Narrow Band Gap Compounds with beta-Pyrochlore Structure in the A(2)O-V2O5-2TeO(3) (A=Rb, Cs) System

Two new isostructural compounds with composition of Rb+V0.1253+V0.6255+Te1.256+O6 and Cs+V0.1253+V0.6255+Te1.256+O6 have been synthesized by solid-state reaction. The crystal structure has been determined by X-ray diffraction analysis using Rietveld refinement. Both compounds possess beta-pyrochlore structure-type with cubic space group Fd-3 m (Z=8); the unit cell parameters: a=10.04120(27) angstrom for RbV0.75Te1.25O6 angstrom and a=10.09119(23) angstrom for CsV0.75Te1.25O6. The oxidation states of vanadium and tellurium have been confirmed by X-ray photoelectron spectroscopy. The compounds contain vanadium in mixed valence state (3+ and 5+). The compounds possess the unusual narrow band gap for oxide compounds - in the infrared (IR) range (similar to 0.6 eV). The schematic electronic structure of the compounds has been estimated using XPS, UPS and electronic conductivity data; however, the shifts of the top of the valence edge and the bottom of the conduction band in the water conditions have been calculated using theoretical approximation. The thermal behavior of obtained compounds has been studied by differential thermal analysis. RbV0.75Te1.25O6 and CsV0.75Te1.25O6 melts at low temperatures (similar to 500 degrees C) with decomposition and weight loss. The RbV0.75Te1.25O6 powder decomposes to amorphous phase, whereas CsV0.75Te1.25O6 transform into new beta-pyrochlore compound with another composition.