Infrared and Raman spectra of Ca-5(PO4)(3)F-2-fluorapatite at high pressures: Compression-induced changes in phosphate site and Davydov splittings

The infrared and Raman spectra of fluorapatite (Ca-5(PO4)(3)F-2) have been examined to pressures of 25 and 23 GPa, respectively; this phase remains stable throughout this pressure range at 300 K. Both the site-group and Davydov (factor-group) splittings of the vibrations of the phosphate group decrease with compression. The shift in site group splitting is consistent with a general increase in phosphate group site symmetry under compression. The anomalous decrease in Davydov splitting can be associated with the decreased distortion of the environment surrounding the phosphorus tetrahedron under compression, and indicates the importance of non-dipole effects in generating the Davydov splitting in apatite. Our results indicate that the change with pressure of the Davydov splitting is not a consequence of simple dipole-dipole interactions, but is rather governed by complex changes in crystal site environments under compression. The Gruneisen parameters of the phosphate-derived vibrations are relatively low, indicating that the bulk of compression takes place within the calcium polyhedra in apatite.