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

Abstract

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.

Formation of luminescence centres under excitation with electron beams in PbWO 4 and ZnWO 4 crystals

“…The STE luminescence is observed in thermoluminesccence peak triggered by hole delocalisation [9], which is an evidence that the hole recombines with an electron localized at a regular lattice site. As shown in [15], the positively charged impurities localize an electron on W 6+ near an impurity and W 5+ center is formed in ZnWO 4 crystal. Similarly to PWO we suggest that, after generation of a geminate electron-hole pair and self-trapping of the hole in ZnWO 4 , its positive charge can stimulate localization of an electron at one of the nearest W 6+ ions, producing a W…”