CRYSTAL STRUCTURE OF MIXED-VALENCE [[alpha]]-CoV3O8 WITH UNUSUAL METAL DISTRIBUTION. Yoshio Oka1, Takeshi Yao2 and Naoichi Yamamoto1, Faculty of Integrated Human Studies1, Faculty of Engineering2, Kyoto University, Kyoto 606 Japan
In the CoO-VO2-V2O5 system studied at 600deg.C only CoV3O8 is found as a V(IV,V) mixed-valence compound1. It was originally formulated as Co1+y(V3O8)2 (O.90<=y<=1) with two polymorphs [[alpha]] and ß phases; [[alpha]] phase transforms reversibly into ß phase at 650 +/-8deg.C. It was reported that a phase crystallizes in the body-centered orthorhombic system but the structure has remained unknown. In the present study single crystals of [[alpha]]-CoV3O8 were grown in the hydrothermal CoI2-VO(OH)2 system. It crystallizes in the orthorhombic system Ibam with a=14.3298(6)Å, b=9.8906(6)Å, c=8.3950(8)Å and Z=8. The structure was refined to R/Rw=0.034/0.030 for 1558 reflections with I>3[[sigma]](I). There are three kinds of metal sites, namely octahedral M (16k) for M=Co, V(1), tetrahedral V(2) (8j) and trigonal-bipyramidal V(3) (8j) where M site is occupied by Co and V atoms evenly. The framework structure is constructed as that zigzag chains of edge-shared MO6 octahedra running along the c-axis are linked by sharing O(5) atoms along the b-axis forming slabs of MO6 octahedra parallel to the bc-plane and the slabs are joined by VO4 tetrahedra and VO5 trigonal bipyramids. The valence states of metal sites were estimated as Co2+ and V4+ for M and V5+ for V(2) and V(3). It is noteworthy that the metal distribution over M site is not random but restricted to Co-O(5)-V in neighboring MO6 through O(5) vertex, which accounts for the even occupancies of Co and V atoms in M site and further ensures the stoichiometric composition CoV3O8. The magnetic susceptibility curve exhibits a sharp peak at 8K suggesting the onset of antiferromagnetic order and the value of effective moment is well corresponding to the formula (Co2+0.5V4+0.5)2 V5+208.
1) A. Cassalot and P. Hagenmuller, J. inorg. nucl. Chem., 31, 3049 (1969).