H4O2+ ion stabilized by pressure

Aquodiium (${\mathrm{H}}_{4}{\mathrm{O}}^{2+}$), an isoelectronic analog of the ammonium ion (${\mathrm{NH}}_{4}^{+}$), can theoretically be formed by combining a molecule of water (${\mathrm{H}}_{2}\mathrm{O}$) with two protons. However, stable aquodiium has never been reported because of the high energy cost during the second protonation after hydronium (${\mathrm{H}}_{3}{\mathrm{O}}^{+}$). Here, by performing ab initio evolutionary structure searches combined with first-principles calculations, stable ionic phases, ${\mathrm{H}}_{4}\mathrm{O}{\mathrm{F}}_{2}$ and ${\mathrm{H}}_{4}\mathrm{O}{\mathrm{F}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{HF}$, were predicted to be thermodynamically stable at high pressure. Analysis of bond lengths and electron density supports the formation of aquodiium under pressure in these two phases. Moreover, ab initio molecular dynamics simulations reveal that these ionic phases will enter the superionic states at lower temperatures compared to water ice. For ${\mathrm{H}}_{4}\mathrm{O}{\mathrm{F}}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{HF}$, there is a plastic phase region where aquodiium ions exhibit free rotation. All aquodiium ions are fully preserved below 1000 K in these ionic phases, while after entering the diffusion state, only the ${\mathrm{H}}_{4}\mathrm{O}{\mathrm{F}}_{2}$ phase keeps ${\mathrm{H}}_{4}{\mathrm{O}}^{2+}$ ions. Our results suggest that pressure stabilizes the ${\mathrm{H}}_{4}{\mathrm{O}}^{2+}$ ion, presenting an important addition to traditional physical and chemical theories such as the valence shell electron pair repulsion model, proton transfer, and acid-base theory.