  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.
