Adsorption and melting of hydrogen in potassium-intercalated graphite

Abstract

Volumetric adsorption and quasielastic neutron scattering are used to study the diffusion and thermodynamics of sorbed ${\text{H}}_{2}$ in the graphite intercalation compound ${\text{KC}}_{24}$. A sorption enthalpy of 8.5 kJ/mol at zero coverage is determined from ${\text{H}}_{2}$ adsorption isotherms. From measurements of total elastic-neutron-scattering intensity as a function of temperature, a melting transition of the ${\text{H}}_{2}$ adsorbate is observed at 35 K for ${\text{KC}}_{24}{({\text{H}}_{2})}_{1}$. Quasielastic-neutron-scattering (QENS) spectra reveal distinct slow- and fast-${\text{H}}_{2}$-diffusion processes which exist simultaneously at temperatures above the transition point. The temperature dependence of the characteristic diffusion times follows an Arrhenius relation $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\text{ }\text{exp}({E}_{a}/T)$, where ${\ensuremath{\tau}}_{0}^{\text{fast}}=1.0\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{ps}$, ${\ensuremath{\tau}}_{0}^{\text{slow}}=21\ifmmode\pm\else\textpm\fi{}2\text{ }\text{ps}$, ${E}_{a}^{\text{fast}}=156\ifmmode\pm\else\textpm\fi{}5\text{ }\text{K}$, and ${E}_{a}^{\text{slow}}=189\ifmmode\pm\else\textpm\fi{}5\text{ }\text{K}$. The fast-diffusion process is attributable to individual motions of ${\text{H}}_{2}$ molecules in a static potassium structure, and the slow-diffusion process could be attributable to fluctuations in ${\text{H}}_{2}$ particle density correlated with jumps of potassium atoms. The QENS spectra at low $Q$ are used to discuss the dimensionality of the diffusion process.