Spectra properties of Cr-doped and Cr, La-co-doped PbWO4 single crystals

We report in this paper the spectra properties of Cr-doped and Cr, La-co-doped PbWO4 (PWO) crystals grown by the Czochralski method. It is found that low-field Cr3+ ions and Cr4+ ions in tetrahedral sites co-exist in the crystals studied. In the case of Cr-doped PWO samples, Cr3+ is supposed to reside primarily at the Pb-site, and this results in the relaxation of lattice, which is confirmed by FT-IR analysis. The crystal-field parameters of Cr3+ in a distorted octahedral environment are estimated, and they are Dq = 1351 cm-1 and Dq/B = 2.11, respectively. The NIR absorption of the crystals confirms the formation of Cr4+ and it is suggested that this is situated in the W6+ position. In particular, the co-addition of La3+ can induce a remarkable increase of absorption coefficient in the NIR region, on the grounds that La3+ ions preferably occupy the Pb2+ sublattice and force Cr to enter the W6+ position in the form of Cr4+. Cr-doped PWO crystals may be of interest as a new passively Q-switched laser material considering their high absorption coefficient in the NIR spectrum