Ab initio stability predictions for rare earth oxyphosphates and experimental confirmation of cerium (III) phases

Rare earth oxyphosphates represent a large family of compounds with the general formula (RE₂O₃)_x(REPO₄), where RE refers to lanthanides and yttrium. At least four known stoichiometries have been established, each with distinct structures. These compounds have potential applications as refractory coatings, catalysts, and magnetic materials. We modeled the stability of RE₃PO₇ [RE₃(PO₄)O₃] with respect to rare earth sesquioxides (RE₂O₃) and orthophosphates (REPO₄) using DFT computations with the GGA-PBE and r2SCAN exchange-correlation functionals. Phase stability predictions were consistent between the two functionals, while r2SCAN calculations of formation enthalpies for REPO₄ showed better agreement with experimental data. RE₃PO₇ phases for La-Dy were predicted to be stable at 0 K, with a space group change from Cm to C2/m starting with Sm. RE₃PO₇ phases for Y, Ho, and Er were found to be stabilized by lattice vibrational entropies at temperatures above 800 K, 1,000 K, and 1,600 K, respectively. The formation of predicted cerium (III) oxyphosphate, isostructural to Nd₃(PO)₄O₃, was confirmed through laser melting of monazite (CePO₄). Ce₃(PO₄)O₃ is monoclinic (space group Cm) with experimentally measured cell parameters a = 12.989(1) Å, b = 13.413(1) Å, c = 12.396(1) Å, β = 108.06(1)°. The existence of a Ce member of RE₇P₃O₁₈ family with unknown structure was experimentally established. This work invites further exploration of RE oxyphosphates as functional materials and indicates the possible formation of Ce oxyphosphate upon monazite melting during impact events and their potential use in geothermometry.