Narrow Band Gap Compounds with β‐Pyrochlore Structure in the A 2 O‐V 2 O 5 ‐2TeO 3 (A=Rb, Cs) System

Two new isostructural compounds with composition of Rb + V 3+ 0.125 V 5+ 0.625 Te 6+ 1.25 O 6 and Cs + V 3+ 0.125 V 5+ 0.625 Te 6+ 1.25 O 6 have been synthesized by solid‐state reaction. The crystal structure has been determined by X‐ray diffraction analysis using Rietveld refinement. Both compounds possess β‐pyrochlore structure‐type with cubic space group Fd‐3 m (Z=8); the unit cell parameters: a=10.04120(27) Å for RbV 0.75 Te 1.25 O 6 Å and a=10.09119(23) Å for CsV 0.75 Te 1.25 O 6 . 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 (∼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. RbV 0.75 Te 1.25 O 6 and CsV 0.75 Te 1.25 O 6 melts at low temperatures (∼500 °C) with decomposition and weight loss. The RbV 0.75 Te 1.25 O 6 powder decomposes to amorphous phase, whereas CsV 0.75 Te 1.25 O 6 transform into new β‐pyrochlore compound with another composition.