Simon Middleburgh

Atomic scale simulations are used to predict how excess oxygen is accommodated across the group II monoxides. In all cases, the preference is to form a peroxide ion centered at an oxygen site, rather than a single oxygen species, although the peroxide ionic orientation changes from <100> to <110> to <111> with increasing host cation radius. The enthalpy for accommodation of excess oxygen in BaO is strongly negative, whereas in SrO it is only slightly negative and in CaO and MgO the energy is positive. Interestingly, the increase in material volume due to the accommodation of oxygen (the defect volume) does not vary greatly as a function of cation radius. The vibrational frequency of peroxide ions in the group II monoxides is predicted with the aim to provide test data for future experimental observations of oxygen uptake. Finally, calculations of the dioxide structures have also been carried out. For these materials the oxygen vacancy formation energy is always positive (1.0–1.5 eV per oxygen removed) indicating that they exhibit only small oxygen defect concentrations.

We have determined the energetics of defect formation and migration in M_n+1AX_n phases with M = Ti, A = Si or Al, X = C, and n = 3 using density functional theory calculations. In the Ti₃SiC₂ structure, the resulting Frenkel defect formation energies are 6.5 eV for Ti, 2.6 eV for Si, and 2.9 eV for C. All three interstitial species reside within the Si layer of the structure, the C interstitial in particular is coordinated to three Si atoms in a triangular configuration (C–Si = 1.889 Å) and to two apical Ti atoms (C–Ti = 2.057 Å). This carbon–metal bonding is typical of the bonding in the SiC and TiC binary carbides. Antisite defects were also considered, giving formation energies of 4.1 eV for Ti–Si, 17.3 eV for Ti–C, and 6.1 eV for Si–C. Broadly similar behavior was found for Frenkel and antisite defect energies in the Ti₃AlC₂ structure, with interstitial atoms preferentially lying in the analogous Al layer. Although the population of residual defects in both structures is expected to be dominated by C interstitials, the defect migration and Frenkel recombination mechanism in Ti₃AlC₂ is different and the energy is lower compared with the Ti₃SiC₂ structure. This effect, together with the observation of a stable C interstitial defect coordinated by three silicon species and two titanium species in Ti₃SiC₂, will have important implications for radiation damage response in these materials.

Simulations using density functional theory were carried out to investigate the defect properties of zirconium diboride (ZrB₂) and also the solution and diffusion of He and Li. Schottky and Frenkel intrinsic defect processes were all high energy as were mechanisms giving rise to nonstoichiometry; this has implications for high‐temperature performance. Li and He species, formed by the transmutation of a ¹⁰B, should therefore mostly be accommodated at the resulting vacant B sites or interstitial sites. Because Li is considerably more stable at the vacant B sites, He will be accommodated interstitially. Furthermore, He was found to diffuse as an interstitial species through the lattice with a low activation energy. This would be consistent with He being lost from the ZrB₂ but with Li being retained to a much greater extent.