Structure and stability of hydroxyapatite: Density functional calculation and Rietveld analysis

A study on the structure and energetics of hydroxyapatite has been carried out using generalized gradient approximation density functional theory. The previously proposed P6₃/m and P6₃ hexagonal structural models have been found to be energetically unfavorable as compared with the previously proposed P2₁/b symmetry and a newly proposed monoclinic P2₁ structural model. A detailed analysis of the symmetry restrictions and inconsistencies between the reported physical properties, such as diffraction and birefringence, with hexagonal symmetry question the validity of these models. Rietveld analysis carried out on a synthetic sample, reported to be predominantly hexagonal at room temperature, shows that the diffraction pattern of this material can be interpreted as a mixed monoclinic phase, 23% monoclinic P2₁/b and 77% monoclinic P2₁. This interpretation reconciles many anomalies in reported experimental observations.