Effect of M 4+ (Ce, Sn, Ti) B-site substitutions on the cation ordering in Ba(Mg 1/3Ta 2/3)O 3

Abstract

The effect of different tetravalent octahedral site substitutions on the cation ordering in Ba(Mg 1/3Ta 2/3)O 3 perovskites were explored by investigating the phase stabilities and structures of their solid solutions with BaTiO 3, BaSnO 3, and BaCeO 3. In the cerate system, the 1:2 ordered form of Ba(Mg 1/3Ta 2/3)O 3 has very limited stability and transforms to a cubic, 1:1 ordered structure with a doubled perovskite repeat between 7 and 25 mol% BaCeO 3. Structure refinements of the Ba(β′ 1/2β″ 1/2)O 3 phases indicate that the 1:1 order can be described by a “random site” model. In this model, the β″ sites are occupied by Ta, the β′sites are occupied by a random distribution of Mg, Ce, and the remaining Ta cations, and the stoichiometry of the structure can be represented by Ba[Mg 2(1−x)/3Ta (1−4x)/3Ce 2x] 1/2[Ta] 1/2O 3. Although the formation of this phase in the cerate system is similar to the results reported previously for BaZrO 3 substitutions, no evidence of 1:1 order was found in the BaTiO 3 solid solutions, which adopt a disordered perovskite structure for >10 mol% Ti. The correlation between the size of the tetravalent cation and the formation of the random site structure extended to the BaSnO 3 system, which also showed no evidence of an extended region of stable 1:1 order.