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.