Rotational tunnelling of ammonia in two-dimensional metal-ammonia solutions

Rotational tunnelling of ammonia in the two-dimensional metal-ammonia solid solutions formed by high-stage caesium graphite intercalates is reported and used to define the adsorption site structure in these systems. Different stages and stoichiometries C_nCs(NH₃)_x have been measured for 1.5 < T/K < 90 using high-resolution neutron scattering to obtain the dependence on stage and temperature of the tunnelling energies. Prominent transitions at 89 μeV (high fillings) and at 89 and 126 μeV (low fillings) indicate sensitivity to the ammonia content with multiple adsorption sites and threefold symmetry for the hindering potential. The activation energy for quenching the 89 μeV transitions is 6 meV, corresponding to a major peak in the vibrational density of states but not the expected E₀₁ torsional transition energy. The bearing of these results on recent theories for the temperature dependence of tunnelling is discussed.