Melting and crystal structure mechanisms of melting of alkali thiocyanates

Abstract

Alkali thiocyanates are remarkable for their low melting-points (KSCN 175°C; RbSCN 195°C). Mechanisms of melting of the sodium and potassium salts have been investigated by determining ionic conductivities of solid and melt, viscosities of the melts and, in the case of the potassium salt, entropy and volume changes on melting. It seems likely that the low melting-points of alkali thiocyanates arise from the contribution of association complexes in the melt to the entropy of fusion. These increase the entropy of fusion without requiring large heat or volume changes. In such cases melting-points tend to be low in accordance with the equation, T f = H f / S f . Marked homophase premelting is observed in solid KSCN; the experiments show hardly any discontinuity in volume at the melting-point.