Triple Guest Occupancy and Negative Compressibility in Hydrogen-Loaded β-Hydroquinone Clathrate.

The molecular interactions and structural behavior of a previously unexplored clathrate system, hydrogen-loaded β-hydroquinone (β-HQ+H2), were investigated under high pressure with synchrotron X-ray diffraction and Raman/infrared spectroscopies. The β-HQ+H2 system exhibits coupling of two independently rare phenomena: multiple occupancy and negative compressibility. The number of H2 molecules per cavity increases from one to three, causing unit cell volume increase by way of unique crystallographic interstitial guest positioning. We anticipate these occupancy-derived trends may be general to a range of inclusion compounds and may aid the chemical and crystallographic design of both high-occupancy hydrogen storage clathrates and novel, variable-composition materials with tunable mechanical properties.