The perovskite-type solid solutions, Sr2-xLaxMg1-yTa1+yOz, were prepared and the cation ordering between Mg2+ and Ta5+ was investigated. In the case of the Sr2-xLaxMgTaOz system, an (NH4)3FeF6-type (1:1 order) phase was formed at x-values greater than 0.4. However, cation ordering between Mg2+ and Ta5+ was also observed for x-values of between 0 and 0.3. The results suggested that cation ordering between Mg2+ and Ta5+ in the Sr2-xLaxMgTaOz system was affected by the average charge of the A-site cation. That is, by the O content. The 1:2-type ordering was observed in Sr2-xLaxMg1-yTa1+yOz solid solutions for x = 0 and y = 0.3, and for x = 0.1 and y = 0.1 or 0.2. In particular, a disappearance of 1:2 ordering due to small amounts of La3+ substitution was observed in stoichiometric Sr2Mg0.67Ta1.33O6-SrLaMgTaO6 solid solution. However, in the case of compositions which contained O vacancies, the 1:2 ordered phase was formed at compositions with Mg/Ta = 0.8/1.2 and 0.9/1.1, even though they had 20 and 35% higher Mg2+ contents, respectively, than in the case of Mg/Ta = 1/2. It was suggested that the 1:2 ordered structure was stabilized by the presence of O vacancies.

Cation Ordering in the Oxygen Deficient Perovskite (Sr,La)2(Mg,Ta)O. S.Kato, E.Ohmori, Y.Suzuki, Y.Ohshima, M.Sugai, H.Takizawa, T.Endo: Journal of the Ceramic Society of Japan, 1999, 107[3], 209-14