〈111〉-Oriented FCl−, FBr−, and FI− Centers in Mixed Alkali Halides

Abstract

$〈111〉$-oriented F${\mathrm{Cl}}^{-}$ molecule ions occupying one negative ion vacancy are produced in ${\mathrm{F}}^{-}$-doped KCl by x or $\gamma$ irradiation at 77°K. At temperatures above 150°K these F${\mathrm{Cl}}^{-}$ centers decay by dissociating into a substitutional ${\mathrm{Cl}}^{-}$ ion and a F° atom which can migrate interstitially. If the crystal also contains substitutional ${\mathrm{Br}}^{-}$ or ${\mathrm{I}}^{-}$ ions, the F° atoms can be retrapped to form $〈111〉$-oriented F${\mathrm{Br}}^{-}$ and F${\mathrm{I}}^{-}$. The ESR spectra of these centers have also been found and analyzed in the other alkali chlorides together with a $〈111〉$-oriented F${\mathrm{Br}}^{-}$ in ${\mathrm{F}}^{-}$-doped KBr. This analysis and comparison with the ${X_2}^{-}$ centers indicates that for the fluorine hyperfine components in ${\mathrm{F}}_2^{-}$ and $\mathrm{F}{X}^{-}$, $A_{\mathrm{II}}\left(\mathrm{F}\right)>\mathrm{}0\mathrm{}$ and $A_{\perp }\left(\mathrm{F}\right)<\mathrm{}0\mathrm{}$, while for ${X_2}^{-}$ and $\mathrm{F}{X}^{-}$, $A_{\mathrm{II}}\mathrm{}\left(X\right)>0\mathrm{}$ and $A_{\perp }\mathrm{}\left(X\right)>0\mathrm{}$ ($X=\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r},\mathrm{I}$). The main optical absorption band of F${\mathrm{Cl}}^{-}$ is situated at 300 mμ, while the approximate positions for the F${\mathrm{Br}}^{-}$ and F${\mathrm{I}}^{-}$ transitions are at 294 mμ and 275 mμ, respectively.