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Using density functional molecular dynamics simulations, we study thebehavior of different hydrogen-oxygen compounds at megabar pressures andseveral thousands of degrees Kelvin where water has been predicted to occur insuperionic form. When we study the close packed hcp and dhcp structures ofsuperionic water, we find that they have comparable Gibbs free energies to thefcc structure that we predicted previously [Phys. Rev. Lett., 110 (2013)151102]. Then we present a comprehensive comparison of different superionicwater candidate structures with P2_1, P2_1/c, P3_121, Pcca, C2/m, and Pa3symmetry that are based on published ground-state structures. We find that theP2_1 and P2_1/c structures transform into a different superionic structure withP2_1/c symmetry, which at 4000 K has a lower Gibbs free energy than fcc forpressures higher than 22.8 +- 0.5 Mbar. This novel structure may also beobtained by distorting a hcp supercell. Finally we show that H2O2 and H9O4structures will also assume a superionic state at elevated temperatures. Basedon Gibbs free energy calculations at 5000 K, we predict that superionic waterdecompose into H2O2 and H9O4 at 68.7 +- 0.5 Mbar.

Ab initio Simulations of Superionic H2O, H2O2, and H9O4 Compounds