Raman data of FePO4

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 kinetics of α-FePO4. The structural evolution across a pressure range of 0.20-27.35 GPa was delineated into three stages: (1) At 2.80-3.65 GPa, α-FePO4 initiates a phase transition, achieving a complete transformation to FePO4-II at 4.60 GPa. (2) Between 4.60-27.35 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 9.80-11.10 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. (3) Upon decompression to 4.65 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.