Neutron scattering study of phonon-libron coupling in potassium-ammonia intercalated graphite

Abstract

We present the results of an inelastic neutron scattering study of the [001]L lattice dynamics of stage-1 and stage-2 K-ammonia intercalated graphite. The results show that a librational mode splits the acoustic-phonon branch of the stage-1 compound at an energy of about 7 meV and that the interaction between the ammonia molecules in different galleries is very weak. This effect can be described by a simple one-force-constant Born--von K\'arm\'an model with the libration treated as an Einstein oscillator and the hybridization follows from a phonon-libron coupling term. The nature of this additional mode, however, remains to be determined. In addition, we report that the elastic constant ${C}_{33}$ is considerably softer in the K-ammonia system than in the analogous binary K compounds.