![]() Phonon calculations and ab initio molecular dynamics simulations at finite temperatures suggest their dynamical stability. ), which are all estimated to be energetically stable below 40 GPa. Based on a machine-learning and graph theory accelerated crystal structure search method and first-principles calculations, we predicted a series of metal nitrides with chain-like poly-nitrogen (P2 1-AlN 6, P2 1-GaN 6, P-1-YN 6 and P4/mnc-TiN 8. Polymeric nitrogen draws much attention on the application as environmentally save high-energy-density materials (HEDMs). This work promotes the study of neon-nitrogen compounds with superior properties and potential applications. Therefore, NeN10, NeN22, t-N22, and t-N20 are promising green high-energy-density materials. Meanwhile, their explosive performance is superior to that of TNT. Moreover, ultra-high energy densities are obtained in NeN10 (11.1 kJ/g), NeN22 (11.5 kJ/g), tetragonal t-N22 (11.6 kJ/g), and t-N20 (12.0 kJ/g) produced from NeN22, which are more than twice the value of trinitrotoluene (TNT). Importantly, both NeN10 and NeN22 not only are dynamically and mechanically stable but also have a high thermal stability up to 500 K under ambient pressure. Especially, NeN22 acquires a duplex host-guest structure, in which guest atoms (Ne and N2 dimers) are trapped inside the crystalline host N20 cages. We find that inserting Ne into N2 substantially decreases the polymeric pressure of the nitrogen and promotes the formation of abundant polynitrogen structures. Here, we identify three new Ne–N compounds (i.e., NeN6, NeN10, and NeN22) under pressure by first-principles calculations. Considering the inertness of neon, whether nitrogen and neon can react has aroused great interest in condensed matter physics and space science. Neon (Ne) can reveal the evolution of planets, and nitrogen (N) is the most abundant element in the Earth's atmosphere. The current results provide theoretical evidence that Xe could be trapped inside planets during their evolution and could help to update models of planetary interiors. This suggests that (NH 3) 2 Xe is a possible constituent of planetary interiors. The superionic phase remains stable in the pressure and temperature region that covers the extreme conditions of the layer outside the core of planets such as Uranus, Neptune, Venus, and Earth. Ab initio molecular dynamics simulations reveal that (NH 3) 2 Xe transforms from a solid to a superionic, and finally to a fluid as the temperature increases. Here, NH 3 in the compound remains in its molecular form up to at least 300 GPa, indicating that the incorporation of Xe could suppress the ionization of NH 3. In this paper, we report combined structure prediction and first-principles calculations to propose an unexpected stoichiometry of (NH 3) 2 Xe that becomes energetically stable >11 GPa. Noble gas elements have been illustrated to exhibit chemical activity to form unconventional compounds at high pressure. The isentropes for Uranus and Neptune (dark green and blue solid lines) and the phase boundary for superionic pure water (white dash-dotted line) are taken from ref. ⁸. The red dashed line distinguishes the two predicted solid phases: I41md and Fd3¯m\documentclass) in the SI-I region. The black dashed lines were fitted to the phase boundaries. The symbols represent four distinct thermodynamic states sampled in our simulations: circle, solid state square, He diffusive state (SI-I) diamond, both He and H diffusive state (SI-II) and triangle, fluid state. Proposed phase diagram of the helium–water system at high pressures obtained from our structure searches and AIMD simulations The insertion of helium atoms substantially decreases the pressure at which superionic states may be formed, compared to those in pure ice. As the He–O interaction is weaker than the H–O interaction, the helium atoms in these superionic states have larger diffusion coefficients and lower ‘melting’ temperatures than those of hydrogen, although helium is heavier than hydrogen. In the second phase, both helium and hydrogen atoms move in a liquid-like fashion within a fixed oxygen sublattice. In the first of these phases, the helium atoms exhibit liquid behaviour within a fixed ice-lattice framework. Surprisingly, we find that they can form two previously unknown types of superionic state under high pressure and high temperature. Here, we use ab initio calculations to show that He and H2O can form stable compounds within a large pressure range that can exist even close to ambient pressure. Helium is the most inert element in nature and it is generally considered to be unreactive. ‘Superionicity’ has attracted much attention in both fundamental science and applications. For example, in superionic ice, hydrogen atoms can move freely while oxygen atoms are fixed in their sublattice. Superionic states are phases of matter that can simultaneously exhibit some of the properties of a liquid and of a solid.
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