Neutron scattering from coupled phonon-impurity modes inKCl1−c(KCN)c

The hybridization of host-lattice phonons with the internal-energy states of C${\mathrm{N}}^{-}$ impurities in KCl has been studied by inelastic neutron scattering as a function of temperature between 10 and 100 K for samples with impurity concentrations in the range $c=0.4\mathrm{} \mathrm{to} 6$ at.%. A temperature- and concentration-dependent coupling between phonons with $E_g$ symmetry and the $E_g$ transitions of C${\mathrm{N}}^{-}$ ions is observed near a frequency of 0.5 THz, a value which is consistent with the energy-level spacings for C${\mathrm{N}}^{-}$ in KCl as deduced by Beyeler. However, an expected coupling of phonons and C${\mathrm{N}}^{-}$ transitions with $T_{2g}$ symmetry near the same frequency was not detected. A simple two-level model for the C${\mathrm{N}}^{-}$ impurity provides a rather good description of the data for the $E_g$ coupled modes for $c<\mathrm{}2\mathrm{}$ at.%, but it deviates significantly for larger concentrations. Quasielastic scattering, which has a strong dependence on impurity concentration, temperature, and phonon wave vector, is also observed.