THE DIPHOSPHOVANADATE CD2VPO7 - A MIXED FRAMEWORK WITH PSEUDO-CLOSE-PACKED OCTAHEDRAL LAYERS CONNECTED THROUGH DITETRAHEDRAL VPO7 GROUPS

A new vanadium (V) phosphate Cd2VPO7 has been synthesized by hydrother mal synthesis. It crystallizes in the space group P2(1/c) with a = 4.7 12(1) angstrom, b = 10.791(1) angstrom, c = 5.620(1) angstrom, and bet a = 97.34-degrees(1). Refinement of the atomic parameters leads to R = 0.032 and R(w) = 0.037. The structure is built from buckled [Cd2O6]in finity layers of edge-sharing CdO6 octahedra parallel to (100), connec ted by layers of tetrahedral VPO7 groups. A comparison with the divana date Cd2V2O7 shows that both structures exhibit [Cd2squareO6]infinity layers derived from the close-packed [Cd3O6]infinity layers observed i n the rock salt CdO structure by elimination of one Cd atom out of thr ee in an ordered way. It results in distorted six-sided windows, which are oriented differently in Cd2VPO7 and in Cd2V2O7 so that the V2O7 a nd VPO7 that block these windows are displayed differently. The distri bution of V and P is discussed. The idea of a random distribution of V and P is discarded in favor of the existence of VPO7 groups oriented at random. (C) 1994 Academic Press, Inc.