Ternary lithium halides, structure maps, characteristic M-X distances, and stability

Ternary lithium halides Li2MCl4 (M = Mg, V, Cr, Mn, Fe, Co, Zn, Cd) and Li2MBr4 (M = Mg, Mn, Fe, Zn, Cd display more or less high ionic conductivity depending on the crystal structure of the respective compound. The structures are mainly governed by geometric factors, viz., the ionic radii of M2+ and the cell volumes of the halides, as shown by various structure maps. Characteristic M-Cl and M-Br distances established for the first time were shown to be a better measure for structural features rather than the ionic radii. The difference Delta r between the sum of the ionic radii and the characteristic M-X distances are a hint of the covalence of the respective bonds. The results of cohesive energy calculations (MAPLE) with respect to the stability of the compounds under study and the conditions of superstructure ordering in inverse spinel-type compounds are reported.