First-principles study of vacancy formation in hydroxyapatite

First-principles plane-wave calculations were performed for hydroxyapatite (HAp) in order to investigate the electronic structure and vacancy formation mechanisms. The HAp unit cell contains PO₄ tetrahedra and OH groups formed by covalent P-O and H-O bonds. Ca ions play a role for connecting the PO₄ tetrahedra and OH groups in an ionic character of bonding. It is found that OH^- vacancies with accompanying H⁺ vacancies are dominantly formed and that their spontaneous formation takes place at temperatures above 10000.3em0exK. This corresponds to the nonstoichiometry of HAp due to OH deficiency that is experimentally observed near 10730.3em0exK.