Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high-resolution neutron powder diffraction. Three dihydrogen molecules (H ₂, D ₂, and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen-doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut-like motion of the HD guest in the center of anisotropic sII-S (sII-S=small cages of structure II hydrates). However, we failed to observe a mass-dependent relationship when comparing D ₂ with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.