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) a nd Cs(TiP)O-5 (CTP) were solved and refined by application of the Riet veld profile technique to high-resolution synchrotron X-ray and neutro n 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 (CT P), and are characterized by an unusual framework consisting of random ly disordered TiO6 octahedra and XO(4) (X As, P) tetrahedra with the C s+ 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 paramete rs for both cations and oxygens. The Ti,X-site can be resolved as a sp lit-atom position where the tetrahedral metal is shifted from the cent er toward the edge formed by the octahedral oxygens. Split-oxygen posi tions were refined from a combination of X-ray and neutron data for CT A. The cubic structure is related to the orthorhombic KTiOPO4-type str ucture in that the latter can be derived from the former by ordering o f the TiO6 octahedra and PO4 tetrahedra. Since different ordering sche mes 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 g amma-NaTiOPO4, cubic CTA/CTP can be viewed as the ''mother of all ABOC O(4)- structures.'' The elucidation of the cubic structure allows us t o propose a quantified ''threatened structure model,'' which helps in the understanding of the mechanism of this phase transition. (C) 1995 Academic Press, Inc.