Thermodynamics of hydroxylbastnaesite

The mineral bastnaesite, REECO₃[F, OH] (REE = rare earth elements), is a major source of rare earth elements and can be found in commercial deposits such as Mountain Pass, California, and Bayan Obo, China. In addition, its natural and synthetic forms have been explored for applications in optical devices, phosphors, batteries, ceramics, and magnets, accounting for 41 % of the world's consumption of rare earth elements. Therefore, investigating and analyzing thermodynamic properties of REE minerals are crucial for understanding origin, improving extraction processes, and optimizing industrial applications. This study presents a detailed reevaluation of structural and thermodynamic properties of well-characterized synthetic hydroxyl bastnaesite compounds: LaCO₃OH, PrCO₃OH, NdCO₃OH, SmCO₃OH, and EuCO₃OH. Their standard enthalpies of formation (ΔH⁰_f) were determined by high temperature oxide melt solution calorimetry and standard entropy (S⁰) was obtained from low temperature heat capacity (c_p) measurement from 1.8 to 300 K, extrapolated to 600 K combined the Einstein and Debye models. These allowed calculation of the Gibbs energy of formation (ΔG⁰_f). Extrapolation of thermodynamic properties to the heavy REEs by relation to the ionic radius of the REE³⁺ cation and to higher temperatures using the experimental c_p fit provides stability constraints for hydroxylcarbonates allowing for modelling REE-H₂O-CO₂ systems.