BETA-BAV2(P2O7)2 - A NEW POLYMORPH OF BARIUM VANADIUM(III) PYROPHOSPHATE CHARACTERIZED BY INTERSECTING TUNNELS

Investigation into the synthesis of reduced vanadium phosphate has led to the formation of a new form of the barium vanadium (III) pyrophosphate compound beta-BaV2(P2O7)2. It is a polymorph of the previously known BaV2(P2O7)2, which is now labeled as the alpha-phase. The title compound crystallizes in the P-1 (No. 2) space group with a = 6.269 (1) angstrom, b = 7.864 (3) angstrom, c = 6.1592 (9) angstrom, alpha = 101.34 (2)degrees, beta = 105.84 (1)degrees, and gamma = 96.51 (2)degrees. The structure consists of comer-shared VO6 octahedra and PO4 tetrahedra that are connected in V-O-P-O-V and V-O-P-O-P-O-V bonding arrangements. This interesting three-dimensional framework is characterized by seven types of intersecting tunnels, three of which are occupied by the barium cation, while the others are empty. It is important to know that one of the empty tunnels has a relatively large window with a minimum diagonal distance of 4.41 angstrom, which facilitates a possible framework for a lithium ion insertion reaction. The barium atom has a 10-coordination sphere, BaO10, in which the oxygen atoms can be viewed as forming two intersecting pseudohexagonal planes. Beta-BaV2(P2O7)2 appears to form at a relatively higher temperature than its polymorph, alpha-BaV2(P2O7)2. A detailed structural analysis and structural comparison with the alpha-phase, as well as a brief comparison with SrV2(P2O7)2, are presented. (C) 1994 Academic Press, Inc.