Exciton luminescence of scheelite- and raspite-structured PbWO4 crystals

The reflection and luminescence spectra of scheelite- and raspite-structured PbWO4 crystals have been studied at 6 K with use of synchrotron radiation as a light source. An intense reflection band due to excitons is found at 4.25 eV for the former and at 3.72 eV for the latter. The scheelite and raspite samples emit a single luminescence peaking at 2.80 eV ("blue" band) and at 2.25 eV ("green" band), respectively, when they are excited with photons in the region of the fundamental absorption including the lowest exciton band. The 2.80-eV luminescence is likely due to radiative decay of the excitons localized on tetrahedral. WO4 groups, while the 2.25-eV luminescence is ascribed to the excitons localized on octahedral WO6 groups. The present result will resolve a long-standing dispute about the origin of the "blue" and "green" emission bands in PbWO4. (C) 2000 Elsevier Science B.V. All rights reserved.