The UV optical absorption edge of pure PbWO4 and a series of La3+-doped PbWO4 crystals was measured at room temperature. Parameters, including the energy gap Eg and the energy of phonon Ep involved in the indirect transition, were calculated, and the dependence of the transition on impurities instead of phonons in heavily doped PbWO4:La crystals was presented. The shift of the absorption edge to higher energy observed in the case of doping at low concentration (<400ppm) and the red-shift of the cut-off edge found for doping at higher concentration (>1600ppm) were explained by the combined effects of ion polarization and localized defect energy level. The FT-IR spectra of the WO4 tetrahedron confirmed the coexistence of interstitial O and Pb vacancies to keep the charge balance in heavily doped PbWO4:La crystals.

Characteristics of the Optical Absorption Edge and Defect Structures of La3+-Doped PbWO4 Crystals. W.Li, X.Feng, Y.Huang: Journal of Physics - Condensed Matter, 2004, 16[8], 1325-33