Stability of the Dipolar (001) Surface of Hydroxyapatite

The features of the ferroelectric (proton ordered) hydroxyapatite HA (001) surface as derived from the P63 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 Ca2+ and PO43– 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).