Premelting in alkali halides due to the thermal formation of dislocations

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Abstract
In a recent paper Allnatt and Sime described some high‐temperature electrical‐conductivity experiments on sodium chloride and potassium chloride in which the activation enthalpy for the conductivity was 110–210 eV and 440 eV, respectively. In this paper we suggest that the rapid rise in conductivity is due to the enhanced diffusion of charged defects along thermally formed dislocations. We present a formula for the conductivity along thermal dislocations and using it we calculate entropies of 1150k–2190k for NaCl and 4750k for KCl. Together with the high enthalpies these entropies are consistent with the Gibbs’s function for the formation of thermal dislocations approaching zero at the melting point, implying that on melting the crystal is highly dislocated.