X-Ray Diffraction Study of Solid Argon

Abstract

Lattice constants for face‐centered cubic argon determined on a precision x‐ray diffractometer were found to be 5.311₈ Å at both 4.2 and 15°K, 5.318₇ at 21°K, 5.403₃ at 64°K, with estimated errors of ±0.0005 Å, and 5.45₈ at 81°K. Least‐squares fitting to a polynomial expression gives, for T≥21°K, a=5.29980+ 5.6389×10^—4T+1.4231×10^—5T²+3.552×10^—8T³ (an expression not intended to be used below 21°K). Lattice constants for the close‐packed hexagonal phase that sometimes coexists with the cubic may be taken as a_hex=0.7071 a_cubic and c_hex=1.633a_hex throughout the temperature range of solid argon. Evidence of rapid diffusion was seen near the melting point; at lower temperatures, cold work produced strain hardening. Agreement with published density measurements is good; values from some prior x‐ray work lie slightly below these values. Techniques were developed which permit plastic deformation and annealing of specimens to produce fine‐grained strain‐free samples suited to powder diffractometry. When grain growth could not be avoided (above about 65°K) a method of single crystal diffractometry was used.