Hydrogen hydrates under extreme conditions. Insights into high-pressure phases and implications for planetary interiors

Hydrogen hydrates, formed from hydrogen and water under high-pressure and low-temperature conditions, exhibit a rich phase behavior governed by intricate intermolecular interactions. Their structural evolution under extreme conditions provides a unique platform to study the interplay between hydrogen bonding, lattice dynamics, and guest-host interactions. This study investigates the complex phase diagram of hydrogen hydrate up to 50 GPa using angle-resolved neutron and x-ray diffraction techniques. We focus on the behavior of key phases: C1, C2, and C3, examining their crystallographic transformations, elastic properties, and pressure-induced structural deformations. Our findings provide critical insights into phase transitions, stability regimes, and the fundamental physics of molecular interactions in dense hydrogen-water systems, enhancing our understanding of their thermodynamic and mechanical properties under extreme conditions. These data are also essential to quantify the role of hydrogen on the thermal state and chemical evolution of water-rich worlds of various sizes.