The Variation with Temperature of the Principal Elastic Moduli of NaCl near the Melting Point

The principal elastic moduli of single crystal rods of NaCl were measured over the temperature range 20°C to 804°C, the melting point. The measurements were carried out by means of a tripartite piezoelectric oscillator consisting of a quartz crystal driver, an intermediate fused silica bar, and the NaCl specimen. The shear constants $C_{44}$ and ($C_{11}-C_{12}$) decrease nearly linearly with temperature, reaching non-zero values at the melting point. The compressibility increases with temperature, but exhibits a sudden drop within a few degrees of the melting point. The results are compared with certain predictions based on Born's theory of the stability of crystal lattices.