Color Centers in Mixed Crystals of Alkali Halides

Abstract

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\ifmmode^\circ\else\textdegree\fi{}, -80\ifmmode^\circ\else\textdegree\fi{}, and -190\ifmmode^\circ\else\textdegree\fi{}C. The influence of radiation dose, coloration temperature, and composition of mixed crystals on spectral position, half-width, and intensity of the ${V}_{1}$, $K$, $F$, ${F}^{\ensuremath{'}}$, 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\ifmmode^\circ\else\textdegree\fi{}C. At -190\ifmmode^\circ\else\textdegree\fi{}C coloration the ${V}_{1}$, $K$, $F$, and ${F}^{\ensuremath{'}}$ bands appeared. In mixed crystals the spectral position of all absorption bands deviates from the Mollwo relation ($\ensuremath{\nu}{d}^{n}=\mathrm{const}$). The deviation in ${V}_{1}$, $K$, $F$, and $M$ is toward lower, and in ${F}^{\ensuremath{'}}$ 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.