Shear Elastic Constant C₄₄ and Phase Transformation of KCl

Abstract

THE shear elastic constant C₄₄ for KCl, KF and the rubidium halides is of special interest because it decreases with pressure. Born¹ and Misra² pointed out that a vanishing shear elastic constant is a sufficient condition for a phase transformation. This suggestion has been elaborated by Anderson and Demarest³ and by Thomsen⁴ to postulate that the phase transformation from the NaCl to CsCl structure at high pressure may be associated with the vanishing of C₄₄. If borne out by experiment, this suggestion would be of considerable physical interest. Based on values of the first pressure derivative of C₄₄ for the halides, measured at relatively low pressure, Demarest⁵ extrapolated C₄₄ to high pressure and showed that the phase transitions actually take place at much lower pressures than those predicted by the vanishing of C₄₄. Such extrapolation, however, does not rule out the possibility of a large downward curvature in the C₄₄/P relation at higher pressures, especially at pressures near the phase transformation leading to a vanishing C₄₄. Thus direct measurement of C₄₄ up to the pressure of phase transformation appears necessary for a test of the hypothesis. I present here experimental results for ultrasonic determination of C₄₄ for a KCl single crystal up to 20 kbar at which pressure it transforms from its NaCl structure to the CsCl structure.