Cation ordering in the oxygen deficient perovskite Sr_2-xLa_xMg_1-yTa_1+yO_z

Perovskite-type solid solutions, Sr_2-xLa_xMg_1-yTa_1+yO_z, were synthesized and cation ordering between Mg²⁺ and Ta⁵⁺ in the solid solutions was investigated. In the Sr_2-xLa_xMgTaO_z system, the (NH₄)₃FeF₆-type (1:1 order) phase was formed in the compositional range, x ≥ 0.4. However, few degree of the cation ordering between Mg²⁺ and Ta⁵⁺was also observed in the solid solutions, 0≤x≤0.3. These results suggest that the cation ordering between Mg²⁺ and Ta⁵⁺ in Sr_2-xLa_xMgTaO_z system is affected by the average charge of A-site cation or, in other words, by the oxygen content. The 1: 2-type ordering was observed in the Sr_2-xLa_xMg_1-yTa_1+yO_z solid solutions, x = 0 for y ≥ 0.3, and x ≤ 0.1 for y = 0.1, 0.2. In particular, the disappearance of the 1: 2 ordering due to small amount of La³⁺ substitution was observed in the stoichiometric Sr₂Mg_0.67Ta_1.33O₆-LaSrMgTaO ₆ solid solution. However, in the case of the composition containing oxygen vacancy, the 1: 2 ordered phase was formed at the composition Mg/Ta = 0.8/1.2 and 0.9/1.1, although they have 20% and 35% higher Mg²⁺ content than in the case of Mg/Ta = 1/2, respectively. These results suggest that the 1 : 2 ordered structure is stabilized by the presence of oxygen vacancy.