First-principles plane-wave calculations were performed in order to investigate electronic structure and vacancy formation mechanisms. The hydroxyapatite unit cell contained PO4 tetrahedra and OH groups, formed by covalent P-O and H-O bonding. The Ca ions played a role in connecting PO4 tetrahedra and OH groups via ionic-type bonding. It was found that OH− vacancies, with accompanying H+ vacancies, were formed predominantly and that their spontaneous formation took place above 1000K. This corresponded to the non-stoichiometry of hydroxyapatite, due to OH deficiency, that was observed experimentally near to 1073K.

First-Principles Study of Vacancy Formation in Hydroxyapatite. K.Matsunaga, A.Kuwabara: Physical Review B, 2007, 75[1], 014102 (9pp)