Ab initio molecular dynamics prediction and experimental validation of the 14:4 rare-earth oxide-phosphate structure

• 成果类型: Article
• 署名作者: Hong, Qi-Jun; Ushakov, Sergey V.; Wang, Ligen; Burkmann, Konrad; Matteucci, Jared; Wu, Jun; Fitch, Andrew; Witharamage, Chathuranga S.; Xu, Hongwu; Opila, Elizabeth J.; Glaum, Robert; Navrotsky, Alexandra
• 署名单位: Arizona State University; Arizona State University-Tempe; Arizona State University; Arizona State University-Tempe; European Synchrotron Radiation Facility (ESRF); University of Virginia; University of Bonn
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.2604858123
• 发表日期: 2026-05-05
• 页码: e2604858123
• 关键词: ab initio molecular dynamics; crystal structure prediction; entropy stabilization; rare-earth oxide-phosphates; phase equilibria; BOND-VALENCE PARAMETERS; crystal; YB8-XYXV2O17

摘要

Rare-earth oxide-phosphates (historically termed oxyphosphates) occupy the compositional space between RE2O3 and REPO4 and form during REPO4 melting and high-temperature degradation of REPO4-based environmental barrier coatings. For several reported stoichiometries, reliable structural models remain unavailable because these phases are low-symmetry, large-unit-cell compounds that seldom form crystals suitable for single-crystal X-ray diffraction. Here, we predict the crystal structure of the compounds reported in the literature as RE8P2O17 (RE: Sm to Lu, Y) by combining finite-temperature ab initio molecular dynamics (AIMD) simulations with targeted experiments. Syntheses and electron microprobe analysis show the correct RE:P ratio is 3.5, corresponding to RE14P4O31 (14:4). Starting from the melt, AIMD simulations in the SLUSCHI framework, followed by symmetry-constrained relaxation, yield a complex (62 distinct oxygen sites on general positions), monoclinic Pc structure which represents a hitherto unknown structure type. It can be described as a defect fluorite (bixbyite, C-type RE2O3) structure penetrated along one direction by tunnels containing (PO4) tetrahedra. The structure was initially predicted for Y14O15(PO4)(4) and was validated for RE = Sm, Eu, Gd, Tb, and Y against synchrotron or laboratory X-ray powder diffraction patterns. Extending the model across the rare-earth series yields consistent lattice trends and places all oxide-phosphates RE14O15(PO4)(4) within 46 meV/atom of the 0 K convex hull. A finite-temperature free-energy analysis from MD trajectories predicts entropy stabilization of Y14O15(PO4)(4) above similar to 1,305 K, reconciling metastability at 0 K with observed synthesis and helping resolve discrepancies among published Y2O3-YPO4 phase diagrams.