Hydrostaticity-Sensitive Structural Phase Transition and High-Pressure Phase Diagram in Fluorite: Evidence of Raman Spectroscopy and Electrical Conductivity

Raman spectroscopic analysis of fluorite was conducted in a diamond anvil cell (DAC) over a pressure range of 0.5–20.5 GPa under different hydrostatic environments, whereas the electrical conductivity was measured at 298–873 K and 1.2–19.6 GPa. High-resolution transmission electron microscopy (HRTEM) observations were performed on both the initial and recovered samples after recovery to ambient conditions. Three representative pressure-transmitting media (PTMs), including silicone oil, the mixture of methanol and ethanol (4:1 volume ratio, ME), and helium, were employed to control the degree of hydrostaticity within the DAC sample chamber. Experimental results indicate that the pressure-induced abrupt change in A1g, A3g, B1g and B2g Raman modes, together with the discontinuities in pressure-dependent Raman shifts, Grüneisen parameters, and electrical conductivity, can efficiently characterize the α (cubic structure, space group Fm3¯m, No 225)-to-γ (cotunnite structure, PbCl2-type, space group Pnma, No 62) phase transition in fluorite. The transition pressures are determined to be 10.4, 9.6, 8.9 and 7.5 GPa under conditions of no PTM, silicone oil, ME and helium, respectively, demonstrating that the structural phase transition of fluorite is highly sensitive to hydrostaticity. Raman spectroscopy and electrical conductivity measurements upon decompression reveal that the phase transition is reversible, which is further confirmed by the HRTEM microstructural observation on both the initial and recovered samples. The linear relationships between electrical current and sinusoidal voltage, with the nonlinearity factors close to 1.00, manifest the Ohmic response of fluorite under high pressure. Finally, our high-temperature and high-pressure electrical conductivity results revealed the negative dependence of transition temperature on pressure, and the phase boundary between cubic and PbCl2-type fluorite was determined as: P (GPa) = 13.057 (±1.008) − 0.008 (±0.001) T (K). The obtained phase diagram of fluorite can be employed to deeply explore the high-pressure phase stability and structural transitions of other similar binary halide family minerals.