Slow Diffusion of Molecular Hydrogen in Zeolite 13X

High-resolution quasi-elastic neutron scattering measurements have been performed on molecular hydrogen in zeolite 13X. Previous NMR measurements suggested that the freezing temperature is suppressed from 14 K down to 8 K. In contrast, previous intermediate resolution quasi-elastic neutron scattering studies suggested freezing occurred between 25 and 35 K. Unfortunately, the limited instrumental resolution available in the previous quasi-elastic neutron scattering study was not sufficient to show this point definitively. We report new quasi-elastic neutron scattering measurements with very high resolution that show no evidence of mobile hydrogen below 25 K, which is well above the bulk liquid-solid transition temperature for hydrogen. A quasi-elastic component appears between 25 and 30 K indicating the presence of mobile H2. However, the width and momentum dependence of the quasi-elastic scattering are much different than would be expected for the diffusive motion of liquid hydrogen in this temperature range. Instead, we find that a slow diffusive component representing jumps between well-defined sites appears first at low temperatures. As the temperature is raised, a faster liquid like diffusive component appears.