Cs(TiAs)O-5 and Cs(TiP)O-5: A disordered parent structure of ABOCO(4) compounds

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 <Fd(3)over barm> with a 10.23415(5) Angstrom (CTA) and alpha = 10.08733(2) Angstrom (CTP), and are characterized by an unusual framework consisting of randomly disordered TiO6 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 KTiOPO4-type structure in that the latter can be derived from the former by ordering of the TiO6 octahedra and PO4 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(Mg1/13Nb2/3)OPO4 and gamma-NaTiOPO4, 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. (C) 1995 Academic Press, Inc.