Equation of State for Solid Argon Valid for Temperatures up to 300 K and Pressures up to 16 GPa

Abstract

A new equation of state (EoS) is presented for solid argon. The EoS is based on the quasi-harmonic approximation and formulated in terms of the Helmholtz energy, with temperature and molar volume as independent variables. To ensure high accuracy over a wide range of pressures, the static energy is represented semi-analytically by a Buckingham potential with three-body corrections. The vibrational modes are represented by Debye and Einstein contributions, and the EoS includes an anharmonic correction. A comprehensive collection of available experimental data has been used in the parameter optimization, including pressure and volume measurements along the co-existence curves, heat capacities, thermal expansivities and isothermal compressibilites. The EoS reproduces the molar volumes along the sublimation coexistence curve within an estimated uncertainty of 0.03%. For the heat capacity, the uncertainty is estimated to 1% in the range 20–50 K, 2% at higher temperatures, and 6% at lower temperatures. The isentropic and isothermal compressibilities have estimated uncertainties of 4% and 3%. For the thermal expansivity, the EoS has an estimated uncertainty of 2% above, and 5% below 30 K. For the pressure along the phase coexistence curves, the EoS has an estimated uncertainty of 0.4% for melting and 5% for sublimation. For the calculation of pressure as function of temperature and molar volume, the average relative deviation with respect to all available data is 5%. The EoS is valid up to pressures of 16 GPa and temperatures of 300 K, yet extrapolates well at temperatures beyond this range. The EoS represents the coexistence of solid argon in argon–hydrogen and argon–helium fluid mixtures nearly within the experimental uncertainty, provided that the EoS used to represent the fluid phase is sufficiently accurate.