New high-pressure van der Waals compound Kr(H₂)₄ discovered in the krypton-hydrogen binary system

The application of pressure to materials can reveal unexpected chemistry. Under compression, noble gases form stoichiometric van der Waals (vdW) compounds with closed-shell molecules such as hydrogen, leading to a variety of unusual structures. We have synthesised Kr(H 2) 4 for the first time in a diamond-anvil high-pressure cell at pressures a ‰ 5.3a €...GPa and characterised its structural and vibrational properties to above 50a €...GPa. The structure of Kr(H 2) 4, as solved by single-crystal synchrotron X-ray diffraction, is face-centred cubic (fcc) with krypton atoms forming isolated octahedra at fcc sites. Rotationally disordered H 2 molecules occupy four different, interstitial sites, consistent with the observation of four Raman active H 2 vibrons. The discovery of Kr(H 2) 4 expands the range of pressure-stabilised, hydrogen-rich vdW solids, and, in comparison with the two known rare-gas-H 2 compounds, Xe(H 2) 8 and Ar(H 2) 2, reveals an increasing change in hydrogen molecular packing with increasing rare gas atomic number.