Hydrothermal synthesis and structure refinements of alkali-metal trivanadates AV3O8 (A = K, Rb, Cs)

Hydrothermal methods for growing single crystals of alkali-metal trivanadates AV3O8 for A = K, Rb, Cs and their single-crystal structural studies are presented. Plate-shaped orange crystals were hydrothermally grown at 250 °C from V2O5 powders dispersed A(NO)3 solutions. RbV3O8, the structure of which has remained undetermined, was found to be isostructural with KV3O8 and CsV3O8: the monoclinic system P21m with a = 4.9864(8) Å, b = 8.442(1) Å, c = 7.8621(7) Å, β = 96.064(9) °, and Z ≖ 2. The crystallographic data for KV3O8 and CsV3O8 have been revised; in particular, the data of KV3O8 has been greatly improved. AV3O8 adopts a layered structure with V3O8 layers consisting of VO6 octahedra and VO5 square pyramids. The oxygen coordination around interlayer A atom is AO8 for A = Rb, Cs but is regarded as AO6 for A = K from the anomalous variation of A − O distances with A+ ionic radii.