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

We demonstrate that doping hydroxyapatite (HAp) with Cr 3+ ions induces oxygen vacancies, contributing to paramagnetism. Cathodoluminescence and photoluminescence analyses reveal increased oxygen vacancy formation in OH⁻ OH - and PO₄³⁻ PO 4 3 - groups with rising Cr 3+ 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 v 1 vibrational mode due to Cr 3+ substitution at Ca(II) sites of the HAp lattice. X-ray photoelectron spectroscopy confirms Cr 3+ in the HAp:Cr. Magnetometry reveals a shift from diamagnetism in pure HAp to increasing paramagnetism in HAp:Cr with higher Cr 3+ content, achieving 0.0460 emu/g at 10 kOe with concentrations higher than 2.9 at.%. This paramagnetism is attributed to Cr 3+ ions and singly ionized oxygen vacancies V^_O V O ′ aligning along an external magnetic field, with V^_O V O ′ formation linked to PO₄^3- PO 4 3 - replacement by PO₃^2- PO 3 2 - in HAp.