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.