Using density functional molecular dynamics free energy calculations , we show that the body-centered-cubic ( bcc ) phase of superionic ice previously believed to be the only phase is in fact thermodynamically unstable compared to a novel phase with oxygen positions in face centered cubic lattice sites .
The novel phase has a lower proton mobility than the bcc phase and may exhibit a higher melting temperature .
We predict a transition between the two phases at a pressure of 1 \pm 0.5 Mbar , with potential consequences for the interiors of ice giants such as Uranus and Neptune .