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

The Hartman–Perdok analysis has been carried out on the monoclinic <i>P</i>2₁/<i>c</i> 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 {1̅02}, 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 {112̅} 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