The Crystal Chemistry of Ferric Oxyhydroxyapatite

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Abstract

Ferric hydroxyapatites (Fe-HAp) and oxyapatites (Fe-OAp) of nominal composition [Ca(10-x)Fe(x)(3+)][(PO(4))(6)][(OH)(2-x)O(x)] (0 Fe(3+) + CO(3)(2-)), while at higher loadings, "interstitial" oxygen is tenanted in the framework (2Ca(2+) + (vac) --> 2Fe(3+) + O(2+)). Although Fe(3+) is smaller than Ca(2+), the unit cell dilates as iron enters apatite, providing evidence of oxygen injection that converts PO(4) tetrahedra to PO(5) trigonal bipyramids, leading to the crystal chemical formula [Ca(10-x)Fe(x)][(PO(4))(6-x/2)(PO(5))(x/2)][(OH)(2-y)O(2y)] (x Fe(2+) aliovalent replacement does not require oxygen penetration and the cell volume contracts with iron loading. All of the materials were paramagnetic, but at low iron concentrations, a transition arising from crystallographic modification or a change in spin ordering is observed at 90 K. The excipient behavior of Fe-OAp was superior to that of HAp and may be linked to the crystalline component or mediated by a ubiquitous nondiffracting amorphous phase. Fe-HAp and Fe-OAp are not intrinsically suitable magnetic agents for drug delivery but may be useful in reactive cements that promote osteoblast proliferation.

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