Thermochemistry of RE3O3(PO4) Rare Earth Oxide-Phosphates

Abstract Rare earth oxide phosphates (oxyphosphates), with extended metal-oxygen-phosphate bonding networks and a high rare-earth metal-to-phosphorus ratio (RE/P > 1), are promising materials for thermal and environmental barrier coatings in aircraft, rockets, and hypersonic vehicles because of their refractory properties. However, challenges such as complex crystal chemistry, phase purity control, and the absence of reliable phase diagrams due to a lack of thermodynamic data complicate their application. We synthesized a full series of rare-earth oxyphosphates with composition RE3O3(PO4), using both a well-established solid-state reaction route and two different solution combustion synthesis procedures. Synthesized materials were analyzed with powder X-ray diffraction (PXRD), FTIR spectroscopy, and Raman spectroscopy to probe phase assemblages, bonding environments, and local coordination changes. 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 ΔtrH = (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.