Neutron time-of-flight patterns at a resolution of ∆d/d = 2 × 10-3 and inelastic scattering spectra at a resolution of 15µeV have been measured for the ternary intercalation compounds which are formed when ammonia gas is absorbed at reduced pressure by stage II caesium-graphite. The diffraction patterns reveal a new staging process giving high stage ternary compounds as the first step in the reaction followed by lower stage compounds and ultimately a true second-stage ternary compound C28Cs(NH3)2·66. The rotational tunnelling spectra at the lowest ammonia concentrations (χ 1·25 in C28Cs(NH3)χ) show transitions at 126µeV and 640µeV, whilst the higher concentrations so far measured show a single transition at 89µzeV up to x = 1·9 when the tunnelling spectrum collapses. The two-line spectrum at low concentrations shows that half of the (on average four) NH3 ligands to each caesium ion have a much weaker V3, hindering potential than the others (about 10/B) compared to 20/B for the 126µeV line showing that two NH3 groups are inclined towards the graphite planes and two others lies parallel to the planes. For the filled lattice unique V3 = 25/B means that all NH3 rotational axes are similarly inclined towards the graphite planes. There is no evidence for a change in NH3-NH3 coupling as the ammonia concentration is varied. The temperature dependence of the 126µeV line shows a shift to higher frequencies over the whole range of temperatures where the line is observable, in contrast to the 89 µeV line. This unusual behaviour indicates a strong modulation of the hindering potential by lattice phonons through a dominant cosine form.
|Number of pages||28|
|Publication status||Published - 1992|