Thermochemistry of RE₃O₃(PO₄) Rare Earth Oxide-Phosphates

We synthesized Yb₃O₃(PO₄) and Lu₃O₃(PO₄), which have not been previously synthesized according to the literature, thereby extending the number of experimentally confirmed compounds with these structures. Additionally, all oxide-phosphates described here exhibit excellent thermal stability up to at least 1100 °C. However, α-La₃O₃(PO₄) undergoes a phase transformation to β-La₃O₃(PO₄) at (972 ± 5) °C, with a phase transition enthalpy of Δ_trH = (10 ± 4) kJ·mol^–1.

To obtain information on the thermodynamic stability of the RE₃O₃(PO₄) series, we experimentally determined their enthalpy of dissolution using oxide melt drop solution calorimetry at 800 °C in a 3 Na₂O·4 MoO₃ solvent. We then calculated their enthalpy of formation using thermodynamic cycles from the respective reactions of rare-earth orthophosphates (REPO₄) and rare-earth sesquioxides (RE₂O₃) at 25 °C. These thermodynamic data will be useful for phase diagram calculations (CalPhaD) and for assessing the phase equilibria data of RE₂O₃–P₂O₅ systems.

Thus, it paves the way for future research in utilizing this exotic class of rare earth metal oxide-phosphate materials in demanding energy, catalysis, and sensing sectors.