title: 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.
abstract: 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 ABO(3-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.
authors.affiliation: Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, United Kingdom.
authors.forename: Tao
authors.initials: T
authors.lastname: Yang
authors.forename: John B
authors.initials: JB
authors.lastname: Claridge
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Rosseinsky