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 Bi2W1−xUxO6 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][An−1BnO3n+1] Aurivillius structures seems yet to be restricted by the n = 1 case.

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