Site-selective doping effect, phase separation, and structure evolution in 1:1:1 triple-cation B-site ordered perovskites Ca 4- x Sr x GaNbO 8 .

Oxygen-deficient perovskites are a family of important materials that may exhibit oxide ionic conductivities. We attempted to introduce oxygen-vacancy disordering in perovskite Ca 4 GaNbO 8 (Ca 4 -type) by substituting Ca 2+ with larger Sr 2+ . Sr 2+ -to-Ca 2+ substitution did not lead to oxygen-vacancy ordering-disordering transition but an interesting Ca 4 -to-Sr 4 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 GaO 4 tetrahedra, which is the origin of the formation of the narrow two-phase region (0.3 ≤ x < 0.65) in Ca 4- x Sr x GaNbO 8 . More importantly, the A- and B-site cavities with large differences in size for both structures resulted in a site-selective doping behaviour for Sr 2+ in Ca 4- x Sr x GaNbO 8 . These structural changes found in Ca 4- x Sr x GaNbO 8 will provide a broad route approaching new oxygen-deficient phases with oxide ionic conductivities.