Hole Self-Trapping in Y3Al5O12 and Lu3Al5O12 Garnet Crystals

The processes of hole localization in Y3Al5O12 and Lu3Al5O12 single crystals are investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It is found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming an O- hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both yttrium aluminum garnet Y3Al5O12 (YAG) and lutetium Lu3Al5O12 garnet (LuAG) crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as Mg2+ and AlY or AlLu antisite ions which leads to an increase in the thermal stability of the trapped hole approximately equal to 150 K. TSL measurements show two composite glow peaks in the temperature range of 77-280 K, the temperature positions of which correlate well with the thermal stability of the O- centers. The hole thermal ionization energy is 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 an O- STH center, and increases to 0.41-0.45 eV for an O- center stabilized by the acceptor. Revealed O- centers can be attributed to O- small polarons formed mainly due to hole stabilization by short-range interactions with the surrounding lattice. Knowledge of the nature of color centers and related trapping sites is critically important for further optimization of all garnet scintillation materials.