Neutron scattering study of coupled defect-phonon modes in KCl_1-x(KCN)_x

Abstract
The hybridization of phonons with the internal modes of CN^- impurities in KCl has been studied by inelastic neutron scattering as a function of temperature between 10 K and 100 K for samples with impurity concentrations in the range x = 0.4 to 6 %. The splitting of the (ζζ0) transverse acoustic phonon branch near 0.5 THz, observed previously [1] for x = 0.4 %, and the damping of the hybridized modes increase substantially with increasing concentration. The hybridization also shows a strong temperature dependence, essentially disappearing above 40 K for all the samples studied. Theoretical studies employing a simple two level impurity model [2] have also been carried out over the same ranges of concentration and temperature. The results are in reasonably good agreement with experiment at the lower concentrations, but the agreement deteriorates at the higher concentrations.