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Session F25 - DMP: THEORY OF MATERIALS IV: OXIDES AND RELATED MATERIALS
Mixed session, Tuesday morning, March 21, 11:00
Regency Ballroom I, Fairmont Hotel
The first-principles modelling of binary oxides with configurational disorder presents a complexity not present in the more studied metallic systems. The creation of charge-compensating defects (vacancies or interstitials) when mixing alio-valent cations, leads to a quaternary configurational problem. We show how the configurational disorder in these materials can be modelled with two coupled binary cluster expansions with reduced symmetry. The configurational energy dependence is treated with a real-space cluster expansion. We show that this expansion can be made to converge rapidly in a limited energy window, even in the case of an unscreened electrostatic 1/r interaction. This makes it possible to compute Madelung sums in real space with only a few terms. The coupled cluster expansion can describe any state of order or association without any of the commonly made a priori assumptions (isolated or associated defects). We apply this formalism to oxide mixtures on the fluorite lattice, such as ZrO _2 -Gd _2 O _3 and ZrO _2 -CaO. Ordering of oxygens and vacancies in these materials is important for their technological applications and is found to be strongly dependent on the state of cation ordering. This is investigated by freezing the cation order parameter while equilibrating the anion species. For quenched cation configurations, there exists a competition between vacancy-cation association (spin-glass behavior) and vacancy ordering. In iso- valent cation mixtures, such as CaO-MgO, where the electrostatic effects do not influence the cation configuration, the variation of the oxygen size is found to be important for correctly predicting phase stability.