The thermodynamic properties of 4f metal trifluorides

Abstract

The experimental low-temperature heat capacities of some solid 4f metal trifluorides were used to reveal the trends in the behavior of variable parameters in the equation that described the lattice heat capacity component in the quasi-harmonic approximation for the whole series of LnF3 (Ln = La, … Lu) compounds. The results were used to describe the temperature dependences of heat capacity over the temperature range from 0 K to the melting point T m. The measured high-temperature enthalpy increments were used to determine corrections to the quasi-harmonic description of heat capacities at T > ∼0.5T m. The reduced Gibbs energies were calculated over the temperature range 298.15–2000 K. The thermodynamic functions of LnF3 in the gaseous state were determined over the same temperature range in the rigid rotator-harmonic oscillator approximation. All calculations were performed taking into account excited electronic states whose energies did not exceed 10000 cm−1. The reliability of the thermodynamic functions obtained was proved by the convergence of the enthalpies of sublimation calculated by the second and third laws of thermodynamics from the experimental data on saturated vapor pressures. The complete set of the consistent thermodynamic properties of these compounds is described.