Anomalies of the thermal expansion of solid argon due to oxygen impurities
Abstract
The coefficient of linear expansion (CLE) of solid argon containing 0.5, 1, 2, and 3% oxygen is measured in the temperature range 2–12 °K. Analysis of the concentration dependence of the CLE led to the conclusion that a two-phase region (presumably fcc-hcp) exists in the range of concentration 1.85–3.4% O2. The temperature dependences of the excess CLE (Δα) in the fcc phase correspond to Schottky curves for a two-level system with a two-fold degenerate upper level and splitting equal to 4.0 °K and 4.4 °K for solutions containing 0.5 and 1% O2, respectively. The magnitudes of Δα at low temperatures turned out to be unusually large, an order of magnitude greater than expected from an analysis of the energy spectra of oxygen molecules in a cubic crystal-field and oriented along the ⟨111⟩ direction. The excess CLE of the hcp phase is calculated from the temperature dependence of the CLE in the two-phase region. The difference between its magnitude and temperature dependence and the corresponding data for the fcc phase suggests the existence of different energy spectra of oxygen molecules in these phases.