Effect of M4+ (Ce, Sn, Ti) B-site substitutions on the cation ordering in Ba(Mg1/3Ta2/3)O3

The effect of different tetravalent octahedral site substitutions on the cation ordering in Ba(Mg1/3Ta2/3)O3 perovskites were explored by investigating the phase stabilities and structures of their solid solutions with BaTiO3, BaSnO3, and BaCeO3. In the cerate system, the 1:2 ordered form of Ba(Mg1/3Ta2/3)O3 has very limited stability and transforms to a cubic, 1:1 ordered structure with a doubled perovskite repeat between 7 and 25 mol% BaCeO3. Structure refinements of the Ba(β′1/2β″1/2)O3 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[Mg2(1−x)/3Ta(1−4x)/3Ce2x]1/2[Ta]1/2O3. Although the formation of this phase in the cerate system is similar to the results reported previously for BaZrO3 substitutions, no evidence of 1:1 order was found in the BaTiO3 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 BaSnO3 system, which also showed no evidence of an extended region of stable 1:1 order.