Color Centers in Mixed Crystals of Alkali Halides

Single crystals of KCl, RbCl, KBr and mixed crystals of KCl-RbCl, KCl-KBr, and RbCl-KBr of various compositions have been colored by 3-MeV electrons with doses 10⁴ to 10⁶ rads at 20^∘{}, -80^∘{}, and -190^∘{}C. The influence of radiation dose, coloration temperature, and composition of mixed crystals on spectral position, half-width, and intensity of the V₁, K, F, F^', and M bands has been studied. At room temperature K, F, and M bands were produced, and at highest coloration N bands; only F and K bands appear at -80^∘{}C. At -190^∘{}C coloration the V₁, K, F, and F^' bands appeared. In mixed crystals the spectral position of all absorption bands deviates from the Mollwo relation (νdⁿ=const). The deviation in V₁, K, F, and M is toward lower, and in F^' toward higher energy. The half-width of all bands in mixed crystals is greater than in pure components except that of V₁ in the KCl-RbCl system, where it is smaller. The intensity of coloration in KCl-RbCl decreases with composition and reaches a minimum at molar ratio 1:1. In KCl-KBr and RbCl-KBr the intensity curves exhibit a relative maximum and a relative minimum. The behavior of the observed color-center bands is correlated to crystal structure and to the configuration of color centers.