Both hydrogen and xenon form unusual phases at very high pressures. Researchers have now observed that an unexpectedly stable compound forms when a hydrogen-rich mixture of the two gases is subjected to pressures in the gigapascal range. Xenon dimers and other unusual bonding states are revealed in this compound, which is stable to megabar pressures.

Encapsulating guest molecules inside host structures ranging from soft, flexible enzymes to rigid, porous zeolites has led to developments in many areas, including catalysis, sensing and separation. This Review highlights how metal-organic frameworks — materials formed by linking metal centres with organic ligands – can combine softness with regularity to produce dynamic, yet crystalline, structures that may prove useful for a range of applications.

Materials built from metal centres and organic ligands have traditionally attracted attention for their channels' host-guest properties. Now, controlling the occupancy of the channels by guest molecules has resulted in a framework that conducts protons under anhydrous conditions and acts as a gas-tight membrane, offering a promising approach to fuel-cell electrolytes.

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