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

TL;DR

This study reveals a new superionic ice phase with different properties than previously thought, impacting models of Uranus and Neptune's interiors by identifying a phase transition at high pressure.

Contribution

We identify a thermodynamically stable superionic ice phase with an fcc lattice, challenging the previous belief that only the bcc phase exists.

Findings

Discovery of a thermodynamically stable superionic ice phase with fcc lattice.

Prediction of a phase transition at approximately 1 Mbar pressure.

Implications for the internal structure and evolution of ice giant planets.

Abstract

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.