Formation of vacancy point-defects in hydroxyapatite nanobelts by selective incorporation of Fe3+ ions in Ca(II) sites. A CL and XPS study

Abstract We report a cathodoluminescence (CL) and x-ray photoelectron spectroscopy (XPS) study of the formation of calcium vacancies (VCa) and hydroxyl ion vacancies (VOH) in hydroxyapatite (HAp) nanobelts selectively doped with Fe3+ ions by substituting Ca(II) ions of the HAp lattice. Additionally, we report a paramagnetic behavior of the nanobelts doped with Fe concentrations lower than 1 atomic percent (at.%), besides the absence of superparamagnetism even at concentrations around 12 at.%. XPS spectroscopy measurements demonstrated that the Fe3+ ions incorporated selectively into the HAp lattice by specific substitution of Ca2+ (II) ions, forming Fe-PO4 bonds. CL spectra obtained on the HAp:Fe powders revealed that Fe doping promotes VCa and VOH formation. We propose that the generation of paramagnetism in the HAp:Fe samples is due to the contribution of both Fe3+ ions and the unpaired electrons of O2− anions adjacent to VCa.