X-Ray Diffraction by Compressed Solid Argon

Abstract

A technique is described to perform precision measurements of the lattice parameter of solid argon under pressure. Details are given of the apparatus which consists of a beryllium pressure vessel, a liquid nitrogen cryostat, a two-circle goniometer, and a cylindrical camera. Small argon crystals are grown in the pressure vessel and one suitable crystal is selected and correctly oriented with a precision of 1 min of arc, using the oscillation method. The single-crystal rotation method is used to measure the lattice parameter with an estimated precision of 0.01%. Measurements were carried out along the melting line in the range from the triple point (83.8 K and 0.7 bar) to 95 K and 450 bar. The result obtained for the lattice parameter of solid argon near the triple point, a0 = 5.4676 ± 0.0005 Å, is in close agreement with the low pressure result of Peterson et al. Some values are calculated for the molar volume, density, coefficient of thermal expansion, and bulk modulus of solid argon, and these results are compared with those obtained by other workers using different methods.