The Crystal Chemistry of Ferric Oxyhydroxyapatite

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 < or = x < or = 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 degrees C to x approximately 0.2 at 1000 degrees 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 (Ca(2+) + OH(-) --> 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 < or = 0.5). A discontinuity in unit cell expansion at x approximately 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 Ca(2+) --> 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.

Crystal Chemistry and Luminescence Properties of Eu-Doped Polycrystalline Hydroxyapatite Synthesized by Chemical Precipitation at Room Temperature

“…O3 interactions discussed above (Figure 2, Table 5), further stabilize the structure (Figure 3b). To describe the hexagonal P63/m HAp structure, we can better write the crystal formula as [Ca(1)4Ca(2)6](PO4)6(OH)2, which underlines the two cationic sites [42]; a close inspection, indeed, suggests that HAp shows a zeolitic character where the framework consists of columns of facesharing Ca1O6 metaprisms corner-connected to six different PO4 tetrahedra down [001] making up [Ca(1)4(PO4)6] 10− anionic moieties ( Figure 2a); this network results in one-dimensional hexagonal tunnels occupied by [Ca(2)6(OH)2] 10+ counter-ions (Figure 3a, Figure 3b). The strong Ca1…O3 interactions discussed above (Figure 2, Table 5), further stabilize the structure (Figure 3b).…”

Nanosized hydroxyapatite and other calcium phosphates: Chemistry of formation and application as drug and gene delivery agents

“…Another interesting direction is doping CAP with magnetic ions, such as Co 369,370 or Fe, 371,372 or even coprecipitation of magnetite. 373 There are indications that magnetic HAP and electrically polarized one (owing to OH groups in HAP crystal lattice) possess a more pronounced osteogenic response.…”

Fe3+/− dual doped nano hydroxyapatite: A novel material for biomedical applications

“…Furthermore, the expansion in a axis could be assigned to the presence of oxyhydroxyapatite. 20,36 However, a small amount of Fe 31 ion may not be sufficient to undergo ionic replacement with Ca 21 ion in SeHA structure. Hence, there may be the possibility of the adsorption of Fe 31 ions on the surface of SeHA.…”