Equation of State for Solid Argon Valid for Temperatures up to 760 K and Pressures up to 6300 MPa

Abstract

Thermodynamic property data for solid argon have been analysed to construct a new fundamental equation of state (EOS) based on the Helmholtz energy. This approach is based on methodologies previously applied to solid CO₂ and benzene (Trusler in J Phys Chem Ref Data 40:043105, 2011; Xiao et al. in J Phys Chem Ref Data 50:043104, 2021). The EOS is capable of predicting thermodynamic properties of solid argon up to 760 K and 6300 MPa, using temperature and cell volume as independent variables. The model incorporates the quasi-harmonic approximation with a Debye oscillator framework for vibrons, along with an anharmonic term to address deviations near the triple point. In addition to literature data, the model was regressed to new measurements of argon’s solid cell volume conducted from (8 to 50) K using a high-intensity neutron diffractometer, the results of which are reported here. This new EOS achieves a high degree of accuracy in representing experimental data, with uncertainties (k = 1) estimated of 0.1 %, 0.5 %, and 0.5 % for the cell volume along the sublimation curve, along the melting curve, and in the compressed solid phase, respectively; 2 % to 10 % for the heat capacity along the sublimation curve in different temperature regions; 1 % to 10 % for the thermal expansivity on the sublimation curve; 2 % for the isothermal bulk modulus, 1 % for the isentropic bulk modulus, 0.2 % for the enthalpy of sublimation, 0.5 % to 2 % for the enthalpy of melting, 1 % for the sublimation pressure (T > 50 K), and 2 % to 5 % for melting pressure. The EOS maintains physically realistic behaviour across the range of conditions from absolute zero to high-pressure.