Exotic FeII/FeIII Local Environments in the Hexagonal Channels of Hydroxyapatite

In this fundamental solid-state chemistry study, two sample series were investigated in depth: iron(III)-doped hydroxyapatite (HA) compounds obtained from a co-sintering process of hematite and pure HA under air and iron(III)-doped HA compounds obtained from a co-sintering process from iron(II) acetate and pure HA under an argon atmosphere. X-ray diffraction, UV-visible, Fourier transform infrared, H-1 and P-31 NMR, electron paramagnetic resonance (EPR,) and Mossbauer spectroscopy methods were coupled to unravel the Fe valence states, the interactions with other anionic species (OH- and PO43-), and finally the complex local environments in hexagonal channels in both the series. In particular, we highlighted the associated mechanism to ensure electroneutrality with a focus on deprotonation versus calcium substitution. By diverging mechanisms, Fe3+ and Fe2+ ions were found to be located in different coordinated sites: 4(+1) coordinated site for Fe(3+ )and 2(+3) coordinated site for Fe2+ and clearly associated with very different Mossbauer and EPR signatures as various absorption bands (leading to different sample colors).

Automated summary

In this study, iron-doped hydroxyapatite compounds were investigated in depth using various methods. It was found that Fe3+ and Fe2+ ions are located in different coordination sites and are associated with different absorption bands.