Uranium substitution for tungsten in the Bi2WO6–Bi2UO6 system: Formation of a broad high-temperature solid solution

Polythermic studies of Bi₂WO₆–Bi₂UO₆ system revealed existence of a broad Bi₂W_1?xU_xO₆ solid solution with the Aurivillius-type structure. Below 750 °C, the homogeneity area is narrow (x ≤ 0.12) but extends significantly at higher temperatures (up to ca. 75% Bi₂UO₆ at 1025 °C). The solid solution retains orthorhombic symmetry over the entire compositional range, but substitution of U for W results in significant lowering of second harmonic intensity and Curie point suggesting, at high substitution degrees, a very close approach to the archetypic body-centered tetragonal centrosymmetric structure. The known Bi₂WO₆-based solid solutions are discussed in terms of thermal stability, and a general geometrical explanation is suggested. Incorporation of large cations into the B perovskitic positions of Bi₂O₂]A_n?1B_nO_3n+1] Aurivillius structures seems yet to be restricted by the n = 1 case.