High-Pressure Raman Spectroscopic Study of Hydroxylbastnäsite-(Ce)

Understanding the physical properties under high pressure of hydroxylbastnäsite-(Ce), an important hydrous rare earth element (REE) fluorocarbonate mineral, can provide key information to explore the effect of fluorine and hydroxyl on high-pressure behavior of carbonate minerals. Here Raman spectroscopy combined with diamond anvil cell (DAC) technology was employed to investigate the high-pressure properties of hydroxylbastnäsite-(Ce). At ambient conditions, the in-plane vibration bands of [CO₃]^2– are observed at 604 cm^–1 and 742 cm^–1, the symmetrical stretching bands are at 1 083, 1 096, and 1 103 cm^–1, and the asymmetric stretching vibration is at 1 430 cm^–1. Six vibration peaks of [OH]^– are at 3 174, 3 197, 3 290, 3 345, 3 526 and 3 648 cm^–1, respectively. The observation of three discrete [CO₃]^2– symmetrical stretching bands, instead of one, indicates that there may be at least three structurally-nonequivalent [CO₃]^2– groups in the hydroxyl-bästnasite-(Ce) structure. On compression, all of the Raman peaks show a continuous shift to the higher frequency and no new peaks appear, suggesting that no phase transition occurs up to 30 GPa at room temperature. The slope of the in-plane bending vibration of [CO₃]^2– is the smallest, about 2(0.06) cm^–1/GPa. Compared with the anhydrous carbonate, it can be inferred that the [OH]^– and F^– in the structures of hydroxylbastnäsite-(Ce) lead to the compression anisotropy. Our results provide new clues for studying the high-pressure physical behavior of carbonates in the deep earth.