Surface relaxations in hydroxyapatite

Relaxation processes due to two opposite surfaces in a slab of hydroxyapati te with two (100) free surfaces are reported. Interatomic potentials for hy droxyapatite, chloroapatite and fluoroapatite were refined using elastic an d vibrational data. Systems with two free (100) surfaces were relaxed. It w as found that the relaxation processes which occurred at the surface includ ed the polarization of layers close to the surface, distortion of the chann els in which the hydroxide ions sit and distortion of the tetrahedral phosp hate ions. Some of these relaxations only occur in the unit cells closest t o the surface; others persist for a few unit cells distance into the bulk. The implications of these results for the growth morphology of hydroxyapati te in bones are discussed.