PARAMAGNETIC CENTERS IN ZNWO4 - TM SINGLE-CRYSTALS

Paramagnetic centers in thulium-doped ZnWO4 single crystals have been characterized by ESR spectroscopy. Most of the dopant is determined to be in the crystal as Tm3+ substitutional for Zn2+ and shows no ESR sp ectrum, however, a low concentration of Tm2+ centers with the 4f(13) e lectron configuration and S = 1/2 is detected in C-1 symmetry. The int ensity of this center is greatly enhanced by a 366-nm UV illumination at 77 K via electron capture. The low symmetry is attributed to an ass ociated defect, probably a Zn vacancy that had contributed to local ch arge compensation of the original Tm3+ ion, where the vacancy remains at the Tm ion even after the electron capture at low temperature. This center is denoted as the Tm2+-V-Zn center. Also observed by ESR after UV illumination at 77 K are some paramagnetic hole-type defects. One of these is characterized as an O- center near a Tm3+ zinc-vacancy pai r and is denoted as the O--V-Zn-Tm3+ center. (C) 1997 Elsevier Science Ltd.