Origin of green luminescence in PbWO4 crystals

Characteristics of two green emission bands, G(I) and G(II), and their origin were investigated within 0.4-300 K under photoexcitation in the 3.4-6.0 eV energy range for undoped and Mo⁶⁺-, Mo⁶⁺ , Y³⁺-, Mo⁶⁺, Nb⁵⁺-, Mo⁶⁺, Ce³⁺-, Cr⁶⁺-, La³⁺-, Ba²⁺- and Cd²⁺-doped PbWO₄ crystals with different concentrations of impurity and intrinsic defects, grown by different methods and annealed at different conditions. The G(I) emission band, observed at low temperatures, located around 2.3-2.4 eV and excited around 3.9 eV, is usually a superposition of many closely positioned bands. The G(I) emission of undoped crystals is assumed to arise from the WO₄²⁻ groups located in the crystal regions of lead-deficient structure. In Mo⁶⁺-doped crystals, this emission arises mainly from the MoO₄²⁻ groups themselves. The G(II) emission band located at 2.5 eV is observed only in the crystals, containing the isolated oxygen vacancies — WO₃ groups. This emission appears at T>160 K under excitation around 4.07 eV as a result of the photo-thermally stimulated disintegration of localized exciton states and subsequent recombination of the produced electron and hole centres near WO₃ groups. The G(II) emission accompanies also thermally stimulated recombination processes in PbWO₄ crystals above 150 K. Mainly the G(II) emission is responsible for the slow decay of the green luminescence in PbWO₄ crystals.