Structural and vibrational behaviour of fluorapatite with pressure, Part II: in situ micro-Raman spectroscopic investigation

摘要

High-pressure Raman investigations were carried out on a synthetic fluorapatite up to about 7 GPa to analyse the behaviour of the phosphate group's internal modes and of its lattice modes. The Raman frequencies of all modes increased with pressure and a trend toward reduced splitting was observed for the PO_4-stretching modes [(v_(3a)(A_g) and v_(3b)(A_g); v_(3a)(E_(2g)) and v_(3b)(E_(2g))] and the PO_4 out-of-plane bending modes [v_(4a)(A_g) and v_(4b)(A_g)]. The pressure coefficients of phosphate modes ranged from 0.0047 to 0.0052 GPa~(-1) for v_3, from 0.0025 to 0.0044 GPa~(-1) for v_4, from 0.0056 to 0.0086 GPa~(-1) for v_2 and 0.0046 for v_1 GPa~(-1), while the pressure coefficients of lattice modes ranged from 0.0106 to 0.0278 GPa~(-1). The corresponding Gruneisen parameters varied from 0.437 to 0.474, 0.428, 0.232 to 0.409 and 0.521 to 0.800 for phosphate modes v_3, v_1, v_4, v_2, respectively, and from 0.99 to 2.59 for lattice modes. The vibrational behaviour was interpreted in view of the high-pressure structural refinement performed on the same crystal under the same experimental conditions. The reduced splitting may thus be linked to the reduced distortion of the environment around the phosphate tetrahedron rather than to the decrease of the tetrahedral distortion itself. Moreover, the amount of calcium polyhedral compression, which is about three times the compression of phosphate tetrahedra, may explain the different Gruneisen parameters.