A Molecular Dynamics Simulation Study of (OH − ) Schottky Defects in Hydroxyapatite

The dynamics of (OH − ) Schottky defects is investigated for a hydroxyapatite / vacuum interface and in the bulk crystal. From our molecular dynamics simulations the mechanism of such defect translocation is identified. The hydroxide ions are arranged in [001] channels formed by staggered triangles of calcium ions. Above 200 °C, the OH − ions start to invert their orientation by passing through the surrounding Ca 2+ triangle. In the neighborhood of an empty calcium triangle, the OH − orientation inversion may also occur by hydroxide ion translocation from the occupied to the unoccupied calcium triangle. The later process causes the hydroxide to jump by c/2 ≈ ±3.5Å along the [001] direction. From an Arrhenius plot the activation energy of (OH − ) Schottky defect diffusion in the bulk crystal was found as 73 kJ mol −1 . At the hydroxyapatite/vacuum interface, OH − defect sintering exhibited a preference of Schottky defects diffusion to the surface.