Computational investigation of formation enthalpies and phase stability for rare earth oxyphosphates

Rare earth phosphates have garnered significant interest due to their versatile properties, including high chemical stability, thermal resistance, luminescence, and the ability to adopt various crystalline structures. Density functional theory (DFT)-based ab initio methods have become essential tools for complementing experimental studies. In this paper, we performed DFT calculations on rare earth (RE; here considered as lanthanides + Y) oxyphosphates to examine their formation enthalpies and phase stability. The calculations were conducted using the GGA-PBE and r2SCAN exchange-correlation functionals. Our results indicate that both functionals predict similar phase stabilities for REPO4 and RE3PO7. However, the r2SCAN functional provides significantly more accurate formation enthalpies for the monazite and xenotime REPO4, aligning closely with experimental data. Furthermore, the inclusion of lattice vibrational entropy enhances the free energy predictions, leading to improved agreement with experimental observations on phase stability.