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 p hosphate group decrease with compression. The shift in site group spli tting is consistent with a general increase in phosphate group site sy mmetry under compression. The anomalous decrease in Davydov splitting can be associated with the decreased distortion of the environment sur rounding the phosphorus tetrahedron under compression, and indicates t he importance of non-dipole effects in generating the Davydov splittin g in apatite. Our results indicate that the change with pressure of th e Davydov splitting is not a consequence of simple dipole-dipole inter actions, but is rather governed by complex changes in crystal site env ironments under compression. The Gruneisen parameters of the phosphate -derived vibrations are relatively low, indicating that the bulk of co mpression takes place within the calcium polyhedra in apatite.