Discovery of a low-density filled-ice phase in nitrogen hydrate at high pressure

Nitrogen, Earth’s most abundant atmospheric component, in the presence of water, is known to form clathrate hydrates, that are relevant to model the evolution of water-rich planets and satellites. Here, we map the high-pressure phase diagram of nitrogen hydrate up to 16 GPa at room temperature by combining neutron diffraction, Raman spectroscopy, and crystal structure prediction. We reveal a rich sequence of structural transformations, from sI/sII clathrates to hexagonal (sH) and tetragonal (sT) phases, culminating in a previously unknown orthorhombic filled-ice structure above 1.8 GPa. This new phase cannot be indexed to any known ice frameworks and exhibits a density approximately 30% lower than that of stable ice VII, pointing to distinctive water-nitrogen interactions. Our results refine the understanding of nitrogen hydrate behavior under extreme conditions and demonstrate the propensity of N2 and H2O to form stable filled-ice structures up to 16 GPa, with important implications for planetary science. While the stability of nitrogen clathrate hydrates at moderate pressures is well established, the behavior of the water-nitrogen system at higher pressures, relevant for the deep interiors of large icy moons and water-rich exoplanets, remains underexplored. Here, the authors use neutron diffraction, Raman spectroscopy and crystal structure prediction to map the high-pressure phase diagram of nitrogen hydrate up to 16 GPa at room temperature, reporting a sequence of structural transformations from sI/sII clathrates to hexagonal and tetragonal phases and orthorhombic filled-ice structures above 1.8 GPa.