Cubic bismuth uranium lanthanum oxide with formula Bi1.76U0.12La0. 12O3.18 has been synthesized by heating at 950 degrees C a mixture of alpha 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 a,= 5.605(l) A has been obtained. The title compound does not undergo any structural transformation on heating up to 800 degrees 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 degrees 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 degrees C for short (100 h) or long time (550 h) produces segregation of alpha-Bi2O3 and phases of lower symmetry (hexagonal or monoclinic, respectively) with a composition different from that of the starting cubic phase. (c) 2005 Elsevier B.V. All rights reserved.