Electron spin-resonance (ESR) and electron-nuclear double-resonance (ENDOR) study of the self-trapped hole in ZnWO4 single crystals

After x-ray irradiation at 20 K, an intrinsic O- centre was identified by ESR and ENDOR spectroscopy as the self-trapped hole centre in ZnWO4. Observation of one Zn and two strong W superhyperfine interactions allows us to distinguish between two possible trapping sites: the hole resides at the B-type oxygen position which has one Zn and two W nearest neighbours. Broadening of the ESR lines and averaging of the g-value is observed and explained as due to thermally activated hopping of the hole between two energetically equivalent oxygen positions. The activation energy of this reorientation is found to be 0.016±0.003 eV. The thermal decay of the intrinsic O- centre, and its connection to thermoluminescence, has been studied; it shows that this centre cannot be the luminescence centre for the typical TL emission at ∼480 nm in ZnWO4. This emission may be due to an intrinsic electron-type defect. (author)