Tunnelling of hydrogen in alkali-metal–graphite intercalation compounds. A systematic study of C24Rb(H2)x and its structural consequences
Abstract
The rotational tunnelling spectrum of hydrogen molecules absorbed in C24Rb(H2)x and the absorption site structure have been investigated systematically over the full range x= 0–2 in the energy region 100 µeV < ΔE < 2.5 meV (1 meV = 8.065 cm–1), together with some librational spectra at selected hydrogen concentrations over the energy range 10 < ΔE/meV <200. The results confirm the two-site absorption model, and the transition energies are consistent with a cylindrically symmetric cos 2θ potential for both sites with barriers to rotation of 57 and 78 meV, respectively (5.5 and 7.5 kJ mol–1). The data for HD are consistent with those for H2. Numbers and dimensions of possible absorption sites are considered in the domain model for C24Rb, and by comparing the tunnelling data with X-ray results, the sites are identified respectively as commensurate (√7 ×√7)R19.1°(metal–metal distance 6.51Å), and triangular incommensurate with a metal–metal distance of 5.98 Å.