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.
@article{arxiv.1211.6482,
title = {Superionic to superionic phase change in water: consequences for the interiors of Uranus and Neptune},
author = {Hugh F. Wilson and Michael L. Wong and Burkhard Militzer},
journal= {arXiv preprint arXiv:1211.6482},
year = {2015}
}