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 P 2 _ { 1 } , P 2 _ { 1 } / c , P 3 _ { 1 } 21 , Pcca , C 2 / m , and Pa \bar { 3 } symmetry that are based on published ground-state structures .
We find that the P 2 _ { 1 } and P 2 _ { 1 } / c structures transform into a different superionic structure with P 2 _ { 1 } / c symmetry , which at 4000 K has a lower Gibbs free energy than fcc for pressures higher than 22.8 \pm 0.5 Mbar .
This novel structure may also be obtained by distorting a hcp supercell .
Finally we show that H _ { 2 } O _ { 2 } and H _ { 9 } O _ { 4 } 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 H _ { 2 } O _ { 2 } and H _ { 9 } O _ { 4 } at 68.7 \pm 0.5 Mbar .