Effect of explicit cationic size and valence constraints on the phase stability of 1:2 B-site-ordered perovskite ruthenates

The related parameters of cation size and valence that control the crystallization of Sr₃CaRu₂O₉ into a 1:2 B-site-ordered perovskite structure were explored by cationic substitution at the strontium and calcium sites and by the application of high pressure. At ambient pressures, Sr₃MRu₂O₉ stoichiometries yield multiphasic mixtures for M = Ni²⁺, Mg²⁺, and Y ³⁺, whereas pseudocubic perovskites result for M = Cu²⁺ and Zn²⁺. For A-site substitutions, an ordered perovskite structure results for Sr_3-xCa_xCaRu₂O₉, with 0 ≤ x ≤ 1.5. In contrast, Ba²⁺ substitution for Sr²⁺ is accompanied by a phase change to a hexagonal BaTiO₃ structure type. At high pressures and temperatures, a 1:2 B-site-ordered perovskite structure is stabilized for Sr_3-xBa_xCaRu₂O ₉, with 0 ≤ x ≤ 3. The scarcity of B-site-ordered perovskite ruthenates at ambient pressure and the metastable nature of the high-pressure phases underscore the strict size and valence requirements that must be met by the constituent cations to achieve these uncommon ordered structures.