Structure and Stability of 7:3 Rare Earth Oxide‐Phosphates: A Combined Ab Initio and Experimental Study
Ligen Wang, Konrad Burkmann, Sergey V. Ushakov, Edric X. Wang, Jared Matteucci, Mara Scheuermann, Erik Melnitschuk, Robert Glaum, Hongwu Xu, Elizabeth J. Opila, Alexandra Navrotsky, Qi‐Jun HongABSTRACT
Rare earth oxide‐phosphates (REOPs) form a largely unexplored family of refractory lanthanides and yttrium compounds with general formula RE x O y (PO 4 ) z. They are of interest for applications ranging from thermal barrier coatings to catalysts and magnetic materials. At least four REOP phases were experimentally identified with RE/P ratios from 7:3 to 6:1, but the structures were solved only for 3:1 phases (RE 3 O 3 (PO 4 )). In this work, we report the structures for the 7:3 phases (RE 7 O 6 (PO 4 ) 3 ) derived from ab initio analysis of models based on previously reported oxide‐vanadate analogues. The most stable structures for all 7:3 REOPs were found to be isotypic, adopting monoclinic symmetry with space group P 2 1 / c . The structures were validated by comparison of their powder x‐ray diffraction patterns to those of synthesized La, Pr, Nd, Sm, Eu, Gd, and Tb 7:3 phases (Rietveld refinement for all except Tb). Ab initio analysis of thermodynamic stability showed that all 7:3 REOPs are unstable at 0 K toward decomposition into REPO 4 and RE 3 PO 7 or RE 2 O 3 . The entropy contribution stabilizes RE 7 O 6 (PO 4 ) 3 phases for light rare earth elements above 1000 K; however, beginning with Dy, the computationally predicted stabilization temperatures are higher than estimated melting points of RE 7 O 6 (PO 4 ) 3 , which is consistent with observed synthesis behavior.