Features of the thermal expansion of hcp and fcc O2–Ar solutions
Abstract
X-ray studies of the lattice parameters, molar volumes and thermal expansion coefficients have been carried out on equilibrium fcc and metastable hcp O2–Ar solutions. Anomalies were found in the thermal expansion at temperatures below 20 K of solutions with 25, 30, and 35% O2, which correlate with the non-monotonic specific heat Cp at these temperatures. It follows from the analysis of the experimental results that in the temperature regions before and after the anomalies, the hcp solution is characterized by appreciably different structural and thermodynamic properties. The contributions of the rotational and magnetic subsystems to the thermodynamic properties of the solutions are estimated and comparison is made between the results of the calculations and the experimental data. The conclusion is drawn that the observed features are, to a considerable extent, due to the increase in the magnetic and orientational order in the impurity oxygen system at low temperatures. Unlike the metastable form, equilibrium fcc solutions containing up to 35% O2 have a monotonic temperature dependence of the thermal expansion coefficient. It can be concluded from the results obtained that the difference in the temperature variation of the properties of fcc and hcp solutions of the same composition is associated with the different nature of the short-range positional order of the oxygen molecules in these two solutions.