Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature

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 ofHe−NH3compounds under pressures up to 500 GPa. These structures may exhibit perovskitelike structures forHeNH3andHe2NH3, and a host-guest crystal structure forHe(NH3)2. Superionic states are found in all theseHe−NH3compounds 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 groupI4cmis 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.

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Plain Language Summary

Helium—the most inert element on the periodic table—and ammonia are major components of icy giant planets. While helium is generally considered to be unreactive, it is not clear whether these two components can react with each other under planetary conditions, or what kinds of states might emerge. Using crystal structure search techniques and ab initio molecular dynamics simulations, we find that three types of helium ammonia compounds can form in eight stable phases over a wide pressure range.

Our investigation identifies that the eight phases belong to three helium ammonia compositions:He(NH3)2,HeNH3, andHe2NH3. Some of the phases resemble host-guest structures, where two or more molecules are held together by forces other than covalent bonds, and perovskitelike systems, in which the compound has a crystalline structure similar to the mineral perovskite. We find that these helium ammonia compounds can even form plastic and superionic states at different pressure and temperature conditions.

Our study provides new and surprising insights into the properties of compounds that may exist in icy giant planets and into new chemistry and physics of helium compounds under extreme conditions.