The cubic Bi1.76U0.12La0.12O3.18 mixed oxide: Synthesis, structural characterization, thermal stability and electrical properties

Cubic bismuth uranium lanthanum oxide with formula Bi1.76U0.12La0.12O3.18 has been synthesized by heating at 950 °C a mixture of α-Bi2O3 and La0.5U0.5O2.25. After heating, the mixture was slowly cooled down to room temperature, and a cubic fluorite-type Fm3m single phase with lattice parameter ac = 5.605(1) Å has been obtained. The title compound does not undergo any structural transformation on heating up to 800 °C, as confirmed by high-temperature X-ray powder diffraction. The electrical conductivity vs. reciprocal temperature shows two regimes with a change in conductivity at ca. 550 °C. Both regimes are associated with conduction of O2− ions. The activation energy of the low-temperature regime and the high-temperature regime is 0.96 and 0.42 eV, respectively. Aging of the cubic phase by annealing at 600 °C for short (100 h) or long time (550 h) produces segregation of α-Bi2O3 and phases of lower symmetry (hexagonal or monoclinic, respectively) with a composition different from that of the starting cubic phase.