Ionic conductivities of spinel-type quaternary lithium chlorides-phase diagrams of LiClM IClM IICl 2 (M ICu, Na; M IIMn, Cd, Mg)

Abstract

The phase relationships of the systems Li 2M IICl 4M 2 IM IICl 4 (M ICu, Na; M IIMg, Mn, Cd) were studied by thermal analyses (DTA), X-ray and Raman methods. The electric conductivities of the Li 2−2xM 2x IM IICl 4 solid solutions obtained were determined by impedance spectroscopy. In the ternary Li 2M IICl 4 as much as 100 mol% of lithium can be replaced by copper (I) (at 650 K, M IICd) and 80 mol% by sodium (at 720 K). At ambient temperature, the mutual solubilities are considerably smaller, i.e., negligible for M INa and only up to 40 mol% for M ICu. In case of the copper containing compounds, the transitions of the spinel-type solid solutions present at ambient temperature to deficient NaCl superstructures at elevated temperatures can be studied by both X-ray and Raman methods. The ionic conductivities of the solid solutions decrease (i) slightly with increase in sodium content, but (ii) strongly with increasing copper content at least for x<0.5. Since copper (I) primarily replaces lithium ions in tetrahedral voids whereas sodium substitutes those in octahedral ones, these conductivity data confirm the conduction mechanism proposed for spinel-type chlorides formerly, namely an interstitialcy diffusion of the lithium ions located in tetrahedral sites including octahedral interstitial sites. The spinel-type Li 2−2xCu 2x IM IICl 4 are a further example of the mixed alkali effect in crystalline materials.