibrium point defect populations influence mass transport in oxides, which in turn affects their stability and performance in hostile environments. In this study a strong dose rate dependence is observed.

Self‐diffusion is a fundamental physical process that, in solid materials, is intimately correlated with both microstructure and functional properties. Local transport behavior is critical to the performance of functional ionic materials for energy generation and storage, and drives fundamental oxidation, corrosion, and segregation phenomena in materials science, geosciences, and nuclear science. Here, an adaptable approach is presented to precisely characterize self‐diffusion in solids by isotopically enriching component elements at specific locations within an epitaxial film stack, and measuring their redistribution at high spatial resolution in 3D with atom probe tomography. Nanoscale anion diffusivity is quantified in a‐Fe₂O₃ thin films deposited by molecular beam epitaxy with a thin (10 nm) buried tracer layer highly enriched in ¹⁸O. The isotopic sensitivity of the atom probe allows precise measurement of the initial sharp layer interfaces and subsequent redistribution of ¹⁸O after annealing. Short‐circuit anion diffusion through 1D and 2D structural defects in Fe₂O₃ is also directly visualized in 3D. This versatile approach to study precisely tailored thin film samples at high spatial and mass fidelity will facilitate a deeper understanding of atomic‐scale diffusion phenomena.

Recent investigations on spinel CoMn₂O₄ have shown its potential for applications in water splitting and fuel cell technologies as it exhibits strong catalytic behavior through oxygen reduction reactivity. To further understand this material, we report for the first time the synthesis of single-crystalline Co_1+x Mn_2−x O₄ thin films using molecular beam epitaxy. By varying sample composition, we establish links between cation stoichiometry and material properties using in-situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry. Our results indicate that excess Co ions occupy tetrahedral interstitial sites at lower excess Co stoichiometries, and become substitutional for octahedrally-coordinated Mn at higher Co levels. We compare these results with density functional theory models of stoichiometric CoMn₂O₄ to understand how the Jahn–Teller distortion and hybridization in Mn–O bonds impact the ability to hole dope the material with excess Co. The findings provide important insights into CoMn₂O₄ and related spinel oxides that are promising candidates for inexpensive oxygen reduction reaction catalysts.

Control of order–disorder phase transitions is a fundamental materials science challenge, underpinning the development of energy storage technologies such as solid oxide fuel cells and batteries, ultra‐high temperature ceramics, and durable nuclear waste forms. At present, the development of promising complex oxides for these applications is hindered by a poor understanding of how interfaces affect lattice disordering processes and defect transport. Here, the evolution of local disorder in ion‐irradiated La₂Ti₂O₇/SrTiO₃ thin film heterostructures is explored using a combination of high‐resolution scanning transmission electron microscopy, position‐averaged convergent beam electron diffraction, electron energy loss spectroscopy, and ab initio simulations. Highly non‐uniform lattice disordering driven by asymmetric oxygen vacancy formation across the interface is observed. Theory calculations indicate that this asymmetry results from differences in the polyhedral connectivity and vacancy formation energies of the two interface components, suggesting ways to manipulate lattice disorder in functional oxide heterostructures.

Understanding the response of ceramics operating in extreme environments is of interest for a variety of applications. Ab initio molecular dynamic simulations have been used to investigate the effect of structure and B-site (=Ti, Zr) cation composition of lanthanum-based oxides (La₂B₂O₇) on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La₂Ti₂O₇ occurs for a lower degree of electronic excitation than for cubic pyrochlore La₂Zr₂O₇. While in each case the formation of O₂-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO₆ octahedra in the monoclinic phase can promote such molecule formation, while such octahedral rotation is not possible in the cubic phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La₂TiZrO₇) is found to be more resistant in the monoclinic than in the cubic phase, which may be related to the lower bandgap of the cubic phase. These results illustrate the complex interplay of structure and composition that give rise to the radiation resistance of these important functional materials.

