Dipolar Reorientation and Order-Disorder Behavior of Pure and Mixed Alkali Cyanides

Abstract

A simple empirical relation connects the critical temperature of the order-disorder transition $T_c$ to the dipolar reorientation rate ${\tau }^{-1\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ of C${\mathrm{N}}^{-}$ molecules by the concept of a critical reorientation rate ${{\tau }_c}^{-1\mathrm{}}\left(T_c\right)$. This relation is derived from experimental data on four pure alkali cyanides, the dipole-diluted ${\left(\mathrm{KBr}\right)}_{1-x}:{\left(\mathrm{KCN}\right)}_x$ system, and the mixed alkali cyanides ${\left(\mathrm{KCN}\right)}_{1-x}:{\left(\mathrm{NaCN}\right)}_x$ and ${\left(\mathrm{RbCN}\right)}_{1-x}:{\left(\mathrm{KCN}\right)}_x$. The observed disappearance of the order-disorder transition at a certain $x$ value occurs in all mixed systems, when the spread of relaxation rates exceeds a particular value.