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.