* Astronomy

Title: A stable all nitrogen metallic salt at terapascal pressures
Authors: Jian Sun (1 and 2), Miguel Martinez-Canales (3), Dennis D. Klug (4), Chris J. Pickard (3), Richard J. Needs (2) ((1) Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Germany, (2) Theory of Condensed Matter Group, United Kingdom, (3) Department of Physics and Astronomy, University College London, United Kingdom, (4) Steacie Institute for Molecular Sciences, National Research Council of Canada, Canada)

Nitrogen, as the seventh most abundant element in the universe, is an important constituent of the atmospheres and interiors of planets such as the Earth and the surfaces of moons such as Triton. The phase diagram and equation of state of dense nitrogen is therefore of interest in understanding the fundamental physics and chemistry of planetary processes and in discovering new materials. We predict stable phases of nitrogen at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N_{2}^{\delta+} pairs and N_{5}^{\delta-} tetrahedra, which is stable in the range 2.5-7.1 TPa. This is followed by a modulated Fdd2 structure at 7.1-11.5 TPa, which also exhibits significant charge transfer. The P4/nbm metallic nitrogen salt and Fdd2 modulated structure exhibit strongly ionic features and charge density distortions, which is unexpected in an element at such high pressures and could represent a new class of nitrogen materials.

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