VIBRATIONAL SPECTROSCOPIC INVESTIGATION OF PHOSPHATE TETRAHEDRON IN FLUOR-APATITE, HYDROXY-APATITE, AND CHLORAPATITES

Data from infrared and Raman spectra of fluorapatite, hydroxyapatite, and chlorapatite were collected and related to structure data. The spe ctroscopic data confirm the P6(3)/m space group of the structure of th e apatite end-members. A shift of the position of column anions from t he mirror plane causes an increase in the distortion of the PO4 tetrah edron both in terms of quadratic elongation and tetrahedral angle vari ation, and results in a systematical decrease of the stretching freque ncies of v(1), v(3) vibrational modes and a clear increase of splittin gs of degenerate v(3), v(4) Raman and infrared active modes. Our resul ts indicate that, on the apatite end-members, the vibrational frequenc y shift is not a direct consequence of the tetrahedral bond length or volume variation, and the change on values of splitting does not resul t simply from a dipole-dipole interaction between the phosphate ions i n a unit-cell. Therefore there should be a vibrational interaction bet ween the tetrahedron and the structural column.