The origin of the conductivity maximum in molten salts. III. Zinc halides

In a continuing effort to master the reasons for conductivity maxima vs temperature in semicovalent molten halides, the structure and some transport properties of molten zinc halide are examined with ab initio molecular dynamics. Molten zinc halides are a special class of molten salts, being extremely viscous near their melting point (with a glassy state below it) and low electrical conductivity, and since they are also known (ZnI₂) or predicted (ZnBr₂ and ZnCl₂) to exhibit conductivity maxima, they would be useful additional cases to probe, in case the reasons for their maxima are unique. Strong attractive forces in ZnX₂ result in tight tetrahedral coordination, and the known mixture of edge-sharing vs corner-sharing ZnX₄ tetrahedra is observed. In the series zinc chloride → bromide → iodide, (i) the ratio of edge-sharing vs corner-sharing tetrahedra increases, (ii) the diffusion coefficient of Zn²⁺ increases, and (iii) the diffusion coefficient of the anion stays roughly constant. A discussion of conductivity, with focus on the Walden product W = ηΛ_e, is presented. With predicted Haven ratios of 1-15 when heated toward their conductivity maxima, the physical chemistry behind molten zinc halide conductivity does not appear to be fundamentally different from other semicovalent molten halides.