Effect of mechanical stress on the Raman and infrared bands of hydroxylapatite: A quantum mechanical first principle investigation

The calcium apatite minerals are among the most studied in the biomaterial field because of their similarity with the mineral phase of bone tissues, which is mainly the hexagonal polymorph of hydroxylapatite. Given the growing interest both in the microscopic processes governing the behaviour of these natural biomaterials and in recent experimental methods to investigate the Raman response of hydroxylapatite upon mechanical loading, we report in the present work a detailed quantum mechanical analysis by DFT/B3LYP-D* approach on the Raman and infrared responses of hydroxylapatite upon deformation of its unit cell. From the vibrational results, the piezo-spectroscopic components Dni = Pi_ijÏ sigma_ij were calculated. For the first time to the authors' knowledge quantum mechanics (QM) was applied to resolve the piezo-spectroscopic response of hydroxylapatite. The QM results on the uniaxial stress responses of this phase on the piezo-spectroscopic components Pi_11and Pi_33of the symmetric P-O stretching mode were 2.54 ± 0.09 cm-1/GPa and 2.56 ± 0.06 cm-1/GPa, respectively (Raman simulation) and 2.48 ± 0.15 cm-1/GPa and Pi_33= 2.74 ± 0.08 cm-1/GPa, respectively, of the asymmetric P-O stretching (infrared spectroscopy simulation). These results are in excellent agreement with previous experimental data reported in literature. The quantum mechanical analysis of the other vibrational bands (not present in literature) shed more light on this new and very important application of both Raman and IR spectroscopies and extend the knowledge of the behaviour of hydroxylapatite, suggesting and addressing further experimental research and analytic strategy.