Two-dimensional caesium-ammonia solid solutions in C28Cs(NH3)x

Abstract

Neutron time-of-flight patterns at a resolution of Δd/d = 2 × 10^-3 and inelastic scattering spectra at a resolution of 15 µeV 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 (x < 1·25 in C₂₈Cs(NH₃) _x ) show transitions at 126 µeV and 640 µeV, whilst the higher concentrations so far measured show a single transition at 89 µeV 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 V ₃ hindering potential than the others (about 10/B) compared to 20/B for the 126 µeV 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 V ₃ = 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 126 µeV 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.