Polarized Micro-Raman Study of Fluorapatite Single Crystals

Abstract

Despite the numerous Raman and infrared (IR) studies of the structure and the vibrational spectra of fluorapatite (FAp), the problem of the correlation of the observed and predicted bands still remains. An IR study of FAp powders was coupled with a micro-Raman polarized study of single-crystal FAp between 45 and 1100 cm⁻¹. In order to observe all the polarization directions and to isolate all the symmetry species, the arrangement of Tsuda and Arends was modified. The results were compared with previously published data. Some discrepancies appeared between the band assignment predictions, the previously published results, and the experimental results. The presence of an E_2g band on the v₁ mode was questioned, two A_g bands were observed for the v₂ mode, and the A_g bands showed different behavior concerning the z(xx)z polarization direction. Such behavior was attributed to the orientation of molecular groups and their vibrations according to the polarization direction. Raman and IR spectroscopies provide dynamic information on the molecular structure, compared with diffraction data, which give information on the average atomic positions in the crystal lattice. This study applies the complementary techniques Raman and IR spectroscopy combined with diffraction data to obtain deeper knowledge of the FAp structure.