Adsorption and melting of hydrogen in potassium-intercalated graphite

Volumetric adsorption and quasielastic neutron scattering are used to study the diffusion and thermodynamics of sorbed H₂ in the graphite intercalation compound KC₂₄. A sorption enthalpy of 8.5 kJ/mol at zero coverage is determined from H₂ adsorption isotherms. From measurements of total elastic-neutron-scattering intensity as a function of temperature, a melting transition of the H₂ adsorbate is observed at 35 K for KC₂₄(H₂)₁. Quasielastic-neutron-scattering (QENS) spectra reveal distinct slow- and fast-H₂-diffusion processes which exist simultaneously at temperatures above the transition point. The temperature dependence of the characteristic diffusion times follows an Arrhenius relation τ=τ₀ exp(Eₐ/T), where τ₀ᶠᵃˢᵗ=1.0±0.1 ps, τ₀ˢˡᵒʷ=21±2 ps, Eₐᶠᵃˢᵗ=156±5 K, and Eₐˢˡᵒʷ=189±5 K. The fast-diffusion process is attributable to individual motions of H₂ molecules in a static potassium structure, and the slow-diffusion process could be attributable to fluctuations in H₂ particle density correlated with jumps of potassium atoms. The QENS spectra at low Q are used to discuss the dimensionality of the diffusion process.