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