Jian Sun, Miguel Martinez-Canales, Dennis D. Klug, Chris J. Pickard, Richard J. Needs
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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http://arxiv.org/abs/1210.4358
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