Cs(TiAs)O 5 and Cs(TiP)O 5: A Disordered Parent Structure of ABOCO 4 Compounds

Abstract

The structures of the two isostructural compounds Cs (TiAs)O 5 (CTA) and Cs(TiP)O 5 (CTP) were solved and refined by application of the Rietveld profile technique to high-resolution synchrotron X-ray and neutron powder data. The structures are cubic, space group F d3̄ m with a = 10.23415(5) Å (CTA) and a = 10.08733(2) Å (CTP), and are characterized by an unusual framework consisting of randomly disordered TiO 6 octahedra and XO 4 ( X = As, P) tetrahedra with the Cs + cation occupying large cavities within this framework. The lack of long-range order between octahedra and tetrahedra simulates partially occupied octahedra shared by Ti and X with large displacement parameters for both cations and oxygens. The Ti, X-site can be resolved as a split-atom position where the tetrahedral metal is shifted from the center toward the edge formed by the octahedral oxygens. Split-oxygen positions were refined from a combination of X-ray and neutron data for CTA. The cubic structure is related to the orthorhombic KTiOPO 4-type structure in that the latter can be derived from the former by ordering of the TiO 6 octahedra and PO 4 tetrahedra. Since different ordering schemes applied to the cubic CTA/CTP structure lead to other known ordered structures of the same stoichiometry such as K(Mg 1/3Nb 2/3)OPO 4 and γ-NaTiOPO 4, cubic CTA/CTP can be viewed as the "mother of all ABOCO 4-structures." The elucidation of the cubic structure allows us to propose a quantified "threatened structure model," which helps in the understanding of the mechanism of this phase transition.