STRUCTURAL AND THERMODYNAMIC CHARACTERIZATION OF THE PEROVSKITE-RELATED BA1+YUO3+X AND (BA, SR)(1+Y)UO3+X PHASES

The perovskite-type BaUO3 structure has been investigated by X-ray and neutron diffraction, The Ba/U ratio, the (Ba, Sr)/U ratio, and the ox ygen stoichiometry in Ba1+yUO3+x were varied, and the integral enthalp ies of formation were determined by solution calorimetry, In addition, equilibrium oxygen partial pressures were measured using a reversible EMF cell. The chemical defect mechanism is discussed, and it is shown that the continuous series BaUO3-Ba1+yUO3+x-Ba3UO6 exists in which ur anium vacancies are gradually filled with barium ions whereas uranium is oxidized via the pentavalent to the hexavalent state in Ba3UO6 (=Ba -2(Ba, U)O-6). (C) 1997 Academic Press.