X-ray luminescence and spectroscopic characteristics of europium ions in glassy and polycrystalline lithium tetraborate matrices


Purpose. The practical use of rare earth oxides as activating compounds is due to the spectroscopic properties of their ions in the structure of lithium tetraborate (LTB), among which special attention is paid to Eu3+ ion, the one of the most actively studied and promising activators for practical application. For Eu3+ radiation spectra, a high quantum yield of luminescence is characteristic, which significantly improves the spectroscopic properties of LTB and its sensitivity to the effects of various radiation fields. Therefore, the development of such materials and the study of their spectroscopic characteristics are important and relevant.
Methods. The glassy and polycrystalline LTB samples activated by europium oxide used in this study were synthesized in platinum crucibles in the air at 950°C from a high optical quality single crystal and a Eu2O3 activator. The X-ray luminescence spectra (XRL) were recorded at room temperature in the spectral range of 200–800 nm, with their excitation carried out by the BSV-21 X-ray tube. Measurements were carried out on an automatic experimental system based on an MDR-23 monochromator having resolution of 1.6 nm/mm. As a radiation receiver, an FEU-106 photoelectron multiplier working in photon count mode was used.
Results. In the work, the concentration dependences of the spectra of glassy LTB with Eu2O3 concentrations of 0.001, 0.003, 0.5 mol %, as well as polycrystalline LTB with a concentration of 1 mol % were studied. It was shown that the energy dependences obtained in the whole spectral range are substantially complicated in comparison with the XRL spectra of non-activated glass and polycrystalline LTB. In the range of 200–550 nm at a nominal concentration of Eu2O3 a band of complex structure with 21 features was recorded. It was found that further increase in the concentration of the activator does not significantly affect the structure of the spectrum or the intensity of luminescence in this frequency region. The transformation of the XRL spectrum at europium oxide concentrations above 0.001 mol % was registered. This is caused by the changes in the number of vacancies, the increase in the concentration of intercellular europium ions, or the difference in charge compensation types, which is due to the change in local symmetry, and leads to a redistribution of intensities and an increase in the heterogeneous broadening of spectral lines without significant displacement of the centers of gravity of individual multiplets of the Eu2+ ion. The identified radiative transitions in this spectral region are due to electronic transitions between the ground state of the Eu2+ ion (the main term 8S7/2) and the terms of the mixed 4f65d configuration, and are electric dipole transitions between the Stark energy levels. In the spectral region of 550–800 nm, there are 7 spectral lines, whose intensity increases insignificantly with increasing activator concentration except for the band at 614 nm, and the structure of the spectrum remains unchanged. The detected luminescent bands are due to radiative transitions between the lowest excited 5D0, 5D1, 5D2 levels of the 5Dj multiplet onto the levels of the 7Fj multiplet of the Eu3+ ion with electronic configuration of 4f6 and the ground multiplet 7F0. In the luminescence spectra of Eu3+ ions, both electric dipole and magnetic dipole transitions are possible. The appearance of electric dipole transitions leads to the mixing of the states of the excited 4fk–15d configurations to the states of the 4fk configurations. The appearance of magnetic dipole transitions, which are prohibited by the selection rules (ΔL = 1, ΔS = 1) between levels 5DJ and 7FJ, is caused by the mixing of 7F and 5D multiplets due to spin-orbital interaction. In the XRL spectrum of glassy LTB, activated by Eu2O3, 40 radiation bands were identified and the types of electron transitions and the nature of their radiation were determined.
Conclusions. It has been shown that in the region of 330–430 nm, X-ray luminescence occurs due to electronic transitions between the ground state of the Eu2+ ion (the main term 8S7/2) and the terms of the mixed 4f65d configuration. X-ray luminescence in the 470–800 nm range is caused by magnetic and electric dipole electron transitions between the ground state of the Eu3+ ion (electronic configuration 4f6, ground term 7F0) and excited levels 5D0, 5D1, 5D2 of the 5Dj multiplet, with the most intense bands being attributed to electric dipole transitions 5D07F2 and 5D0 7F4. Prohibited transitions 5D07F0 and 5D07F3 appear very weakly

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