Effect of Pressure on Dislocation Damping in Alkali Halide Crystals

Abstract

Attenuation of longitudinal pulsed ultrasound at frequencies between 27 and 99 MHz has been measured in LiF and NaCl crystals under hydrostatic oil pressure up to 3500 kg/cm² at room temperature. Specimens used were in two different states: deformed to introduce dislocations, and irradiated by γ rays to eliminate the effect of dislocation vibration. From the pressure dependences of sound attenuation in the two states, the pressure dependence of the damping constant for dislocation motion was determined by an analysis based on the Granato-Lücke theory of overdamped resonance of dislocations. The results for LiF and NaCl crystals of high purity were well understood by considering the non-linearity mechanism or the fluttering mechanism of dislocation damping. For an impure LiF crystal, the pressure dependence of damping constant was found to be very different from that for the pure crystal.