SYNTHESIS AND CHARACTERIZATION OF THE NEW CUBIC PHASE CSVTEO5 AND REINVESTIGATION OF THE PYROCHLORE CSVTEO6

The compound CsVTeO5 has been synthesized by solid state reactions and studied by X-ray powder diffraction. CsVTeO5 is polymorphic with a cu bic high temperature variety, space group Fd (3) over bar m and a = 10 .482(2) Angstrom. The observed X-ray powder pattern of this cubic phas e is similar to the AB(2)O(6) pyrochlore-type structure and also to th ose reported for ATiXO(5) phases (A = Rb, Ca, Tl and X = P, As). These phases correspond to the high temperature cubic polymorph of KTiOPO4 (KTP). The framework of these cubic phases consists of randomly disord ered TiO6 octahedra and XO4 (X = P, As) tetrahedra with the A(+) catio n occupying large cavities within this framework. The cubic phase 'CsV TeO6' [a = 10.483(1) Angstrom], previously reported as a pyrochlore-ty pe structure, corresponds in fact to CsVTeO5. The cubic CsVTeO6 phase, with a pyrochlore related structure, has also been synthesized and th e structure was refined by Rietveld profiling of the X-ray data. The s pace group is Fd (3) over bar m with a = 10.077(1) Angstrom. The value of the unit cell parameter is in good agreement with those reported f or the ABTeO(6) pyrochlore series (A = K, Rb, Cs and B=Nb, Ta). The th ermal stabilities of CsVTeO5 and CsVTeO6 under argon and oxygen atmosp heres are discussed.