Al- and OH-related paramagnetic impurity centres in UV- or x-irradiated ZnWO4single crystals

Using electron paramagnetic resonance (EPR) and in one case also electron nuclear double resonance new trapped electron and hole centres are characterised in ZnWO4 single crystals. In undoped and Al-doped ZnWO4 a new W5+-type defect is produced by UV- or x-irradiation at 77 K. The centre is stabilised by a nearby Al3+ impurity that is substituting for Zn2+. In ZnWO4:Al in addition to the above electron-type defect three kinds of hole-type centres created by x-irradiation at 77 K are also studied: one of these is assigned to an O− ion near a VZn–AlZn3+ complex, and the second to an O− ion near a VZn–OH− pair. The - and hydrogen super hyperfine (SHF) tensors of the latter centre have been published previously. Due to the higher concentration in the Al-doped crystal, SHF tensors of two W neighbours could also be determined. The third hole-type defect shows quartet SHF (I = 3/2) with a 100% abundant nucleus and therefore the O− in this centre is most likely stabilised by an Na+ impurity. Complete sets of EPR parameters are compared for trapped electron and trapped hole centres including previously characterised defects.