Superionic to superionic phase change in water: consequences for the interiors of Uranus and Neptune

摘要

Using density functional molecular dynamics free energy calculations, we showthat the body-centered-cubic phase of superionic ice previously believed to bethe only phase is in fact thermodynamically unstable compared to a novel phasewith oxygen positions in fcc lattice sites. The novel phase has a lower protonmobility than the bc phase and may exhibit a higher melting temperature. Wepredict 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 andNeptune.