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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. 

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. 

We present the effect of sodium ions (Na+) on the nucleation process and phase selectivity for the formation of hexagonal molybdenum trioxide crystals (h-MoO3). The phase selectivity during the reaction is attributed to the interaction of Na+ with the molecules in our precursor solution formed by metallic molybdenum dissolved in a mix of hydrochloric and nitric acids. The vibrational characteristics of the precursor solutions were studied by Raman spectroscopy in combination with density functional theory modeling, showing the presence of [MoO2Cl3(H2O)] ions within the solutions. The symmetric stretching vibration of the Mo-O bonds found at 962 cm-1 in [MoO2Cl3(H2O)]- proved that the addition of Na+ (in the form of dissolved NaCl) to the precursor solutions resulted only in an electrostatic interaction with the aquo (H2O) and chloro (Cl-) ligands in the complex. After heating the precursor solutions, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy of the obtained powders showed that adding NaCl contributed to the phase selectivity of the reaction, with the Na+ ions playing a vital role in the formation of h-MoO3 over other crystalline phases. Based on the nature of the molybdenum complexes found in the precursor solutions and the structural characteristics of the powders, a formation mechanism to obtain h-MoO3 is proposed. Additionally, the phase stability of the h-MoO3 crystals was studied by calorimetry techniques showing that h-MoO3 transforms to a-MoO3 at ~649 K. These results provide important insights into phase control to selectively form hexagonal MoO3.