Superionic to Superionic Phase Change in Water: Consequences for the Interiors of Uranus and Neptune

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 $1\pm 0.5\mathrm{Mbar}$, with potential consequences for the interiors of ice giants such as Uranus and Neptune.

Figure 1

Gibbs free energy difference per molecule between the bcc and fcc phases (solid line) shown as a function of pressure for a temperature of 3000 K and as a function of temperature for a pressure of 40 Mbar. Also shown are the internal energy $\Delta U$ and pressure-volume $P\Delta V$ components of the free energy difference.

Figure 2

Phase diagram of water showing the superionic regime. The phase boundary from solid ice to superionic water indicated by a shading gradient indicating the degree of uncertainty due to the error bars. The melting line of bcc superionic ice is from Redmer et al. [7], as are the indicated isentropes of Uranus and Neptune. The superionic regime is shaded to show the transition from bcc to fcc stability at pressures of $1\pm 0.5\mathrm{Mbar}$. The fcc-superionic to fluid (red) and solid-to-superionic (green) lines were obtained by single-phase simulations in which the system was heated and cooled; we estimate a slightly higher melting temperature for the fcc lattice than was found by Redmer et al. for the bcc lattice.

Figure 3

Isosurfaces of constant hydrogen density in molecular dynamics simulations of superionic ice in the bcc (upper) and fcc (lower) phases at 40 Mbar and 5000 K. Two different maxima representing the tetrahedral and octohedral sites are seen in the bcc lattice; in the fcc lattice, by contrast, the hydrogens are concentrated at the tetrahedral interstitial sites.