A mechanical instability hypothesis for melting in the alkali halides

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Abstract
Attention is drawn to the essential first-order and heterogeneous character of melting. Elastic moduli are then examined as a function of dilatation from room temperature to the melting-point and in the melt. The bulk modulus and the two shear moduli each have a Gruneisen-type dependence on crystal volume. The moduli appear to be continuous with respect to volume through the melting expansion and in particular the shear modulus 1/2(c 11 - c 12) for the rock salt alkali halides and c 44 for CsBr and CsI falls continuously to zero in the melt at the melting-point. Born's elastic instability theory of melting is modified, such that the shear modulus vanishes in the melt at the melting-point rather than in the solid; with this alteration the theory becomes a two-phase theory and therefore contains both the first-order and heterogeneous character of melting. More precisely, it is shown that the modulus does not quite vanish in the melt but becomes sufficiently small that, at any instant, about one-tenth of all ions are in a state of spontaneous flow.

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