X-Ray Diffraction Study of Solid Argon

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 ahex=0.7071 acubic and chex=1.633ahex 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.