Identifying surface phonons in the vibrational spectra of carbonated apatite using density functional theory

Vibrational spectroscopy is widely used to examine the mineralogy of bone apatite. Yet, these spectra may be significantly influenced by the nanometre size of the crystallites through either phonon confinement or surface phonon contributions. This could lead to misinterpretations of the implications of non-apatitic environments that have been described previously as additional bands in the vibrational spectra. Here we use density functional theory to simulate bulk and slabs of hydroxyapatite as well as A-type, B-type, and AB-type carbonated apatite to test for eventual contributions of surface phonons. The analysis showed that surface phonons can have a significant intensity in the vibrational spectra. They are expected at both higher and lower wavenumbers than their bulk counterparts, unlike phonon confinement which has been linked with only lower wavenumber shifts. The band shift of surface phonons was up to 40 cm-1, which is determinable by both Raman and Infrared spectroscopy. All internal modes of evaluated molecular groups (OH, CO3, PO4) were affected by the surface presence. Therefore, it is expected that surface phonons are likely to be present in the vibrational spectra of bone minerals and contribute to spectral effects such as line broadening, presenting a crucial factor in their interpretation and application.