Daurzhan Kenbayev

It was demonstrated that various defects can be induced in halide crystals by irradiation with swift heavy ions. Here, we irradiated barium fluoride (BaF₂) single crystals with 220 MeV xenon ions at room temperature and performed stepwise thermal annealing up to the temperature of 825 K to study the kinetics of ion-induced defects at different temperatures. Optical spectroscopy was utilized for the measurement of the wide range of absorption spectra from NIR to VUV. A sharp decrease in the F₂ absorption peak was observed for the samples annealed in the temperature range of 400–450 K. This result can be explained by their recombination with anion interstitials during thermal decay of the complex hole centers. The mobile interstitials, those did not recombine with the F₂ centers, increase the absorption peaks in the 9–10 eV region, which can be associated with interstitial aggregates.

The photoluminescent properties of BaFBr single crystals irradiated with xenon ions at a specific energy of 1.75 MeV/nucleon at a temperature of 300 K have been investigated. The crystals were grown using the Steber method, which allows for reduced oxygen impurity content. It was found that with an increase in ion irradiation fluence (F = 1·10¹⁰–1·10¹² ions/cm²), the photoluminescence intensity increases. The spectra revealed bands associated with various oxygen-vacancy centers, the presence of oxygen was also confirmed by elemental analysis performed on the BaFBr crystals. Decomposition of the spectra into Gaussian components enabled the identification of the contributions from specific types of oxygen-related defects. The results indicate the formation of color centers associated with bromine vacancies and a shift in luminescence maxima. This study contributes to the understanding of energy accumulation and relaxation mechanisms in BaFBr crystals.

This study presents theoretical and experimental investigations into the electron and hole color centers in BaFBr crystals, characterizing their electronic and optical properties. Stoichiometric BaFBr crystals grown by the Steber method were used in the experiments. Radiation defects in BaFBr crystals were created by irradiation with 147 MeV ⁸⁴Kr ions with up to fluences of 10¹⁰–10¹⁴ ions/cm². The formation of electron color centers (F(F⁻), F₂(F⁻), F₂(Br⁻)) and hole aggregates was experimentally established by optical absorption spectroscopy. Performed measurements are compared with theoretical calculations. It allows us to determine the electron transition mechanisms and investigate the processes involved in photoluminescence emission in Eu-doped BaFBr materials to enhance the understanding of the fundamental electronic structure and properties of electron and hole color centers formed in BaFBr crystals.

Today, BaFBr crystals activated by europium ions are used as detectors that store absorbed energy in metastable centers. In these materials, the image created by X-ray irradiation remains stable in the dark for long periods at room temperature. As a result, memory image plates are created, and they are extended to other types of ionizing radiation as well. Despite significant progress towards X-ray storage and readout of information, the mechanisms of these processes have not been fully identified to date, which has hindered the efficiency of this class of phosphors. In this study, using photoluminescence (PL), optical absorption (OA), Raman spectroscopy (RS), and atomic force microscopy (AFM), the luminescence of oxygen vacancy defects to BaFBr crystals irradiated with 147 MeV ⁸⁴Kr and 24.5 MeV ¹⁴N ions at 300 K to fluences (10¹⁰–10¹⁴) ion/cm² was investigated. BaFBr crystals were grown by the Shteber method on a special device. Energy-dispersive X-ray spectroscopy (EDX) analysis revealed the presence of Ba, Br, F, and O. The effect of oxygen impurities present in the studied crystals was considered. The analysis of the complex PL band, depending on the fluence and type of ions, showed the formation of three types of oxygen vacancy defects. Macrodefects (tracks) and aggregates significantly influence the luminescence of oxygen vacancy defects. The creation of hillocks and tracks in BaFBr crystals irradiated with 147 MeV ⁸⁴Kr ions is shown for the first time. Raman spectra analysis confirmed that BaFBr crystals were amorphized by 147 MeV ⁸⁴Kr ions due to track overlap, in contrast to samples irradiated with 24.5 MeV ¹⁴N ions. Raman and absorption spectra demonstrated the formation of hole and electron aggregate centers upon swift heavy ions irradiation.