Electron-Paramagnetic-Resonance Study of the Structure and Motions of $H_{\mathrm{AA}}$ and $H_{A^{\prime }A}$ Centers in ${\mathrm{Na}}^{+}$- and ${\mathrm{Li}}^{+}$-Doped KCl

Interstitial chlorine atoms in KCl can be stabilized by pairs of ${\mathrm{Na}}^{+}$ or ${\mathrm{Li}}^{+}$ impurity ions which are nearest neighbors (nn) and next nearest neighbors (nnn) of each other, forming what are called, respectively, $H_{\mathrm{AA}}$ and $H_{A^{\prime }A}$ centers. These centers have a higher thermal stability than the $H_A$ (or $V_1$) and $H$ centers. Analysis of the electron-paramagnetic-resonance (EPR) spectra shows that the symmetries of $H_{\mathrm{AA}}\left({\mathrm{Na}}^{+}\right)$ and $H_{\mathrm{AA}}\left({\mathrm{Li}}^{+}\right)$ are the same and very similar to those of the $H$ center: They each consist of a $〈110〉$-oriented $\mathrm{Cl}_2^{}{}_{}{}^{-}$ molecule ion occupying a single negative-ion site and possessing weak molecular bonds with two neighboring substitutional ${\mathrm{Cl}}^{-}$ ions along $〈110〉$. In contrast to the $H$ center, the three molecular bonds of the $\mathrm{Cl}_4^{}{}_{}{}^{3-}$ are bent in a {001} plane. It is concluded that the axis defined by the two adjoining nn impurity alkali ions is parallel to the $〈110〉$-oriented $\mathrm{Cl}_2^{}{}_{}{}^{-}$ internuclear axis. This model does not allow any motion for the $H_{\mathrm{AA}}$ centers, and none is observed in the EPR spectra as the temperature is raised. The $H_{A^{\prime }A}$ centers consist of a $\mathrm{Cl}_2^{}{}_{}{}^{-}$ molecule ion, occupying a single negative-ion site associated with two ${\mathrm{Na}}^{+}$ or ${\mathrm{Li}}^{+}$ ions which are nnn of each other. However, the geometries of $H_{A^{\prime }A}\left({\mathrm{Li}}^{+}\right)$ and $H_{A^{\prime }A}\left({\mathrm{Na}}^{+}\right)$ are different. The