Nature of the Phase Transition in KCN at 168 K

Neutron inelastic scattering line shapes for [100] TA phonons in KCN have been measured for temperatures between 169 and 300 K. The results are compared to the predictions of a model incorporating a strong indirect ${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}^{-}$-${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}^{-}$ ion interaction mediated by elastic strains. The present experiments confirm the essential features of the model and elucidate the nature of the mode softening which leads to the phase transition at 168 K. In particular, the softening is shown to exist even for wave vectors one-half the distance to the Brillouin zone boundary.