Interstitial and di-interstitial BrCl22−in KCl

In KCl crystals containing both ${\mathrm{Na}}^{+}$ and ${\mathrm{Br}}^{-}$ ions as impurities, three types of ${\mathrm{}\left(\mathrm{ClBrCl}\right)\mathrm{}}^{2-}$ centers are identified by ESR after x irradiation at 77 K followed by a suitable thermal treatment. The first Br${\mathrm{Cl}}_2^{2-}$ is [110] oriented and possesses $C_{2v}$ symmetry. It is an $H_A\left({\mathrm{Na}}^{+}\right)$-type center: the Br occupies the interstitial position between two substitutional Cl along [110] and next to the Br is a substitutional ${\mathrm{Na}}^{+}$ impurity along [$1\overline{1}0$] which induces a bending of the molecule of about 14°. The second Br${\mathrm{Cl}}_2^{2-}$ is also oriented along [110] but possesses $D_{2h}$ symmetry. It is an $H_{\mathrm{AA}}\left({\mathrm{Na}}^{+}\right)$-type center: the interstitial Br is now flanked by two ${\mathrm{Na}}^{+}$ impurities along [$1\overline{1}0$]. These two centers, which may be considered as impurity stabilized interstitial bromine atoms ${\mathrm{Br}}_i^0$, are thought to be formed by the trapping of a mobile ${\mathrm{Cl}}_i^0$ by a pair of substitutional ${\mathrm{Br}}^{-}$ and ${\mathrm{Na}}^{+}$ ions, or by a complex of two ${\mathrm{Na}}^{+}$ ions adjoining a ${\mathrm{Br}}^{-}$, respectively. The third Br${\mathrm{Cl}}_2^{2-}$ has $C_{1h}$ symmetry. It is a di-interstitial center: the Br${\mathrm{Cl}}_2^{2-}$ occupies a single negative-ion site next to a substitutional ${\mathrm{Na}}^{+}$ impurity and the molecular axis makes a 22° angle with $〈100〉$ in a {100} plane. The latter center is thought to be formed by the trapping of both a ${\mathrm{Cl}}_i^0$ and a ${{\mathrm{Cl}}_i}^{-}$ by a pair of adjoining substitutional ${\mathrm{Br}}^{-}$ and ${\mathrm{Na}}^{+}$ ions. A short discussion of the Br${\mathrm{Cl}}_2^{2-}$ electronic structure is given.