No abstract

An apparatus for the measurement of sound velocity in gases up to pressures of 10 kbar and down to temperatures of 77 K is described. Important features of the experimental setup include a specially designed ultrasonic cell with double reflectors, based on the phase comparison pulse-echo technique, and a vacuum type of cryostat using a cold finger system for temperature control. Detailed measurements on argon down to the melting line show that the method is capable of giving a precision of 0.02% for the sound velocity.

The shear modulus C44 of platinum is measured at temperatures from 77 to 298 K and at pressures up to 253 MPa using the previously developed torsional‐bar resonance technique. The anomalous temperature variation of the shear modulus, observed previously at zero pressure, is found to persist at higher pressures. The pressure derivative of C44 is found to decrease linearly with decrease of temperature.

The shear modulus of polycrystalline silver is measured at temperatures from 77 to 298 K and at pressures up to 253 MPa using the previously developed torsional‐bar resonance technique. The experimental data are used for a critical evaluation of several averaging methods which relate the shear modulus of a polycrystal with the elastic shear constants of single crystals. The observed temperature dependence of the shear modulus as well as the temperature dependence of the pressure derivative are analyzed in terms of the theory of Leibfried and Ludwig based on lattice dynamics in the quasi‐harmonic approximation.