1:1:1 Triple-Cation B‑Site-Ordered and Oxygen-Deficient Perovskite Ca4GaNbO8: A Member of a Family of Anion-Vacancy-Based Cation-Ordered Complex Perovskites

Exploration of the Ca–Ga–Nb–O phase diagram by solid-state reaction in air led to isolation of Ca4GaNbO8. The crystal structure was determined ab initio by synchrotron X-ray and high-resolution neutron powder diffraction. Ca4GaNbO8 adopts a heavily distorted oxygen-deficient perovskite structure with the rare feature of complete ordering of the three B-site cations, driven by their distinct chemistries. One of the calcium cations occupies a distorted octahedral cavity and together with tetrahedrally coordinated Ga and octahedrally coordinated Nb is considered as a B-site cation in the ABO3–x perovskite. This interpretation of the structure reveals Ca4GaNbO8 is part of a family of B-site and vacancy-ordered perovskites related to mineral structures. The anion-vacancy ordering pattern in Ca4GaNbO8 is driven by the coordination preferences of the three structurally distinct cations and correlated with the ordering of each cation on a distinct site. Alternating current impedance spectra show Ca4GaNbO8 is insulating (bulk conductivity 10–5–10–7 S·cm–1) over the measured temperature range 550–950 °C with an activation energy of 1.10(3) eV.