PREPARATION, CHARACTERIZATION, AND THERMAL-BEHAVIOR OF A NEW HIGH OXIDE-ION CONDUCTOR - BISMUTH URANIUM LANTHANUM OXIDE

The bismuth-based mixed oxide with composition Bi2U0.5La0.5O5.25 has been prepared by a mixed ceramic/organic precursor method. Material quenched from 950 degrees C crystallizes with the fluorite-type structure, which reversibly transforms into a trigonal P (3) over bar low-temperature phase. The chemical and structure characterization, thermal behavior, and long-term stability studies of this material have been undertaken. For the low-temperature phase, a random distribution of cations in 1a and 2d sites of the P (3) over bar space group leads to the best fit for the structure Rietveld refinement. From electron diffraction studies, structural relationships between the fcc high-temperature phase and the low-temperature form have been established. The latter can be described as alpha(c) root 2/2 x alpha(c) root 2/2 x alpha c root 3 superstructure of the fluorite cell. The presence of microdomains of the low-temperature form embedded within the fluorite matrix has also been shown. The mechanism that can account for the thermal decomposition of this oxide above 1000 degrees C has been pointed out. The oxide ion conductivity of both cubic and trigonal Bi2U0.5La0.5O5.25 has also been measured, the total conductivity at 500 degrees C being 2.6 x 10(-3) and 5.2 x 10(-4) S cm(-1), with activation energies 71.7 and 84.6 kJ mol(-1), respectively.