Librations of hydrogen in stage II caesium graphite, C24Cs

Abstract

Neutron spectra of hydrogen and hydrogen–nitrogen mixtures in the second-stage caesium graphite intercalation compound C₂₄Cs(H₂)₁, C₂₄Cs(H₂)₂ and C₂₄Cs(H₂)₁(N₂)_x, where x= 0.18, 0.45 and 1.00, in the librational energy region 10< ΔE/meV <200 (80–1600 cm^–1) are reported. The results confirm the two-site absorption model deduced from earlier tunnelling data, but indicate a cos 2θ potential with axial symmetry. The barriers to rotation are 51 and 78 meV (4.9 and 7.5 kJ mol^–1) in the A and B sites. The librational spectra using HD and D₂ show isotopic shifts which are largely in agreement with this model.