Cation ordering transformations in Ba(Mg Ta )O3-BaZrO3 perovskite solid solutions

The effect of the substitution of BaZrO3 on the cation ordering in Ba(Mg13Ta23)O3 was studied using TEM and X-ray and neutron diffraction. Almost no solubility of Zr was found in the 1:2 ordered, trigonal structure of the Ba(Mg13Ta23)O3 end-member (P 3̄ml), and a transformation to a 1:1 ordered, cubic (Fm3m) phase with a = 2aper occurs for substitution levels between ~10 and 25 mol% BaZrO3. The structure of this Ba(β′12β″sol12)O3-type phase consists of two distinct octahedral sites, β′ and β″. The occupancies of the two cation positions, refined using the Rietveld method, were found to be consistent with a “random site” model in which β″ is occupied by Ta, and β′ by a random distribution of the remaining cations. The homogeneity range of the 1:1 solid solutions predicted by this model, Ba{[Mg(2 − y)3Ta(1 − 2y)3 Zry]12[Ta]12}O3 (0.0 < y ≤ 0.5), is in excellent agreement with that observed experimentally.