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 ${10}^4$ to ${10}^6$ 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 $V_1$, $K$, $F$, $F^{\prime }$, 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 $V_1$, $K$, $F$, and $F^{\prime }$ bands appeared. In mixed crystals the spectral position of all absorption bands deviates from the Mollwo relation ($\nu d^n=\mathrm{const}$). The deviation in $V_1$, $K$, $F$, and $M$ is toward lower, and in $F^{\prime }$ toward higher energy. The half-width of all bands in mixed crystals is greater than in pure components except that of $V_1$ 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.