Discovery of a low-density filled-ice phase in nitrogen hydrate at high pressure
Abstract
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 in the Pnma space group, which we designate as NH-V. This new phase cannot be indexed to any known ice frameworks - such as the high-pressure methane hydrates MH-III (Imma) or MH-IV (Pmcn) - 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 nitrogen and water to form stable filled-ice structures up to 16 GPa, with important implications for planetary science.
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Cite
@article{arxiv.2508.09771,
title = {Discovery of a low-density filled-ice phase in nitrogen hydrate at high pressure},
author = {Selene Berni and Sophie Espert and Tomasz Poreba and Simone Di Cataldo and Richard Gaal and Gabriel Tobie and Erwan Le Menn and Thomas C. Hansen and Roberto Bini and Livia Eleonora Bove},
journal= {arXiv preprint arXiv:2508.09771},
year = {2025}
}