Site-selective doping effect, phase separation, and structure evolution in 1:1:1 triple-cation B-site ordered perovskites Ca4−xSrxGaNbO8

Abstract

Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca₄GaNbO₈ (Ca₄-type) by substituting Ca²⁺ with larger Sr²⁺. Sr²⁺-to-Ca²⁺ substitution did not lead to oxygen-vacancy ordering–disordering transition but an interesting Ca₄-to-Sr₄ type structure transition. Rietveld refinements revealed that the two-type structures exhibit similar oxygen-vacancy ordering and identical 1:1:1 triple-cation B-site ordering. Close inspection of the two-type structures revealed the subtle structure difference lies in the orientations of GaO₄ tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca_4−xSr_xGaNbO₈. More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr²⁺ in Ca_4−xSr_xGaNbO₈. These structural changes found in Ca_4−xSr_xGaNbO₈ will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities.