Abstract
Both helium and ammonia are main components of icy giant planets. While ammonia is very reactive, helium is the most inert element in the universe. It is of great interest whether ammonia and helium can react with each other under planetary conditions, and if so, what kinds of structures and states of matter can form. Here, using crystal structure prediction methods and first-principles calculations, we report three new stable stoichiometries and eight new stable phases of He−NH3 compounds under pressures up to 500 GPa. These structures may exhibit perovskitelike structures for HeNH3 and He2NH3, and a host-guest crystal structure for He(NH3)2. Superionic states are found in all these He−NH3 compounds under high pressures and temperatures in which the hydrogen atoms are diffusive while the nitrogen and helium atoms remain fixed. Such dynamical behavior in helium ammonia compounds is quite different from that in helium water compounds, where weakly interacting helium is more diffusive than stronger bound hydrogen. The low-density host-guest phase of space group I4cm is found to be stable at very low pressures (about 3 GPa) and it enters into a plastic state, characterized by freely rotating ammonia molecules. The present results suggest that plastic or superionic helium ammonia compounds may exist under planetary conditions, and helium contributes crucially to the exotic physics and chemistry observed under extreme conditions.Received 25 November 2019Revised 25 February 2020Accepted 10 March 2020DOI:https://doi.org/10.1103/PhysRevX.10.021007Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCrystal meltingCrystal structureLiquid-solid phase transitionPressure effectsPhysical SystemsCore of giant planetsMolecular solidsTechniquesFirst-principles calculationsMolecular dynamicsCondensed Matter, Materials & Applied Physics
Highlights
Helium is the second most abundant element in the universe, and it occupies a large fraction of the atmospheres of Uranus and Neptune
We have explored the crystal structures in helium ammonia compounds below 500 GPa using variablecomposition structure prediction methods in conjunction with density functional theory (DFT) total energy calculations
The appearance of superionic phases in both HeðNH3Þ2 and HeðND3Þ2 indicates that nuclear quantum effects do not affect the existence of the superionic state in the helium ammonia compounds, we do not know whether the nuclear quantum effects affect the onset of the superionic state
Summary
Helium is the second most abundant element in the universe, and it occupies a large fraction of the atmospheres of Uranus and Neptune. Helium is generally considered to be unreactive, several groups have reported stable helium compounds under high pressure. It can react with metals [1,2], metal oxides, sulfides, or fluorides [3,4,5], in which the insertion of helium atoms. Helium can react with inert gases [6,7,8], nitrogen [9,10], water [11,12,13], and ammonia [14], where it exhibits negligible charge transfer and plays a role in increasing the internal pressure. The hot ice is composed of water, ammonia, and methane under extreme pressures up to several megabars and temperatures up to several thousand kelvin, and is below the atmospheres of hydrogen and
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
