COMPUTER-SIMULATION STUDIES OF TERNARY URANATE PHASES WITH ALKALI AND ALKALINE-EARTH METALS .1. MUO3

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, MIIO, in MIIUO3 is shown to occur via a mechanism in which the M(II) ions substitute onto both the M(II) and U sublattices.