Luminescent properties of Tb and Eu activated AxB1-xAlO3 (A = Y, Lu, Gd; B = Lu; x = 0, 0.5, 1) mixed oxides crystals prepared by micro-pulling-down method

Development of the Composite Thermoluminescent Detectors Based on the Single Crystalline Films and Crystals of Perovskite Compounds

This work is dedicated to the development of new types of composite thermoluminescent detectors based on the single crystalline films of Ce-doped GdAlO₃ perovskite and Mn-doped YAlO₃ and (Lu_0.8Y_0.2)AlO₃:Mn perovskites as well as Ce and Pr-doped YAlO₃ single crystal substrates. These detectors were obtained using the Liquid Phase Epitaxy growth method from the melt solution based on the PbO-B₂O₃ fluxes. Such composite detectors can by applied for the simultaneous registration of different components of mixed ionization fluxes using the differences between the thermoluminescent glow curves, recorded from the film and crystal parts of epitaxial structures. For creation of the new composite detectors, we considered using, for the film and crystal components of epitaxial structures (i) the different perovskite matrixes doped with the same type of activator or (ii) the same perovskite host with various types of activators. The thermoluminescent properties of the different types of epitaxial structures based on the abovementioned films and crystal substrates were examined in the conditions of β-particles and X-ray excitation with aim of determination of the optimal combination of perovskites for composite detectors. It was shown that, among the structures with all the studied compositions, the best properties for the simultaneous thermoluminescent detection of α- and X-rays were the GdAlO₃:Ce film/YAlO₃:Ce crystal epitaxial structure.

Band Gap Engineering and Trap Depths of Intrinsic Point Defects in RAlO3 (R = Y, La, Gd, Yb, Lu) Perovskites

The possibility of band gap engineering (BGE) in RAlO₃ (R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials, E _g, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect of E _g reduction from ∼8.5 to ∼5.5 eV in RAlO₃ perovskites with increasing R³⁺ ionic radius was confirmed by the DFT electronic structure calculations performed for RM^IIIO₃ crystals (R = Lu, Y, La; M^III = Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO₃:Mn⁴⁺ phosphors, which confirmed decreasing of the trap depths in the cation sequence Y → Gd → La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO₃) were considered to be formed by the energy level of oxygen vacancy (V_O) with different arrangement of neighboring Y_Al and V_Y, while shallower electron traps of 0.9-1.0 eV were related to the energy level of Y_Al antisite complexes with neighboring V_O or (V_O + V_Y). The effect of the lowering of electron trap depths in RAlO₃ was demonstrated for the V_O-related level of the (Y_Al + V_O + V_Y) complex defect for the particular case of La substituting Y.

Thermoluminescence Enhancement of LiMgPO4 Crystal Host by Tb3+ and Tm3+ Trivalent Rare-Earth Ions Co-doping

We investigated the influence of terbium and thulium trivalent rare-earth (RE) ions co-doping on the luminescent properties enhancement of LiMgPO₄ (LMP) crystal host. The studied crystals were grown from the melt by micro-pulling-down (MPD) technique. Luminescent properties of the obtained crystals were investigated by thermoluminescence (TL) method. The most favorable properties and the highest luminescence enhancement were measured for Tb and Tm double doped crystals. A similar luminescence level can be also obtained for Tm, B co-doped samples. In this case, however, the low-temperature TL components have a significant contribution. The measured luminescent spectra showed a typical emission of Tb³⁺ and Tm³⁺ ions of an opposite trapping nature, namely the holes and electron-trapping sites, respectively. The most prominent transitions of ⁵D₄ → ⁷F₃ (550 nm for Tb³⁺) and ¹D₂ → ³F₄ (450 nm for Tm³⁺) were observed. It was also found that Tb³⁺ and Tm³⁺ emissions show temperature dependence in the case of double doped LMP crystal sample, which was not visible in the case of the samples doped with a single RE dopant. At a low temperature range (up to around 290 °C) Tm³⁺ emission was dominant. At higher temperatures, the electrons occupying Tm³⁺ sites started to be released giving rise to emissions from Tb-related recombination centers, and emissions from Tm³⁺ centers simultaneously decreased. At the highest temperatures, emission took place from Tb³⁺ recombination centers, but only from deeper ⁵D₄ level-related traps which had not been emptied at a lower temperature range.