Phase Transition of α-FePO₄ under High Pressure: A Raman Spectroscopy Study

Given the topological isomorphism between α-FePO₄ and α-quartz, this study employed a diamond anvil cell coupled with Raman spectroscopy to examine the phase transition of α-FePO₄. The structural evolution across a pressure range of 0.2−27.3 GPa was delineated into three stages. At 2.8−3.6 GPa, α-FePO₄ initiates a phase transition, achieving a complete transformation to FePO₄-Ⅱ at 4.6 GPa. Between 4.6−27.3 GPa, the (meta) stability of FePO₄-Ⅱ is predicated on the cooperative deformation of the adaptable [FeO₆] octahedra and the rigid [PO₄] tetrahedra. The progressive increase in structural disorder and the slowing of vibrational frequency shifts signify a transition to a non-linear compression regime. Notably, in the 9.8−11.1 GPa threshold, discontinuous variations in P―O bond lengths and mode widths serve as evidences of pressure-induced heterogeneous strain within the [FeO₆]-[PO₄] network, suggesting entry into a metastable region. Upon decompression to 4.6 GPa, FePO₄-Ⅱ exhibits partial recovery of structural order, maintaining metastability at ambient conditions, which underscores its unique pressure memory characteristics. This study demarcates the stability boundary of α-FePO₄, elucidates the fundamental mechanisms underpinning stability in orthophosphates, and forecasts structural evolution pathways. The findings offer insights into high-pressure dynamic response of quartz-like minerals.