Incorporation of Iron(II) and (III) in Hydroxyapatite—A Theoretical Study

Abstract

Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) doped with various transition metals has generated great interest in view of its potential application in a wide variety of fields, including in catalysis as a support with a series of attractive properties. Despite a large number of experimental works devoted to the synthesis and application of iron-substituted hydroxyapatites, problems concerning the location, introduced defects, and charge compensation schemes for Fe2+ and/or Fe3+ cations in the crystal structure of HAp remain unclear. This paper is devoted to the comprehensive analysis of iron (II) and (III) introduction into the HAp lattice by density functional theory (DFT) calculations. We show that the inclusion of Fe2+ in the Ca(1) and Ca(2) positions of HAp is energetically comparable. For the Fe3+, there is a clear preference to be included in the Ca(2) position. The inclusion of iron results in cell contraction, which is more pronounced in the case of Fe3+. In addition, Fe3+ may form a shorter linkage to oxygen atoms. The incorporation of both Fe2+ and Fe3+ leads to notable local reorganization in the HAp cell.

The Mutual Incorporation of Mg2+ and CO32− into Hydroxyapatite: A DFT Study

“…Previously, for both pure HA and HA containing foreign ions, it was shown that one monoclinic cell (space group P21/b) was sufficient to reproduce the g-factors and A-hyperfine values for various paramagnetic species [57,58] and phonon spectral density [59]. It was possible to adequately describe HA systems containing foreign cations, such as aluminum, iron, and magnesium cations [18,38,60,61]. It is important to note, the descriptions of monoclinic and hexagonal cells (usually found in the experiment) use different axis notation which should not lead to confusion, as demonstrated by our results [60].…”

“…The morphology, stability, solubility, mechanical properties, and biological behaviors of HA synthesized in vitro are also modulated by Materials 2022, 15, 9046 2 of 12 the incorporation of foreign ions. According to their size and charge, cations, such as Na + , Zn 2+ , Mg 2+ , and Fe 3+ substitute Ca 2+ [18][19][20][21]. In contrast, anions, such as CO 3 2− , SiO 4 4− , F − , and Cl − can theoretically substitute PO 4 3− (B position) or OH − (A position) (Figure 1).…”

Nutrient-Doped Hydroxyapatite: Structure, Synthesis and Properties

“…Further, the release of H + from OH − was also considered to maintain a neutral overall charge and to minimize the cell contraction caused by charge compensation. The presence of OHwithin the anion channel, surrounded by calcium ions, can show a minimal impact on the overall packing and density of the crystal [86]. In addition, they reported that Fe 3+ substitution is more reactive with the surrounding.…”

RF Magnetron Sputtering of Substituted Hydroxyapatite for Deposition of Biocoatings

“… Hydroxyapatite unit cell. Color coding: phosphorus—orange, oxygen—red, hydrogen—light grey, Ca(1)—purple, Ca(2)—blue [ 11 ]. …”

“…Substitutions in the apatite structure have been developed for a wide range of biomedical applications, such as bone repair and tissue regenerations; bioactive and/or antibacterial coating for medical devices, biomarkers, or carriers in drug/gene delivery systems; and biomagnetic agents for cancer treatment [ 10 ]. These properties are due to the unique structural characteristics of HA, the unit cell of which is represented in Figure 1 [ 11 ]. HA belongs to the class of minerals whose composition has a general formula: M 10 (ZO 4 ) 6 X 2 , where M is one-, two-, three-valent cations (K + , Ca 2+ , Sr 2+ , Ba 2+ , Pb 2+ , Na + , Mn 2+ , Mg 2+ , Th 3+ , Ni 2+ , etc.…”