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 E SR and ENDOR spectroscopy as the self-trapped hole centre in ZnWO4. Observa tion of one Zn and two strong W superhyperfine interactions allows us to di stinguish between two possible trapping sites: the hole resides at the B-ty pe oxygen position which has one Zn and two W nearest neighbours. Broadenin g of the ESR lines and averaging of the g-value is observed and explained a s 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- cent re, and its connection to thermoluminescence, has been studied; it shows th at this centre cannot be the luminescence centre for the typical TL emissio n at similar to 480 nm in ZnWO4. This emission may be due to an intrinsic e lectron-type defect.