Synthesis and characterisation of the new cubic phase CsVTeO5 and re-investigation of the pyrochlore CsVTeO6

Abstract

The compound CsVTeO5 has been synthesized by solid state reactions and studied by X-ray powder diffraction. CsVTeO5 is polymorphic with a cubic high temperature variety, space group Fd3m and a=10.482(2). The observed X-ray powder pattern of this cubic phase is similar to the AB2O6 pyrochlore-type structure and also to those reported for ATiXO5 phases (A=Rb, Cs, 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 disordered TiO6 octahedra and XO4 (X=P, As) tetrahedra with the A+ cation occupying large cavities within this framework. The cubic phase 6' [a=10.483(1)], previously reported as a pyrochlore-type structure, corresponds in fact to CsVTeO5. The cubic CsVTeO6 phase, with a pyrochlore related structure, has also been synthesized and the structure was refined by Rietveld profiling of the X-ray data. The space group is Fd3m with a=10.077(1). The value of the unit cell parameter is in good agreement with those reported for the ABTeO6 pyrochlore series (A=K, Rb, Cs and B=Nb, Ta). The thermal stabilities of CsVTeO5 and CsVTeO6 under argon and oxygen atmospheres are discussed.