Effects of Cr doping on the optical characteristics of PbWO4 crystals

We report here the optical properties of PWO crystals doped with Cr3+ at a series of concentration levels. It is found that deep traps diminish and the luminescence deteriorates with the addition of Cr3+ ion, which is interpreted by the combined effects of self-absorption, large distortion of WO4 tetrahedron and the formation of “killing site”, defect pairs [2(Cr3+Pb)·-(Cr4+W)″]. Upon doping at low concentration, the violet-shift of optical absorption edge occurs and it is explained by ion polarization effect. Cr3+ is suggested to substitute for Pb2+ and results in the relaxation of crystal lattice, which is confirmed by Raman spectra. When heavily doped, some Cr go into W-site in the form of Cr4+ in addition to (Cr3+Pb) to keep the charge neutrality and scheelite-type structure integrity. It seems that W6+ tetrahedral site is suitable for hosting Cr4+, which is an important laser active ion.