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We report the influence of vacancy point defects on the conductivity and piezoelectricity of hydroxyapatite (HAp) nanofibers. A combination of experimental techniques, including conductive atomic force microscopy, electrostatic force microscopy, and switching spectroscopy piezoresponse force microscopy, along with computational modeling, was employed to elucidate the conduction mechanisms and charge accumulation effects in HAp. Our findings demonstrate that oxygen and calcium vacancy defects play a crucial role in the conduction mechanism of HAp nanofibers, specifically through charge-trapping and de-trapping processes, as well as in charge accumulation and the piezoelectric response. The Poole-Frenkel conduction mechanism was confirmed by fitting experimental current-voltage data to a theoretical model, revealing a dielectric constant consistent with previously reported theoretical values. These insights contribute to a deeper understanding of the role of point defects in the electrical and piezoelectric properties of HAp, which is essential for optimizing its performance in biomedical applications. 

Abstract 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 thev₁vibrational 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.