Electron-spin-resonance study of complex interstitial halogenHD-type defects in KCI doped with divalent cations

An extensive electron-spin-resonance study has been made of trapped interstitial halogen defects in KCl doped with divalent cations ($M^{2+}={\mathrm{Ca}}^{2+},{\mathrm{Ba}}^{2+},{\mathrm{Sr}}^{2+},{\mathrm{Cd}}^{2+},{\mathrm{Pb}}^{2+},{\mathrm{Mg}}^{2+}, or {\mathrm{Sn}}^{2+}$). It is shown that $x$-irradiation-produced interstitial halogens can be trapped by monomers and dimers of $M^{2+}$, positive-ion vacancy complexes. These defects, which manifest themselves as ${\mathrm{Cl}}_2^{-}$ or ${\mathrm{Cl}}_3^{2-}$ species, are called $H_D$-type centers and altogether five different centers have been detected and studied. One is a di-interstitial halogen defect associated with an $M^{2+}$-vacancy monomer. Two other defects involve dimers of $M^{2+}$-vacancy complexes in configurations possessing inversion symmetry. The last two defects involve a ${\mathrm{Cl}}_2^{-}$ associated with a monomer complex in which the $M^{2+}$ is either nearest neighbor or next-nearest neighbor to the positive-ion vacancy. A careful analysis of the electron-spin-resonance spectra reveals that in both these centers the ${\mathrm{Cl}}_2^{-}$ molecule performs a librational motion with respect to the symmetry plane of the defect.