POLYMORPHISM, PHASE-TRANSFORMATIONS, AND OXIDE-ION CONDUCTIVITY IN BI1.56U0.22LA0.22O3.33

This paper deals with the polymorphism, phase transitions, and the oxi de ion conduction of Bi1.56U0.22La0.22O3.33. Thermal treatment at 950 degrees C of a mixture of alpha-Bi2O3 and LaUO4+x, followed by slow co oling to room-temperature, yields a hexagonal phase of that compositio n with cell parameters a(H) = 4.0066(7) and c(H) = 9.543(2) Angstrom. Quenching of the reaction mixture from 950 degrees C leads to the form ation of a cubic fluorite-type phase with a(c) = 5.6273(8) Angstrom. A nnealing of both cubic and hexagonal phases at 600 degrees C for 500 h yields a new polymorph that is indexed with a monoclinic lattice, the cell parameters being a(M) = 7.778(3), b(M) = 7.834(4), c(M) = 5.763( 3) Angstrom, and beta = 89.71(2)Angstrom. Phase transitions experience d by each polymorph with temperature are followed by high-temperature X-ray powder diffraction. The three phases are transformed into a new C cubic phase at temperatures above 820 degrees C. The oxide ion cond uction exhibited by each Bi1.56U0.22La0.22O3.33 polymorph is rather di fferent. At 300 degrees C the cubic phase, which is the best conductin g one, shows a value of sigma = 7.2 x 10(-5) S cm(-1), the conductivit y of the hexagonal phase is 2.5 x 10(-5) S cm(-1), and the monoclinic phase shows the lowest conductivity, 6.6 x 10(-7) S cm(-1). The plot o f conductivity vs inverse temperature shows a linear dependence for ea ch phase. The nonlinear dependencies also observed in the conductivity plots are related to structural transformations taking place during t hermal treatments.