Die Kristallstruktur von β-Li₃VF₆

Abstract

The crystal structure of the metastable high temperature β-phase of Li₃VF₆ has been determined from 1404 independent reflections (R = 0.035). The space group is C2/c, Z = 12, a = 1440.5(6)pm, b = 868.8(3)pm, c = 1008.2(4) pm, β = 95.84(3)°, d_c = 2.949 g cm⁻³, d_m = 2.94 g cm⁻³. The hitherto unknown β-form of Li₃CrF₆ has been prepared using a mineralisator [lattice constants: a = 1438.2(5) pm, b = 859.4(3) pm, c = 1001.1(2)pm, β = 94.81(2)°]. In contrast to the cryolithe related room temperature α-form β-Li₃VF₆ represents a new structure type. While the V-atoms and 7/9 of the Li-atoms have octahedral coordination, the remaining 2/9 of the Li-atoms show a distorted tetrahedral environment and form binuclear [Li₂F₆]-groups. The octahedra are linked to units of one [VF₆] and two [LiF₆] groups having one common vertex and sharing three edges like the octahedra in some heteropoly anions. These units form a 3-dimensional network by additional edge and corner sharing with each other and the [Li₂F₆]-groups. The unusual polymorphism of the isotypic Li₃MF₆ compounds (M = Al, Ga, Cr, Fe, V, Ti) is discussed on the base of these new results.