Stability of the dipolar (001) surface of hydroxyapatite

The features of the ferroelectric (proton ordered) hydroxyapatite HA (001) surface as derived from the P6 ₃ hexagonal HA bulk have been studied by periodic density functional calculations using the hybrid B3LYP functional and Gaussian basis set of polarized double-ζ quality. Geometry, surface energy, and electronic features of HA (001) slab models of thickness from 1 nm to almost 50 nm have been computed, by keeping under careful control numerical errors due to the very large system size. The present results reveal that the ferroelectric OH ^- alignment does not compromise the stability of the HA (001) surface up to the nanometric scale. Indeed, a slab thickness of 43 nm, containing 2640 atoms in the unit cell, exhibits a dipole moment across the slab of 0.73 D, a wide band gap of 7.60 eV, and a surface energy of 1.344 J·m ^-2. No sign of "metallization" occurs as for the classical macroscopic polar zinc- or oxygen-terminated ZnO (0001) surfaces, due to counterpolarization of the electronic density of the Ca ²⁺ and PO ₄ ^3- moieties surrounding the monodimensional OH ^- polar arrays. These findings may be relevant to explain why, experimentally, HA nanocrystals orient along the main axis of the proto-collagen fibrils with their crystallographic c axis (perpendicular to the {001} crystal plane family).