Optical, Electrical, and EPR Studies of KBr Crystals Doped with Cadmium

Abstract

The peak positions, half-widths, and oscillator strengths of Cd (and Zn) bands in KBr and other uncolored alkali halides are given. In uncolored KBr, Cd introduces three optical absorption bands at 202, 215, and 280 nm, respectively. Two new bands, $D_1$ and $D_2$, at 250 and 285 nm, respectively, are observed in the irradiated KBr: Cd crystals. The growth of absorption coefficients ${\alpha }_F$ and ${\alpha }_M$ at the peaks of $F$ and $M$ bands, respectively, was studied as a function of radiation dose. The value of $S$ in the relation ${\alpha }_M=S{{\alpha }_F}^2$ is suppressed by a factor ∼100 in crystals containing more than 20 ppm of Cd. In the crystals irradiated for short times, the $M$ band cannot be produced even by optically bleaching the $F$ band. The second-stage growth of the F band is also suppressed considerably by the Cd impurity, which supports the view that the second-stage $F$ centers are necessary for the formation of $M$ centers. The $D_1$ and $D_2$ bands observed in x-irradiated crystals are absent in the additively colored crystals. In the crystals colored additively at 460°C, an ultraviolet band at 330 nm and a band at 750 nm are observed in addition to the F band. The $F$ band can be converted to the 750-nm band by optical or partial thermal bleaching of the crystal. In a crystal colored at 560°C the 750-nm band is absent, and only the 330-nm band and a very stable $F$ band are present. On heating the crystals to 600°C, the 330-nm band, $F$ band, and 750-nm band disappear, and a new band at 300 nm appears. The crystal containing the 300-nm band shows an EPR absorption line with Lorentzian shape and a $g$ value of 1.987, and enhanced conductivity above 500°C. These results suggest that the centers responsible for this band are metallic colloidal particles of Cd. The equilibrium concentration of $F$ centers in crystals doped with 28 ppm of Cd and colored additively at 560°C is 15% less than that in a pure crystal. At 560°C the rate of diffusion of $F$ centers in the Cd-doped crystals is less by a factor ∼5 as compared to the rate in a pure crystal, and is less by a factor ∼50 as compared to rate the of diffusion of 330-nm band centers.