Optical properties of scheelite and raspite PbWO4 crystals

Reflection spectra of lead tungstate (PbWO4) crystals with scheelite and raspite structures have been measured at 6 K in the spectral range from 3 to 25 eV. The optical constants of both phases have been derived through the Kramers-Kronig analysis. The absorption spectra thus obtained exhibit a strong doublet exciton structure at around 4.30 eV for the scheelite phase and a weak exciton band at 3.75 eV for the raspite phase. The scheelite sample gives rise to a luminescence band peaking at 2.80 eV, while the raspite sample emits a band at 2.25 eV. The decay behaviors of these bands are found to be similar to each other. The 2.80- and 2.25-eV luminescence bands are intrinsic to the respective phases. The above results are discussed on the bases of a recent band-structure calculation of scheelite PbWO4 and of a difference in crystal structure between the two phases. Furthermore, it is suggested that the controversial "green" luminescence in scheelite is due to the raspite-type inclusions introduced by thermal stress during the crystal growth.