✍ Scribed by Yoshio Oka; Takeshi Yao; Naoichi Yamamoto; Yutaka Ueda
No coin nor oath required. For personal study only.
Single crystals of -CoV 3 O 8 , a mixed-valence compound of V IV/V , were hydrothermally synthesized. It crystallizes in the orthorhombic system Ibam with a ؍ 14.3298(6) A > , b ؍ 9.8906(6) A > , c ؍ 8.3950(8) A > , and Z ؍ 8. The structure was refined to R ؍ 0.034 and R w ؍ 0.030 for 1558 reflections with I'3 (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 MO 6 slabs and V(2)O 4 -V(3)O 5 units. The MO 6 slab is constructed by linking zigzag chains of edge-sharing MO 6 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 bc plane. The V(2)O 4 -V(3)O 5 unit, playing a role in bridging MO 6 slabs, is made of an edge-sharing V(3)O 5 pair to which two V(2)O 4 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 MO 6 chains, which accounts for the half-occupancies of Co and V(1) as well as a positional shift of O(5). The structure is compared with those of -ZnV 3 O 8 and -MgV 3 O 8 in particular reference to metal distributions. The magnetic susceptibility curve of -CoV 3 O 8 exhibits a sharp peak at 8 K, suggesting the onset of antiferromagnetic order.
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