Experimental and theoretical evidence for an ionic crystal of ammonia at high pressure

Abstract

We report experimental and theoretical evidence that solid molecular ammonia becomes unstable at room temperature and high pressures and transforms into an ionic crystalline form. This material has been characterized in both hydrogenated (NH${}_{3}$) and deuterated (ND${}_{3}$) ammonia samples up to about 180 and 200 GPa, respectively, by infrared absorption, Raman spectroscopy, and x-ray diffraction. The presence of a new strong infrared absorption band centered at 2500 cm${}^{\ensuremath{-}1}$ in NH${}_{3}$ (1900 cm${}^{\ensuremath{-}1}$ in ND${}_{3}$) is in line with previous theoretical predictions regarding the ionization of ammonia molecules into ${{\mathrm{NH}}_{2}}^{\ensuremath{-}}$ and ${{\mathrm{NH}}_{4}}^{+}$ ions. The experimental data suggest the coexistence of two crystalline ionic forms, which our ab initio structure searches predict to be the most stable at the relevant pressures. The ionic crystalline form of ammonia appears stable at low temperatures, which contrasts with the behavior of water in which no equivalent crystalline ionic phase has been found.