Abstract
The photoinduced redistribution of charge has been characterized in Bi12GeO20 and Bi12SiO20 crystals using electron spin resonance, thermally stimulated luminescence, and optical absorption techniques. Excitation with 350-nm light at 77 K converts Fe3+ ions to Fe2+ ions. The source of electrons (i.e., the hole traps) is not known, but they may be other impurities or intrinsic defects such as vacancies or antisite cations. As the crystals are warmed to room temperature, recovery of the Fe3+ ions correlates with thermoluminescence peaks at 145, 165, and 245 K. We postulate that Fe2+ ions are the recombination site and that each thermoluminescence peak corresponds to the release of holes from a different trap. Our results suggest that Fe3+ ions may play an important role in the photorefractive effect in these materials.
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