  Using density functional molecular dynamics simulations, we study the
behavior of different hydrogen-oxygen compounds at megabar pressures and
several thousands of degrees Kelvin where water has been predicted to occur in
superionic form. When we study the close packed hcp and dhcp structures of
superionic water, we find that they have comparable Gibbs free energies to the
fcc structure that we predicted previously [Phys. Rev. Lett., 110 (2013)
151102]. Then we present a comprehensive comparison of different superionic
water candidate structures with P2_1, P2_1/c, P3_121, Pcca, C2/m, and Pa3
symmetry that are based on published ground-state structures. We find that the
P2_1 and P2_1/c structures transform into a different superionic structure with
P2_1/c symmetry, which at 4000 K has a lower Gibbs free energy than fcc for
pressures higher than 22.8 +- 0.5 Mbar. This novel structure may also be
obtained by distorting a hcp supercell. Finally we show that H2O2 and H9O4
structures will also assume a superionic state at elevated temperatures. Based
on Gibbs free energy calculations at 5000 K, we predict that superionic water
decompose into H2O2 and H9O4 at 68.7 +- 0.5 Mbar.
