Computer simulation studies of ternary uranate phases with alkali and alkaline-earth metals. Part 1.—MUO3

Abstract

Solid-state computer simulation techniques have been used to study the alkali and alkaline-earth metal MUO₃ uranate phases. These compounds display an interesting gradation in their structures which, it is shown, is accompanied by a variation in their intrinsic defect chemistry. For example, in the alkali-metal series, LiUO₃ adopts the lithium niobate structure and lithium Frenkel disorder dominates whereas KUO₃ and RbUO₃ adopt regular perovskite structures with Schottky defects being dominant.

For the alkaline-earth metal compounds, both the calculations and experiment show that only SrUO₃ and BaUO₃ are stable with respect to the binary oxides. Both of these phases adopt the GdFeO₃ distorted perovskite structure and both have anti-site defects as the dominant intrinsic disorder. The tendency for anti-site disorder is also seen in the oxidation behaviour of these compounds. The calculations suggest that the oxidation will occur through the formation of a secondary UO₂ fluorite phase by the movement of alkaline-earth ions onto uranium sites, leaving behind M vacancies. The calculated energies for such oxidation processes are particularly favourable. The solution of alkaline-earth oxide, M_IIO, in M_IIIUO₃ is shown to occur via a mechanism in which the M_II ions substitute onto both the M_II and U sublattices.