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

Abstract 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 ≥5.3 GPa and characterised its structural and vibrational properties to above 50 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.