Al- and OH-related paramagnetic impurity centres in UV- or x-irradiatedZnWO4 single crystals

Abstract

Using electron paramagnetic resonance (EPR) and in one case also electron nucleardouble resonance new trapped electron and hole centres are characterised inZnWO4 single crystals. Inundoped and Al-doped ZnWO4 a new W5+-typedefect is produced by UV- or x-irradiation at 77 K. The centre is stabilised by a nearbyAl3+ impurity that issubstituting for Zn2+.In ZnWO4:Al in addition to the above electron-type defect three kinds of hole-type centrescreated by x-irradiation at 77 K are also studied: one of these is assigned to anO− ion near aVZn–AlZn3+ complex, andthe second to an O− ion near a VZn–OH− pair. The - and hydrogen super hyperfine (SHF) tensors of the latter centre have been publishedpreviously. Due to the higher concentration in the Al-doped crystal, SHF tensors of two Wneighbours could also be determined. The third hole-type defect shows quartet SHF(I = 3/2) with a 100% abundant nucleus and therefore theO− in this centre is mostlikely stabilised by an Na+ impurity. Complete sets of EPR parameters are compared for trapped electron and trappedhole centres including previously characterised defects.