Structural order and dielectric behaviour of hydroxyapatite

Disorder of the OH^- ion orientation is inherent in the currently accepted structure of hydroxyapatite (HA), hexagonal P6₃/m. In the case of local ordering in the OH^- ion orientation, the resulting structure will be a monoclinic P2₁/b, with an alternate orientation of OH^- columns in the b-direction. Both of these proposed structures are electrically non polar and fail to explain the polarisation phenomena observed in HA. Here we report some theoretical considerations of structural order in HA and show that in addition to the non polar P2₁/b phase HA can also exist as a polar phase. Various physical properties including static dielectric constant and polarisation are simulated from empirical interatomic potential calculations and used to explain the observed dielectric anomaly in sintered hydroxyapatite.