Color Centers in KI Containing Impurity Electron Acceptors: KI:Tl

Abstract

The optical and chemical properties of KI:Tl after x‐raying and subsequent treatments at dry ice temperature or higher, or after coloration by electrolysis or chemical addition, have been investigated. Analysis of the experimental data yields the following conclusions: (1) KI:Tl contains more electron traps than KI; (2) an electron‐surplus center (T) is formed when KI:Tl is F‐bleached; (3) at concentrations of 10^—2 mole percent Tl⁺, metallic thallium, and not F centers, is produced by additive or electrolytic coloration; (4) KI:Tl, KI, and other alkali halides undergo photochemical changes as a result of x‐raying which are related to the production of V centers. Arguments are presented which lead to the following deductions: (1) Tl⁺ ions act as electron traps; (2) the greater stability of the T center relative to the F center is correlated to the greater electron‐accepting nature of the impurity ion (whether it be Tl⁺ or some other cation) relative to K⁺, which renders the F, K, R, and M centers unstable; (3) F bleaching at room temperature of x‐rayed crystals results in the formation of vacancy aggregates which are stable at room temperature via a transitory stage involving negative‐ion vacancies; at low temperatures these aggregates cannot be ``decomposed'' by electrons because of the lack of thermal activation. Similar properties are exhibited by KBr:Tl. Many of the color center properties of KI:Tl should occur with other uni‐ and polyvalent cationic impurities, since they are most naturally explained on the basis of a greater electron affinity of the impurity ion than of K⁺.