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Formation of ammonia–helium compounds at high pressure

Jingming Shi,W. Cui,J. Hao,Meiling Xu,Xianlong Wang,Yinwei Li

Published 2020 in Nature Communications

ABSTRACT

Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres. Here, we report the possibility of helium being fixed in the upper mantles of these planets in the form of NH 3 –He compounds. Structure predictions reveal two energetically stable NH 3 –He compounds with stoichiometries (NH 3 ) 2 He and NH 3 He at high pressures. At low temperatures, (NH 3 ) 2 He is ionic with NH 3 molecules partially dissociating into (NH 2 ) − and (NH 4 ) + ions. Simulations show that (NH 3 ) 2 He transforms into intermediate phase at 100 GPa and 1000 K with H atoms slightly vibrate around N atoms, and then to a superionic phase at  ~2000 K with H and He exhibiting liquid behavior within the fixed N sublattice. Finally, (NH 3 ) 2 He becomes a fluid phase at temperatures of 3000 K. The stability of (NH 3 ) 2 He at high pressure and temperature could contribute to update models of the interiors of Uranus and Neptune. Helium is generally considered too inert to be present in giant ice planet mantles. The authors, by first-principles calculations and crystal structure searches, find stable ammonia–helium compounds at the conditions of Uranus and Neptune’s upper mantles, with possible implications in the planet composition models.

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