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

Abstract

AbstractWe demonstrate that doping hydroxyapatite (HAp) with Cr3+ ions induces oxygen vacancies, contributing to paramagnetism. Cathodoluminescence and photoluminescence analyses reveal increased oxygen vacancy formation in $${\text{O}}{\text{H}}^{-}$$ OH - and $${\text{P}}{\text{O}}_{4}^{3-}$$ 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 O ′ aligning along an external magnetic field, with $$V^{\prime}_{{\text{O}}}$$ V O ′ formation linked to $${\text{PO}}_{4}^{{3}-}$$ PO 4 3 - replacement by $${\text{PO}}_{3}^{{2}-}$$ PO 3 2 - in HAp.