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) pyrophosp hate compound beta-BaV2(P2O7)2. It is a polymorph of the previously kn own BaV2(P2O7)2, which is now labeled as the alpha-phase. The title co mpound 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 tetrahe dra that are connected in V-O-P-O-V and V-O-P-O-P-O-V bonding arrangem ents. This interesting three-dimensional framework is characterized by seven types of intersecting tunnels, three of which are occupied by t he 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 min imum 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 comp arison with the alpha-phase, as well as a brief comparison with SrV2(P 2O7)2, are presented. (C) 1994 Academic Press, Inc.