CRYSTAL-STRUCTURES OF MGCRO4-TYPE LI2VCL4 AND SPINEL-TYPE LI2MGCL4 AND LI2CDCL4

The crystal structures of the ternary lithium chlorides Li2MCl4 (M = M g, V, Cd) have been determined firstly by X-ray single-crystal experim ents. Li2MgCl4 and Li2CdCl4 crystallize in an inverse spinel structure (space group Fd3mBAR, Z = 8, a = 1 040.1(2) and 1 062.06(9) pm, struc tural parameters u = 0.25699(2) and 0.2550(1), R = 1.7 and 3.7% for 21 8 and 211 unique reflections). The Li-Cl distances of the tetrahedrall y coordinated Li+ ions are significantly greater than calculated with Shannon's crystal radii (> 238 +/- 1 instead of 233 pm). Contrary to t he results of X-ray powder data reported in the literature, Li2VCl4 cr ystallizes in the distorted spinel structure of MgCr2O4 type (space gr oup F43mBAR, Z = 8, a = 1037.49(2) pm, R = 5.9% for 217 unique reflect ions). The decrease of the site symmetry of the octahedrally coordinat ed ions (V2+, Li+) from 3mBAR to 3m resulting in contracted and widene d tetrahedral M, entities of the spinel structure is obviously caused by V-V metal-metal bonds (shortest V-V distance 366.2(7) pm).