Synthesis and structural characterization of AMV2O8 (A = K, Rb, Tl, Cs; M = Nb, Ta) vanadates: a structural comparison of A+M5+V2O8 vanadates and A+M5+P2O8 phosphates

Abstract

Eight new quaternary vanadates of niobium and tantalum, AMV₂O₈ (A = K, Rb, Tl, Cs; M = Nb, Ta), have been prepared by solid state reactions and structurally characterized by single crystal and powder X-ray diffraction (XRD) techniques. The two cesium compounds, unlike the known CsSbV₂O₈ with a layered yavapaiite structure, have a new three-dimensional structure and the other six compounds possess the known KSbV₂O₈ structure type. The three types of [(MV₂O₈)⁻]_∞ anionic frameworks of twelve A⁺M⁵⁺V₂O₈ (A = K, Rb, Tl, Cs; M = Nb, Ta, Sb) vanadates could be conceived to be built by different connectivity patterns of M₂V₄O₁₈ ribbons, which contain MO₆ octahedra and VO₄ tetrahedra. A structural comparison of these twelve vanadates and the nineteen A⁺M⁵⁺P₂O₈ phosphates has been made. The spectroscopic studies of these eight new quaternary vanadates are presented.