Low Symmetry Polymorph of Hydroxyapatite. Theoretical Equilibrium Morphology of the Monoclinic Ca5(OH)(PO4)3

摘要

The Hartman?Perdok analysis has been carried out on the monoclinic P21/c polymorph of Hydroxyapatite(HAp) in order to find the character of the main {hkl} forms and foresee the possible profiles of the related surfaces. Then, the corresponding specific surface energies have been calculated, at 0 K, by ab initio quantum-mechanical calculations, and the HAp equilibrium shape has been obtained: a striking pseudohexagonal prism comes out from the close similarity of the surface energy of the three pinacoids {001}, {100}, and {102}, all developing around the [010] zone axis which runs parallel to the OH channels of the structure bulk. The most stable form, the pinacoid {010}, truncates the pseudohexagonal prism perpendicularly to the OH channels, while the less morphologically important monoclinic prisms {012}, {110}, and {112} complete the HAp equilibrium shape. Preliminary considerations are exposed about the morphological path: monoclinic polymorph → twinning → hexagonal polymorph. Finally, a simple kinetic model based on 2D nucleation is proposed to explain the rod-shaped morphology of the solution grown monoclinic HAp, as we obtained at room temperature.