Observations on the thermal defect structure of solid argon

Abstract

Accurate measurements of the x-ray lattice constant of a pure argon crystal yield an x-ray density of ρ _x =[1.74974 + (4.349 × 10⁻⁴ deg⁻¹)T—(2.338 × 10⁻⁵ deg⁻²)T ²] g cm⁻³ in the temperature interval 60 to 83°K, with an r.m.s. deviation of 9.4 × 10⁻⁵ g cm⁻³. Comparison of this density with macroscopic values in the literature obtained by volumetric methods leads to the conclusion that the volumetric specimens were generally rather imperfect and that the equilibrium vacancy content of solid argon is less than 2 × 10⁻³ mole fraction at the triple point. The present coefficient of thermal expansion is significantly different from previous work. The conclusions about vacancies in Ar drawn from heat capacity data by Foreman and Lidiard and by Kuebler and Tosi therefore require revision. In particular, predictions using two-body interactions for the vacancy free energy are not inconsistent with available data within their estimated accuracy. Finally, x-ray observations very near the triple point are given which preclude any significant pre-melting phenomena in pure argon.