Work

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

Year: 2026

Type: article

Abstract: 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.

Source: Communications Chemistry

DOI: 10.1038/s42004-026-02030-6