Heat capacity and thermodynamic functions of stoichiometric rare earth oxychlorides (REOCl)

This work investigates the heat capacity and thermodynamic properties of three rare earth oxychlorides (REOCl), TmOCl, NdOCl, and YOCl. To understand their thermodynamic landscapes, we measured their heat capacities from 1.8 to 300 K. Our results indicate the presence of several Schottky anomalies at low temperatures (below 7 K), and TmOCl had an additional Schottky anomaly centered around 25 K, which we attribute to a contribution from f-block electrons. We fitted the data to theoretical functions and used these models to derive the standard entropy, enthalpy, and Gibbs energy. Using previously published enthalpy of formation data, we determined the Gibbs energies of formation from the elements and the oxides and chlorides at selected temperatures. At 298.15 K, the Gibbs energy of formation relative to oxides and chlorides was calculated to be −59.4 kJ·mol⁻¹, −41.0 kJ·mol⁻¹, and − 11.1 kJ·mol⁻¹ for NdOCl, YOCl, and TmOCl, respectively. The Gibbs energy of formation of REOCl relative to the elements at 298.15 K was calculated to be −952.3 kJ·mol⁻¹, −967.4 kJ·mol⁻¹, and − 938.9 kJ·mol⁻¹ for NdOCl, YOCl, and TmOCl, respectively. These results confirm the stability of REOCl relative to the elements and the binary chlorides and oxides.