Structure, ionic conductivity and phase transformation of double chloride spinels

New double chlorides, Li2MCl4 (M=Cr, Co, Fe) were prepared and were characterized by X-ray structure refinement, differential thermal analysis, high-temperature X-ray diffraction analysis and electrical conductivity measurement. The new phases have a distorted spinel structure; their orthorhombic and monoclinic distortions determined respectively for the cobalt and iron spinels, and for the chromium spinel were caused by a 1:1 ordering of lithium and metal divalent ions on the octahedral B sites. Conductivity measurements on both the distorted spinels Li2MCl4 (M=Cr, Co, Fe) and the cubic spinels Li2MCl4 (M=Mg, Ti, Mn, Fe, Cd) proved that the distortion causes a considerable degradation of lithium ionic conduction. The ionic conduction of the halide spinels is discussed in terms of dimensionality of conduction pathway, position of lithium ion, polarizability, cation vacancies, and bottleneck size.