Neutron inelastic scattering study of the molecular motions in solid silane

The molecular motions of SiH4 have been studied by the neutron scattering technique both in the plastic phase I above 63.45 K as well as in phase II. In the plastic phase the spectra are characterized by a very broad quasielastic component. From the Q-behaviour of the quasielastic and elastic intensities and from lineshape fits it is found that a model involving random 1200-reorientations around the threefold molecular axes is able to describe the results satisfactorily. The residence times and an activation energy of (1.05 +- 0.25) kJ mol-1 have been derived. In phase II the spectra are characterized by a structure in the inelastic region. The peaks have to be assigned to periodic motions; it is concluded that transitional and librational modes are separated, with perhaps an overlap at approximately 12 meV. (Auth.)