Optical and Electrical Studies of Colloidal andF-Aggregate Centers in Highly Pure KBr Crystals

The optical and thermal stability of $F$, $M$, $R$ bands and colloids in KBr is measured. Extensive measurements on conductivity of the additively colored crystals are reported. Excess electronic conductivity is observed; it is due to $M$ centers below 150°C and to colloids between 300 and 450°C. The background divalent cation impurities suppress the excess conductivity due to $M$ centers and colloids, and reduce the ionic conductivity at ∼300°C to a value much less than that of a similar uncolored crystal. The influence of light on the excess and the ionic conductivity in the colored KBr crystals is also discussed. O${\mathrm{H}}^{-}$ impurity enhances both the optical colloidal band and the excess conductivity associated with it. The results are compared with those obtained by us recently for KCl crystals.