Formation of Oxygen Vacancies in Cr³⁺-Doped Hydroxyapatite Nanofibers and Their Role in Generating Paramagnetism

We demonstrate that doping hydroxyapatite (HAp) with Cr³⁺ ions induces oxygen vacancies, contributing to paramagnetism. Cathodoluminescence and photoluminescence analyses reveal increased oxygen vacancy formation in \({\text{O}}{\text{H}}^{-}\) ${\text{OH}}^{-}$ and \({\text{P}}{\text{O}}_{4}^{3-}\) ${\text{PO}}_4^{3-}$ groups with rising Cr³⁺ concentrations, highlighted by stronger cathodoluminescence emissions at 2.57 and 2.95 eV and the photoluminescence emission at 3.32 eV. Raman spectroscopy shows new modes at 900 and 970 cm⁻¹, indicating distortion of the v₁ vibrational mode due to Cr³⁺ substitution at Ca(II) sites of the HAp lattice. X-ray photoelectron spectroscopy confirms Cr³⁺ in the HAp:Cr. Magnetometry reveals a shift from diamagnetism in pure HAp to increasing paramagnetism in HAp:Cr with higher Cr³⁺ content, achieving 0.0460 emu/g at 10 kOe with concentrations higher than 2.9 at.%. This paramagnetism is attributed to Cr³⁺ ions and singly ionized oxygen vacancies \(V^{\prime}_{{\text{O}}}\) $V_{\text{O}}^{\prime }$ aligning along an external magnetic field, with \(V^{\prime}_{{\text{O}}}\) $V_{\text{O}}^{\prime }$ formation linked to \({\text{PO}}_{4}^{{3}-}\) ${\text{PO}}_4^{3-}$ replacement by \({\text{PO}}_{3}^{{2}-}\) ${\text{PO}}_3^{2-}$ in HAp.