V. V. Laguta

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.

The processes of hole localization in double-doped PbWO_4:Mo,La and PbWO_4:Mo,Y single crystals have been studied by continuous wave and pulse electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. We show that the holes created by the UV irradiation are preferably trapped at lattice oxygen ions in the vicinity of perturbing defects such as lead vacancies, impurity ions (La, Y, Mo), and other lattice imperfections. This leads to a variety of O^- centers, which differ both by thermal stability (from about 170 K up to 240 K) and ESR parameters. The hole centers of this type were not observed neither in PbWO_4:Mo nor in PbWO_4:La(Y) crystals. The recombination processes of thermally released holes with electrons stored at different traps, including Pb^+ - WO_3 and (MoO_4)^3- centers, are systematically studied by TSL. Thermal stability parameters are defined by ESR and TSL techniques for different O^- type defects.