The authors report on the chemical, structural, and optical properties of molecular beam epitaxy synthesized thin films of multifunctional Fe2CrO4 on (001)-oriented MgAl2O4 (MAO). Substrate temperatures near 500 °C are required to obtain smooth films with an out-of-plane lattice parameter consistent with the 3.8% compressive strain induced by the film and substrate lattice mismatch. Mg diffusion from the MAO substrate is kinetically suppressed up to 500 °C. They discuss antiphase boundaries in symmetry matched epitaxial systems. This research provides new insight into the epitaxial growth and crystalline properties of crystal symmetry matched Fe2CrO4/MAO(001) heterostructures.

Radioactive waste immobilization is a means to limit the release of radionuclides from various waste streams into the environment over a timescale of hundreds to many thousands of years. Incorporation of radionuclide-containing wastes into borosilicate glass during vitrification is one potential route to accomplish such immobilization. To facilitate comparisons and assessments of reproducibility across experiments and laboratories, a six-component borosilicate glass (Si, B, Na, Al, Ca, Zr) known as the International Simple Glass (ISG) was developed by international consensus as a compromise between simplicity and similarity to waste glasses. Focusing on a single glass composition with a multi-pronged approach utilizing state-of-the-art, multi-scale experimental and theoretical tools provides a common database that can be used to assess relative importance of mechanisms and models. Here we present physical property data (both published and previously unpublished) on a single batch of ISG, which was cast into individual ingots that were distributed to the collaborators. Properties from the atomic scale to the macroscale, including composition and elemental impurities, phase purity, density, thermal properties, mechanical properties, optical and vibrational properties, and the results of molecular dynamics simulations are presented. In addition, information on the surface composition and morphology after polishing is included. Although the existing literature on the alteration of ISG is not extensively reviewed here, the results of well-controlled static alteration experiments are presented here as a point of reference for other performance investigations.

The behavior of polar LaMnO₃ (LMO) thin films deposited epitaxially on nonpolar SrTiO₃(001) (STO) is dictated by both the LMO/STO band alignment and the chemistry of the Mn cation. Using in situ X‐ray photoelectron spectroscopy, the valence band offset (VBO) of LMO/STO heterojunctions is directly measured as a function of thickness, and found that the VBO is 2.5 eV for thicker (≥3 u.c.) films. No evidence of a built‐in electric field in LMO films of any thickness is found. Measurements of the Mn valence by Mn L‐edge X‐ray absorption spectroscopy and by spatially resolved electron energy loss spectra in scanning transmission electron microscopy images reveal that Mn²⁺ is present at the LMO surface, but not at the LMO/STO interface. These results are corroborated by density functional theory simulations that confirm a VBO of ≈2.5 eV for both ideal and intermixed interfaces. A model is proposed for the behavior of polar/nonpolar LMO/STO heterojunctions in which the polar catastrophe is alleviated by the formation of oxygen vacancies at the LMO surface.

Current piezoelectric sensors and actuators are limited to operating temperatures less than ~200 °C due to the low Curie temperature of the piezoelectric material. Strengthening the piezoelectric coupling of high-temperature piezoelectric materials, such as La₂Ti₂O₇(LTO), would allow sensors to operate across a broad temperature range. The crystalline orientation and piezoelectric coupling direction of LTO thin films can be controlled by epitaxial matching to SrTiO₃(001), SrTiO₃(110), and rutile TiO₂(110) substrates via pulsed laser deposition. The structure and phase purity of the films are investigated by x-ray diffraction and scanning transmission electron microscopy. Piezoresponse force microscopy is used to measure the in-plane and out-of-plane piezoelectric coupling in the films. The strength of the out-of-plane piezoelectric coupling can be increased when the piezoelectric direction is rotated partially out-of-plane via epitaxy. The strongest out-of-plane coupling is observed for LTO/STO(001). Deposition on TiO₂(110) results in epitaxial La_2/3TiO₃, an orthorhombic perovskite of interest as a microwave dielectric material and an ion conductor. La_2/3TiO₃can be difficult to stabilize in bulk form, and epitaxial stabilization on TiO₂(110) is a promising route to realize La_2/3TiO₃for both fundamental studies and device applications. Overall, these results confirm that control of the crystalline orientation of epitaxial LTO-based materials can govern the resulting functional properties.