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 H_2 in the graphite intercalation compound KC24. A sorption enthalpy of 8.5 kJ/mol at zero coverage is determined from H_2 adsorption isotherms. From measurements of total elastic-neutron-scattering intensity as a function of temperature, a melting transition of the H_2 adsorbate is observed at 35 K for KC_(24)(H_2)_1. Quasielastic-neutron-scattering (QENS) spectra reveal distinct slow- and fast-H_2-diffusion processes which exist simultaneously at temperatures above the transition point. The temperature dependence of the characteristic diffusion times follows an Arrhenius relation tau=tau_0 exp(E_a/T), where tau_0^(fast)=1.0±0.1 ps, tau_0^(slow)=21±2 ps, E_a^(fast)=156±5 K, and E_a^(slow)=189±5 K. The fast-diffusion process is attributable to individual motions of H_2 molecules in a static potassium structure, and the slow-diffusion process could be attributable to fluctuations in H_2 particle density correlated with jumps of potassium atoms. The QENS spectra at low Q are used to discuss the dimensionality of the diffusion process.