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

The molecular interactions and structural behavior of a previously unexplored clathrate system, hydrogen-loaded beta-hydroquinone (beta-HQ+H-2), were investigated under high pressure with synchrotron X-ray diffraction and Raman/infrared spectroscopies. The beta-HQ+H-2 system exhibits coupling of two independently rare phenomena: multiple occupancy and negative compressibility. The number of H-2 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.