The crystal chemistry of ferric oxyhydroxyapatite

Ferrie hydroxyapatites (Fe-HAp) and oxyapatites (Fe-OAp) of nominal composition [Ca_10-xFe_x³⁺][(PO₄) ₆][(OH)_2-xO_x] (0 ≤ x ≤ 0.5) were synthesized from a coprecipitated precursor calcined under flowing nitrogen. The solid solubility of iron was temperature-dependent, varying from x = 0.5 after firing at 600°C to x ∼ 0.2 at 1000°C, beyond which Fe-OAp was progressively replaced by tricalcium phosphate (Fe-TCP). Crystal size (13-116 nm) was controlled by iron content and calcination temperature. Ferric iron replaces calcium by two altervalent mechanisms in which carbonate and oxygen are incorporated as counterions. At low iron loadings, carbonate predominantly displaces hydroxyl in the apatite channels (Ca²⁺ + OH^- → Fe³⁺ + CO₃^2-), while at higher loadings, "interstitial" oxygen is tenanted in the framework (2Ca²⁺ + □_vac → 2Fe³⁺ + O²⁺). Although Fe³⁺ is smaller than Ca²⁺, the unit cell dilates as iron enters apatite, providing evidence of oxygen injection that converts PO ₄ tetrahedra to PO₅ trigonal bipyramids, leading to the crystal chemical formula [Ca_10-xFe_x][(PO₄) _6-x/2(PO₅)_x/2][(OH)_2-yO _2y] (x ≤ 0.5). A discontinuity in unit cell expansion at x ∼ 0.2 combined with a substantial reduction of the carbonate FTIR fingerprint shows that oxygen infusion, rather than tunnel hydroxyl displacement, is dominant beyond this loading. This behavior is in contrast to ferrous-fluorapatite where Ca²⁺ → Fe²⁺ 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.