Velocity of Ultrasonic Waves in Single Crystals of Calcium Fluoride

Abstract

THE elastic constants of crystals are commonly determined by measuring the velocities of propagation of acoustic waves in different principal crystal directions and applying well-known formulae¹. For crystals of high perfection it has generally been reported that the measured velocity of sound of a given substance is substantially independent of impurity concentration and is a slowly varying function of temperature. There have been relatively few investigations of the dependence of acoustic velocity on impurity density due to the fact that, until recent years, the preparation of highly doped single crystals of high perfection was virtually impossible. We have investigated ultrasonic propagation in a number of calcium fluoride monocrystals grown in the <111> direction by the Czochralski technique. Some of the crystals were of high purity and others were doped with up to 2 per cent significant impurity for experiments on laser action. They were all of comparable perfection, with measured dislocation densities lying in the range 10⁴–10⁵ cm⁻². Samples for measurement were cylinders with opposite ends cleaved and finely polished to plane-parallelism.