NovelH2-H2O clathrates at high pressures

High-pressure optical and x-ray studies of ${\mathrm{H}}_2$-${\mathrm{H}}_2$O mixtures have revealed the formation of the first hydrogen clathrate hydrates. A rhombohedral hydrate with a ${\mathrm{H}}_2$O sublattice similar to ice II and a ${\mathrm{H}}_2$:${\mathrm{H}}_2$O ratio of 1:6 is stable between 0.75 and 3.1 GPa (295 K). Above 2.3 GPa, a novel hydrate forms with the ${\mathrm{H}}_2$O molecules in a cubic diamond structure and with a very high ${\mathrm{H}}_2$:${\mathrm{H}}_2$O stoichiometry of 1:1. The ${\mathrm{H}}_2$ molecules occupy voids in the ${\mathrm{H}}_2$O framework, thus improving the packing efficiency and stabilizing this hydrate to very high pressures of at least 30 GPa.