Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune

Silica, water, and hydrogen are known to be the major components of celestial bodies, and have significant influence on the formation and evolution of giant planets, such as Uranus and Neptune. Thus, it is of fundamental importance to investigate their states and possible reactions under the planetary conditions. Here, using advanced crystal structure searches and first-principles calculations in the $\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}$ system, we find that a silica-water compound ${({\mathrm{SiO}}_2)}_2({\mathrm{H}}_2\mathrm{O})$ and a silica-hydrogen compound ${\mathrm{SiO}}_2{\mathrm{H}}_2$ can exist under high pressures above 450 and 650 GPa, respectively. Further simulations reveal that, at high pressure and high temperature conditions corresponding to the interiors of Uranus and Neptune, these compounds exhibit superionic behavior, in which protons diffuse freely like liquid while the silicon and oxygen framework is fixed as solid. Therefore, these superionic silica-water and silica-hydrogen compounds could be regarded as important components of the deep mantle or core of giants, which also provides an alternative origin for their anomalous magnetic fields. These unexpected physical and chemical properties of the most common natural materials at high pressure offer key clues to understand some abstruse issues including demixing and erosion of the core in giant planets, and shed light on building reliable models for solar giants and exoplanets.

Figure 1

The energetics and crystal structures of the predicted $\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}$ compounds at high pressures. (a) $\mathrm{Si}\text{−}\mathrm{O}\text{−}\mathrm{H}$ ternary phase diagram at 700 GPa. The known and newly found phases are marked by green and red, respectively. Solid and open circles represent the stable and metastable phases, respectively. The crystal structure of (b) the silica-water compound $\mathrm{C}2/\mathrm{c}$ ${\mathrm{Si}}_2{\mathrm{O}}_5{\mathrm{H}}_2$ [corresponding to ${({\mathrm{SiO}}_2)}_2({\mathrm{H}}_2\mathrm{O})$], (b) the silica-hydrogen compound $P{2}_1/m$ ${\mathrm{SiO}}_2{\mathrm{H}}_2$, and (d) the $P{2}_1/m$ ${\mathrm{SiOH}}_2$. The convex hulls for (e) the ${\mathrm{SiO}}_2\text{−}{\mathrm{H}}_2\mathrm{O}$ system and (f) the ${\mathrm{SiO}}_2\text{−}\mathrm{H}$ system with and without zero-point energy.

Figure 2

Electronic properties of the silica-water compound. (a) ELF in a plane of ${\mathrm{Si}}_2{\mathrm{O}}_5{\mathrm{H}}_2$ at 600 GPa. (b) The $p\mathrm{COHP}$ analysis for $\mathrm{O}─\mathrm{H}$ interactions in ${\mathrm{Si}}_2{\mathrm{O}}_5{\mathrm{H}}_2$ at 600 GPa.

Figure 3

Dynamical properties of silica-water and silica-hydrogen compounds at high pressure and high temperature from ab initio molecular dynamics simulations. MSD of hydrogen, oxygen, and silicon in ${\mathrm{Si}}_2{\mathrm{O}}_5{\mathrm{H}}_2$ (a) at 485 GPa and 6000 K (corresponding to the isentropic condition of Uranus) and ${\mathrm{SiO}}_2{\mathrm{H}}_2$ (b) at 794 GPa and 5000 K (corresponding to the isentropic condition of Neptune). Trajectory snapshots of the corresponding superionic phase are shown in (c) and (d), respectively, where the blue and red balls represent equilibrium sites of silicon and oxygen atoms, respectively. The cyan points represent instantaneous positions of hydrogen atoms.

Figure 4

Phase diagrams of silica-water and silica-hydrogen compounds and their impact on the interior structure of Uranus and Neptune. Phase diagrams of ${\mathrm{Si}}_2{\mathrm{O}}_5{\mathrm{H}}_2$ (a) and ${\mathrm{SiO}}_2{\mathrm{H}}_2$ (b). Each solid symbol represents an AIMD simulation. The black dashed lines are phase boundaries. The blue and magenta lines represent interior isentropic profiles of Uranus and Neptune, respectively. For each planet, two different models from Ref. [73] marked by solid and dashed-dotted lines are presented. The open triangles are the states of core-mantle boundaries for each model. (c) Sketches of the internal structures of Uranus and Neptune based on the U2 and N1 models from Ref. [73]. The gray and cyan regions represent the $\mathrm{H}/\mathrm{He}$ atmosphere and the “hot ice” layer composed by water, ammonia, and methane, respectively, the yellow regions represent rocky cores. Both models with compact (right) and diluted/fuzzy (left) cores are shown. The cyan-yellow gradual change region beneath the white dashed curves represents the possible region for the silica-water compound predicted in this work.