Crystal structure and metal distribution of α-CoV3O8

Single crystals of alpha-CoV3O8, a mixed-valence compound of V-IV/V, were hydrothermally synthesized. It crystallizes in the orthorhombic system Ibam with a = 14.3298(6) Angstrom, b = 9.8906(6) Angstrom, c = 8.3950(8) Angstrom, and Z = 8. The structure was refined to R = 0.034 and R-w = 0.030 for 1558 reflections with I > 3 sigma(I). There are three kinds of metal sites, namely, octahedral M (16 k) for M = Co, V(1), tetrahedral V(2) (8j), and trigonal-bipyramidal V(3) (8j), where the M site was found to consist of half-occupied Co and V(1), Its polyhedral framework is built up of MO6 slabs and V(2)O-4-V(3)O-5 units. The MO6 slab is constructed by linking zigzag chains of edge-sharing MO6 octahedra running along the c axis, The linkage is made along the b axis by sharing common vertices of O(5) and thus the slab stands in the be plane. The V(2)O-4-V(3)O-5 unit, playing a role in bridging MO6 slabs, is made of an edge-sharing V(3)O-5 pair to which two V(2)0, are attached. Valence states of metals were evaluated as Co-II, V(1)(IV), V(2)(V), and V(3)(V). The metal distribution over the M site is basically random but is constrained by a rule to form a Co-O(5)-V(1) group at the linkage of MO6 chains, which accounts for the half-occupancies of Co and V(1) as well as a positional shift of 0(5), The structure is compared with those of alpha-ZnV3O8 and beta-MgV3O8 in particular reference to metal distributions. The magnetic susceptibility curve of alpha-CoV3O8 exhibits a sharp peak at 8 K, suggesting the onset of antiferromagnetic order. (C) 1998 Academic Press.