  Using density functional molecular dynamics free energy calculations, we show
that the body-centered-cubic 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 fcc lattice sites. The novel phase has a lower proton
mobility than the bc phase and may exhibit a higher melting temperature. We
predict a transition between the two phases at a pressure of 1 +/- 0.5 Mbar,
with potential consequences for the interiors of ice giants such as Uranus and
Neptune.
