Structure Prediction of Ordered and Disordered Multiple Octahedral Cation Perovskites using SPuDS

The software package SPuDS has previously been shown to accurately predict crystal structures of AMX ₃ and A _{1 - x} A ' _x MX ₃ perovskites that have undergone octahedral tilting distortions. This paper describes the extension of this technique and its accuracy for A ₂ MM ' X ₆ ordered double perovskites with the aristotype Fm3̄m cubic structure, as well as those that have undergone octahedral tilting distortions. A survey of the literature shows that roughly 70% of all ordered double perovskites undergo octahedral tilting distortions. Of the 11 distinct types of octahedral tilting that can occur in ordered perovskites, five tilt systems account for ~97% of the reported structures. SPuDS can calculate structures for the five dominant tilt systems, Fm3̄m ( a ⁰ a ⁰ a ⁰ ), I 4/ m ( a ⁰ a ⁰ c ^- ), R3̄ ( a ^- a ^- a ^- ), I 2/ m ( a ⁰ b ^- b ^- ) and P 2 ₁ / n ( a ^- a ^- b ⁺ ), as well as two additional tilt systems, Pn3̄ ( a ⁺ a ⁺ a ⁺ ) and P 4/ mnc ( a ⁰ a ⁰ c ⁺ ). Comparison with reported crystal structures shows that SPuDS is quite accurate at predicting distortions driven by octahedral tilting. The favored modes of octahedral tilting in ordered double perovskites are compared and contrasted with those in AMX ₃ perovskites. Unit-cell pseudosymmetry in Sr- and Ca-containing double perovskites is also examined. Experimentally, Sr ₂ MM 'O ₆ compounds show a much stronger tendency toward pseudosymmetry than do Ca ₂ MM 'O ₆ compounds with similar tolerance factors.