Formation of ammonia-helium compounds at high pressure.

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 NH3-He compounds. Structure predictions reveal two energetically stable NH3-He compounds with stoichiometries (NH3)2He and NH3He at high pressures. At low temperatures, (NH3)2He is ionic with NH3 molecules partially dissociating into (NH2)- and (NH4)+ ions. Simulations show that (NH3)2He 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, (NH3)2He becomes a fluid phase at temperatures of 3000 K. The stability of (NH3)2He at high pressure and temperature could contribute to update models of the interiors of Uranus and Neptune.