A basic understanding of Zn incorporation on bulk and hydrated (001) surfaces of hydroxyapatite was attained through electronic structure calculations which use a combined first principles density functional and extended Hückel tight binding methodology. A Zn substituted hydroxyapatite relaxed structure was obtained through a periodic cell density functional theory geometry optimization method. Electronic structure properties were calculated by using both cluster density functional theory and periodic cell extended Hückel tight binding methods. Bond order calculations showed that Zn preference for the Ca2 vacancy, near the OH channel and with greater structural flexibility, was associated with the formation of a four-fold (bulk) and nearly four-fold (surface) coordination, as in ZnO. When occupying the octahedral Ca1 vacancy, Zn remained six-fold in the bulk, but coordination decreases to five-fold in the surface. In the bulk and surface, Zn2 was found to be more covalent than Zn1, due to a decrease in bond lengths at the four-fold site, which approach the 1.99 Å ZnO value. Zn was however considerably less bound in the biomaterial than in the oxide, where calculated bond orders were twice as large as in HA. Surface phosphate groups (PO4) and hydroxide ions behave as compact individual units as in the bulk; no evidence was found for the presence of HPO4. Ca–O bond orders decrease at the surface, with a consequent increase in ionicity. Comparison between density functional theory and extended Hückel tight binding results showed that the latter method gave a good qualitative account of charge and bonding in these systems.

Mechanism of Zn Stabilization in Hydroxyapatite and Hydrated (001) Surfaces of Hydroxyapatite. M.Matos, J.Terra, D.E.Ellis: Journal of Physics - Condensed Matter, 2010, 22[14], 145502