Superionicity in Ammonium Polyhydrides at Extreme Pressures

Abstract: Polyhydrides have been shown to form novel structures at high pressure, which may be found in the interiors of giant planets. With density functional molecular dynamics simulations we studied the behavior of ammonium polyhydride compounds with stoichiometries of NH$7$, NH$_9$, NH${10}$, NH${11}$, NH${14}$, NH${20}$, and NH${24}$ which were predicted with crystal structure search methods to be metastable at 100-300~GPa. For every compound, we performed simulations at a range of temperatures (and for several compounds, pressures) covering the solid, superionic and liquid phases. We show that when heated, high pressure ammonium polyhydride compounds exhibit hydrogen superionic diffusion. We demonstrate a number of metrics by which the solid-to-superionic and superionic-to-liquid transitions can be detected from simulation data, including changes in the internal energy and pressure, formation of new chemical species, and atomic diffusion rates. We find that both the solid-to-superionic and the superionic-to-liquid transitions decrease in temperature as proton fraction increases. These trends indicate that above a proton fraction of $\sim$0.97, ammonium hydride structures are likely to directly melt instead of first exhibiting a superionic phase. Our observed melting trend further indicates that at the extreme conditions of ice giant interiors, hydrogen rich ammonium hydrides such as those studied in this work would exist predominantly as liquids rather than exhibiting a superionic phase.