Clathrate hydrates of methane and other gaseous alkanes are crystalline host-guest compounds resembling ice, in which the guest gaseous molecules are trapped inside host cages of hydrogen bonded frozen water molecules. These materials are a natural part of the deep-sea and permafrost environments and important gas reservoirs. Still other gas hydrate phases, not yet known in nature, have been obtained and characterized in the laboratory. In this study, a combination of high pressures and high temperatures (above 0.4 Mbar and 1000 K) was used to synthesize a novel hydrogen hydrate phase which has the structure of cubic ice for the water framework and twice as many hydrogen as water molecules in the unit cell, namely the highest gas-to-water molar ratio reported to date in any crystalline solid made of water and gas. This water–hydrogen compound is most likely stable up to pressures beyond the limit of the current experiments and might be abundant inside extra-terrestrial objects. The experimental study has been conducted through laser heated diamond anvil cells, coupled to optical spectroscopy and synchrotron (ESRF) X-ray diffraction materials characterization methods. The description of the novel hydrate was also enriched by ab initio computer simulations.

The collaborative research team included scientists from several European institutions: University of Rome “La Sapienza”, Italy; University of Edinburgh, UK; Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica, CNR-INO, and European Laboratory for Nonlinear Spectroscopy, LENS, Sesto Fiorentino, Italy; Institut für Festkörperphysik, Wien, Austria; École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland; Sorbonne Université, Paris, France.

Main publication: U. Ranieri, S. Di Cataldo, M. Rescigno, L. Monacelli, R. Gaal, M. Santoro, L. Andriambariarijaona, P. Parisiades, C. De Michele, and L. E. Bove, Observation of the most H2-dense filled ice under high pressure, Proc. Natl. Acad. Sci. U. S. A. 120, e2312665120 (2023). DOI: 10.1073/pnas.2312665120.

We acknowledge the project PRIN 2022 NRBLPT, Funded by the European Union – Next Generation EU