Paramagnetic centers in ZnWO 4: Tm single crystals

Abstract

Paramagnetic centers in thulium-doped ZnWO 4 single crystals have been characterized by ESR spectroscopy. Most of the dopant is determined to be in the crystal as Tm 3+ substitutional for Zn 2+ and shows no ESR spectrum, however, a low concentration of Tm 2+ centers with the 4 f 13 electron configuration and S = 1 2 is detected in C 1 symmetry. The intensity 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 associated defect, probably a Zn vacancy that had contributed to local charge compensation of the original Tm 3+ ion, where the vacancy remains at the Tm ion even after the electron capture at low temperature. This center is denoted as the Tm 2+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 Tm 3+ zinc-vacancy pair and is denoted as the O −V ZnTm 3+ center.