Micro-Raman and FTIR studies of synthetic and natural apatites

B-type synthetic carbonate hydroxyapatite (CHAp), natural carbonate fluorapatite (CFAp) and silicon-substituted hydroxyapatite (SiHAp) have been studied by using micro-Raman and infrared (IR) spectroscopy. It was found that while B-type carbonate substitution predominates in carbonate apatites (CAps), A-type is also detected. B-type carbonate substitution causes a broadening of the v1 P–O stretching mode that is associated with the atomic disorder and lowering of the local symmetry in CAps from C6h2 to C3h. An ∼15 cm−1 shift of the v3c PO4 stretching IR mode was observed upon decarbonation of the CFAp. Such shift which was confirmed by lattice dynamics calculations points out that the P–O bond lengths on the mirror plane increase when carbonate leaves the apatite structure. The present results support the substitution mechanism proposed on the basis of neutron powder diffraction studies of the same samples whereby carbonate substitutes on the mirror plane of the phosphate tetrahedron. The intensity ratios of the v2 IR CO3 and v1 PO4 bands in samples of various carbonate contents provide a measure of the degree of carbonation for CAps.