Hydroxyapatite crystallographic structure

Figure 10.7 (a) Crystallographic structure of hydroxyapatite, showing the orthohexagonal unit cell with a = 943.2 pm, b = a /3, c = 688.14 pm (Posner et al., 1958). Color coding of ions dark blue spheres = 0 light... 

These contradictory results about the sequence of calcium phosphate phase nucleation and growth demonstrate vividly how complex and in many important details not yet understood mechanisms appear to govern biomimetic formation of bone-like hydroxyapatite. In particular, the transformation of OCP to HAp was shown to be crystallographically controlled (Fernandez et al., 2003) because hydroxyapatite and octacalcium phosphate can form an epitaxial interface. A new OCP-HA interface model based on an earlier configuration model (Brown, 1962) and using the minimum interface free-energy optimisation was presented. In this new model a structure is formed that consists of half a unit cell of HAp and one unit cell of OCP whereby [0001]HAp is parallel to [001]OCp and [1210]HAp is parallel to [010]Ocp (Figure 7.66). It was shown by self-consistent field methods that the atoms of this model possess similar environments as in the HAp and OCP unit cells and that, as a result of the differences between HAp and OCP unit cell parameters, this interface displays misfit dislocation-like features. 

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