Diffusion and conductivity in the NaCl-ZnCl2system

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Abstract
The diffusion of Zn tracer and the ionic conductivity have been measured in pure and Zn-doped NaCl single crystals. Writing D 0 is 2 × 10−2 cm2 cm2/sec for pure and 1·5 × 10−4 cm2/sec for heavily doped crystals. Q is 23·8 kcal/mol for pure and 11·9 kcal/mol for heavily doped crystals. The free energy of association for the Zn++-cation vacancy pair is 11 kcal/mol at 976°K, which is much higher than predicted by theory; a lower free energy of association is obtained if next-nearest neighbour association is included in the calculation. Precipitation occurs at much higher temperatures and lower Zn concentrations than expected; the heat of solution of ZnCl2 in NaCl is 22·6 kcal/mol. The trapping of Zn++, probably by OH─ ions, was also noted.