Birefringence in heavily Li-doped and La-doped PbWO4 single crystals

Li- and La-doped PbWO4 single crystals were prepared by Czochralski technique and their optical properties, including birefringence and optical absorption spectra, were investigated. For La-containing sample, it was found that the refractive indices decrease and the birefringence property deteriorates, which was explained by the substitution of La3+ for Pb2+. On the other hand, lithium ion was supposed to primarily localize at interstitial site and result in slight increase of refractive indices of the crystal. However, the birefringence performance is hardly improved. The impedance spectra confirmed the difference of doping mechanisms between Li+ and La3+. It is expected that the improvement of birefringence for PWO crystal could be realized through the substitution of Pb2+ by larger ions (more than that of Pb2+) and/or raising amount of interstitial ions in the crystal.