Neutron scattering studies of potassium-ammonia layers in graphite

Neutron scattering investigations of the structural and dynamical properties of ammonia molecules in the stage 1 compound KC24(NH3)4.3 are discussed. The K-NH3 intercalate layers represent the two-dimensional analogue of the well-known metal-ammonia solutions. At room temperature the intercalate liquid structure factor can be described by a model in which the K ions are surrounded on the average by four NH3 molecules, and the remaining molecules are essentially free. Quasi-elastic neutron scattering revealed two independent rotational motions of the NH3 molecule, one associated with the rotation about the C3 symmetry axis and the other one about the K ions. Above 200 K translational diffusion also becomes noticeable. The phonon dispersion of the (00q) longitudinal modes show clear signs of a phonon-libron coupling which causes the acoustic branch to split