Orientational Micro-Raman Spectroscopy on Hydroxyapatite Single Crystals and Human Enamel Crystallites

Single crystals of synthetic hydroxyapatite have been examined by orientational micro-Raman spectroscopy. The observed Raman bands include the PO4 3-/OH- internal and external modes over the spectral range from 180 to 3600 cm-1. The Raman-active symmetry tensors (A, E1, and E2) of crystal-class C6 have been assigned to these bands. Intensities of the Raman bands are invariant to the orientation of the a- or b-axis of a single crystal, but depend solely on the c-axis orientation. Changes in the spectral profile thus reflect changes in the c-axis orientation, which, with respect to the known excitation/observation coordinates, can be determined through the angular dependencies of Raman bands due to A and E2 symmetries. The useful bands are those of the PO43- v4 vibrational mode, which contains relatively well-defined bands due to E2 (581 cm-1) and A (593 cm-1) symmetries. A preliminary Raman study on crystallites near the intact outer surface of human enamel has also been carried out. Strong orientational d...