Quasielastic Neutron Scattering of Hydrogen Adsorbed in KC$_{24}$

Quasielastic neutron scattering (QENS) and volumetric techniques were used to study the adsorption of H$_{2}$ by the stage-2 potassium graphite intercalation compound KC$_{24}$. A zero-coverage sorption enthalpy of 8.5~kJ/mol was measured from H$_{2}$ isotherms recorded at 77~K and 87~K. The saturation H$_{2}$ adsorption amount at 77~K was 1.2~mass\%, corresponding to a stoichiometry of KC$_{24}$(H$_{2}$)$_{2.0}$. Quasielastic neutron scattering spectra for KC$_{24}$(H$_{2}$)$_{1.0}$ were collected at temperatures between 40~K and 80~K on a chopper spectrometer and a backscattering spectrometer. Two distinct H$_{2}$ diffusion processes were identified with characteristic times of approximately $\tau = 10$~ps and $\tau = 350$~ps at 60~K, respectively. By operating the backscattering spectrometer in fixed window mode, the total elastic scattering of KC$_{24}$(H$_{2}$)$_{1.0}$ was measured as a function of temperature. A sharp decrease in elastic intensity was observed at 35~K due to the onset of quasielastic scattering. This was interpreted as a melting transition of the H$_{2}$ adsorbate in KC$_{24}$.