V. Babin

Single crystals of LiCaAlF6 undoped and Eu, Na co-doped were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation exhibited two hole-like charge trapping centers creation, the molecular ions of the form: ClF- and F2^- - F2^- dimer. Their trap depths and frequency factors were determined as follows: Et1=1.7 eV and Et2=1.1. eV for trap depths and f ~ 10^13 s-1 for frequency factor, respectively. It was found that the europium preferable charge state is 2+ in the LiCaAlF6:Eu,Na samples, however, some amount of the Eu3+ is also present. Moreover, there were two Eu2+ centers: the dominating Eu2+(Ca) and the low-content Eu2+(Li). The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF6 host.

The processes of hole localization in the Y3Al5O12 and Lu3Al5O12 single crystals were investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It was found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming O- hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both YAG and LuAG crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as Mg2+ and Al_{Y} or Al_{Lu} antisite ion that leads to increase of the thermal stability of the trapped hole to app. 150 K. TSL measurements show two composite glow peaks in the temperature range of 77 - 280 K, the temperature positions of which well correlate with the thermal stability of the O- centers. The hole thermal ionization energy was determined from a numerical fit of the TSL peaks within the model of second order kinetics. It is in the range of 0.25 - 0.26 eV for the O- STH center, and increases to 0.41 - 0.45 eV for O- center stabilized by the acceptor. Revealed O- centers can be attributed to O- small polarons formed mainly due to the hole stabilization by short-range interaction with the surrounding lattice.

Photoluminescence, thermally stimulated luminescence (TSL) and EPR characteristics of the Ce3+ doped single crystals of multicomponent Y1Lu2GaxAl5-xO12 and Lu3GaxAl5-xO12 garnets with different Ga contents (x = 0, 1, 2, 3, 4, 5) excited in the Ce3+ - related absorption bands are investigated in the 9 - 500 K temperature range. The distribution of Ga3+ and Al3+ ions in the crystal lattice is determined by the NMR method. The relative number of Ga3+ ions in the tetrahedral crystal lattice sites, the maxima positions of the TSL glow curve peaks and the corresponding trap depths are found to decrease linearly with the increasing Ga content. At the same time, the reduction of the activation energy Ea of the TSL glow curve peaks creation under irradiation in the 4f - 4d1 absorption band of Ce3+ is strongly nonlinear. To explain this effect, the suggestion is made that Ea is the energy distance between the excited 5d1 level of Ce3+ and a defect level located between the 5d1 level and the bottom of the conduction band and arising from the Ga3+ ion perturbed by the nearest neighboring Ce3+ ion. The electrons thermally released from the excited Ce3+ ions are suggested to be trapped at the perturbed Ga3+ ions resulting in the appearance of electron Ga2+ centers. In spite of the fact that the paramagnetic Ga2+ ions were not detected by EPR, the described above process was found for Fe3+ impurity ions, namely the electron transfer from the 5d1 excited levels of Ce3+ to Fe3+ was directly detected by EPR.