Thermodynamic and mechanical properties of the rutherfordine mineral based on density functional theory

Abstract

The thermodynamic and mechanical properties of rutherfordine, a uranyl carbonate mineral, were studied by means of first principles calculations based on density functional theory. Thermodynamic properties, including enthalpy, free energy, entropy, heat capacity, and Debye temperature, were evaluated as a function of temperature and compared with experimental data in the 300-700 K range. Our calculations show very good agreement with experimental data, and based on them, the knowledge of these properties is extended to the temperature range from 0 to 1000 K, including the full range of thermal stability (0-700 K). The computed values of the heat capacity, entropy, and free energy at 298 K deviate from the experimental values by about 8, 0.3, and 0.3%, respectively. At 700 K, the corresponding differences remain very small, 3.9, 2.3, and 1.3%, respectively. The equation of state and mechanical properties were also computed. The crystalline structure is seen to be mechanically and dynamically stable. Rutherfordine is shown to be a highly anisotropic and brittle material with a very large compressibility along the direction perpendicular to the sheets characterizing its structure. The computed bulk modulus is very small, B ≈ 20 GPa, in comparison to the values obtained in previous calculations.