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

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 PO4-stretching modes [(nu (3a)(A(g)) and nu (3b)(A(g)); nu (3a)(E-2g) and nu (3b)(E-2g)] and the PO4 out-of-plane bending modes [nu (4a)(A(g)) and nu (4b)(A(g))]. The pressure coefficients of phosphate modes ranged from 0.0047 to 0.0052 GPa(-1) for nu (3), from 0.0025 to 0.0044 GPa(-1) for nu (4), from 0.0056 to 0.0086 GPa(-1) for nu (2) and 0.0046 for nu (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 nu (3), nu (1), nu (4), nu (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.