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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. 

We present a luminescence study investigating the dissolution of rare-earth-doped hydroxyapatite scaffolds in simulated body fluid (SBF), aiming to assess the luminescence stability of Tb-, Ce-, and Eu-doped scaffolds over time. Our findings reveal a consistent decrease in luminescence emission intensity across all samples over a four-week period in which the scaffolds were immersed in the SBF. In addition, energy-dispersive spectroscopy confirms a decrease in rare-earth ion concentration in the scaffolds with respect to time, whereas fluorescence spectroscopy shows the presence of rare-earth ions in the SBF, indicating the partial dissolution of the scaffolds over time. The use of rare-earth ions as luminescence markers provides insights into the mechanisms of apatite formation in hydroxyapatites. Thus, these scaffolds may find wider use in regenerative medicine, particularly in targeted drug delivery systems, where their luminescent properties have the potential to noninvasively track drug release. 

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. 

We describe the triboluminescence response of undoped (BaAl2Si2O8, h–BAS) and Eu-doped (h–BAS:Eu) barium hexacelsian powders and show that the triboluminescence behavior is dependent on the formation of barium vacancies. X-ray photoelectron spectroscopy of the h–BAS:Eu powders confirms the presence of Eu3+ and Eu2+ in the compound, leading to the formation of significant vacancy point defects in excess of those found in h–BAS as a result of the charge imbalance caused by the substitution of Eu3+ in Ba2+ sites. From electron paramagnetic resonance measurements and density functional theory (DFT) calculations, we demonstrate that the vacancy defects correspond to singly ionized barium vacancies. DFT-calculated thermodynamic transitions and electronic structure calculations reveal deep energy levels within the compound’s energy band gap, with a strong emission at 3.33 eV correlated to an electron exchange between the conduction band minimum and a barium vacancy center. Time-resolved triboluminescence spectra show that the increased concentration of barium vacancies in h–BAS:Eu enhances the signal by about 75% compared to the signal from h–BAS. These results play an important role in the understanding of fundamental mechanisms behind the triboluminescence response of ceramic materials as well as the role of different types of defects in this process.