Ionic and superionic phases in ammonia dihydrate NH3·2H2O under high pressure

Abstract

Water and ammonia have long been seen as the main species of extraterrestrial space, especially on solar giants, moons, comets, and numerous extrasolar planets. The phases formed by their admixtures under temperature and pressure conditions of the giant planets are important for understanding many observable properties (gravitational moments, atmospheric composition, and magnetic field). Here we employ a Monte Carlo packing algorithm combined with first-principles calculations to search the low-energy crystal structures of ammonia dihydrate (ADH). At high pressure above 11.81 GPa, we predict an unusual ionic phase (tetragonal, $I{4}_{1}cd)$ consisting of three alternating layers of ${\mathrm{H}}_{2}\mathrm{O},{\mathrm{NH}}_{4}{}^{+}$, and $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}$. The occurrence of ionic phase is attributed to the ${\mathrm{NH}}_{4}{}^{+}$ and $\mathrm{O}{\mathrm{H}}^{\ensuremath{-}}$ electrostatic interaction induced volume reduction, which lowers the energy barrier of molecular to ionic phase transition. Analysis of proton transfer under pressure further supports the transformation mechanism between molecular and ionic phase. According to the mobility of hydrogen atoms from ab initio molecular dynamics, this ionic crystal will transform into a superionic phase under high temperature and high pressure. The existence of ionic or superionic ADH may have important implications for understanding the interiors of Neptune, Uranus, and many extrasolar planets.