Novel H2-H2O clathrates at high pressures

High-pressure optical and x-ray studies of H2-H2O mixtures have revealed the formation of the first hydrogen clathrate hydrates. A rhombohedral hydrate with a H2O sublattice similar to ice II and a H2:H2O 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 H2O molecules in a cubic diamond structure and with a very high H2:H2O stoichiometry of 1:1. The H2 molecules occupy voids in the H2O framework, thus improving the packing efficiency and stabilizing this hydrate to very high pressures of at least 30 GPa.