Some physical properties of solid argon

Abstract

The velocities of ultrasonic waves in solid argon were measured by means of a pulse technique for temperatures between 74°K and the triple point (84.80°K). One longitudinal, v�, and two transverse, v[subscript t]� and v[subscript t]₂ , velocities, assumed to be single crystal velocities in the 110-direction, were observed. The values at 77.5°K were: v� = 1334 m/s; v[subscript t]� = 700 m/s; and v[subscript t]₂ = 642 m/s. the elastic constants obtained at 77.5°K were c�� = 3.045x10¹� dynes/cm²; and c₄₄ = 0.682x10¹� dynes/cm². Values of the velocities and elastic constants indicated that the solid became increasingly soft near the melting point and thus gave no indication of hole formation. Volume expansion measurements for temperatures from 67.7°K to 78.6°K were in close agreement with previously reported values and also gave no indication of hole formation. The vapor pressure was measured for temperatures from 63.5°K to 77.5°K under various conditions designed to observe the existence of the hcp phase reported to be in the solid argon. It was estimated that less than 1% (if any) of the hcp phase was formed at 77.5°K. Experiments with crystal growth techniques suggested that growing large single crystals is a formidable task. In conclusion, the hypothesis of hole formation near the melting point was not substantiated. The concentration of the hcp phase in solid argon seemed to be small or nonexistent. Crystals of 1 cc were consistently grown, if etch patterns on the free surface of the solid indicated grain boundaries. However, velocity measurements on optically clear solids of about 2 cc indicate that they were essentially single crystals.