Electrical conductivity and colour centres in Tl-doped KCl crystal

The electrical conductivity and optical absorption of potassium chloride crystals doped with different concentrations of thallium have been measured before and after X-irradiation. The optical absorption spectrum exhibits the characteristic Tl⁺ ion band at 247 nm. The extrinsic conductivity and the absorption coefficient at 247 nm increase with impurity addition upto a certain concentration. Further increase in impurities decreases them. Room temperature X-irradiation decreases the Tl⁺ ion band and produces the F band. F centre concentration is higher in lightly doped crystal compared to pure or heavily doped KCl. These results have been interpreted in terms of formation of interstitial potassium ions and positive ion vacancies in the Tl-doped KCl lattice due to large ionic radius of Tl⁺ ions. The impurity ions precipitate into TlCl phase when the doping is heavy.