Melting conditions and entropies of superionic water ice: Free-energy calculations based on hybrid solid/liquid reference systems

Superionic (SI) water ices-high-temperature, high-pressure phases of water in which oxygen ions occupy a regular crystal lattice whereas the protons flow in a liquid-like manner-have attracted a growing amount of attention over the past few years, in particular due to their possible role in the magnetic anomalies of the ice giants Neptune and Uranus. In this paper, we consider the calculation of the free energies of such phases, exploring hybrid reference systems consisting of a combination of an Einstein solid for the oxygen ions occupying a crystal lattice and a Uhlenbeck-Ford potential for the protonic fluid that avoids irregularities associated with possible particle overlaps. Applying this approach to a recent neural-network potential-energy landscape for SI water ice, we compute Gibbs free energies as a function of temperature for the SI fcc and liquid phases to determine the melting temperature T-m at 340 GPa. The results are consistent with previous estimates and indicate that the entropy difference between both phases is comparatively small, in particular due to the large amplitude of vibration of the oxygen ions in the fcc phase at the melting temperature.Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0138987

Automated summary

Superionic water ices, where the oxygen ions occupy a crystal lattice while the protons flow in a liquid-like manner, have gained increasing attention due to their potential role in the magnetic anomalies of ice giants. In this paper, the free energies of these phases are calculated using a hybrid reference system, and the melting temperature is determined for the face-centered cubic (fcc) and liquid phases. The results show that the entropy difference between the two phases is relatively small, primarily due to the large amplitude of vibration of the oxygen ions in the fcc phase at the melting temperature.