Decomposition and Terapascal Phases of Water Ice

Computational searches for stable and metastable structures of water ice and other H:O compositions at TPa pressures have led us to predict that ${\mathrm{H}}_2\mathrm{O}$ decomposes into ${\mathrm{H}}_2{\mathrm{O}}_2$ and a hydrogen-rich phase at pressures of a little over 5 TPa. The hydrogen-rich phase is stable over a wide range of hydrogen contents, and it might play a role in the erosion of the icy component of the cores of gas giants as ${\mathrm{H}}_2\mathrm{O}$ comes into contact with hydrogen. Metallization of ${\mathrm{H}}_2\mathrm{O}$ is predicted at a higher pressure of just over 6 TPa, and therefore ${\mathrm{H}}_2\mathrm{O}$ does not have a thermodynamically stable low-temperature metallic form. We have also found a new and rich mineralogy of complicated water ice phases that are more stable in the pressure range 0.8–2 TPa than any predicted previously.

Figure 1

Variation of the enthalpies with pressure of high-pressure water-ice structures. Previously known structures are indicated by dashed lines, and phases that we have predicted are shown as solid lines. ZP motion is included at the quasiharmonic level.

Figure 2

Phase diagram of water ice including vibrational motion.

Figure 3

Top: $P{3}_{1}21$ structure of water ice at 1 TPa. Bottom: $Pa\overline{3}$ structure of ${\mathrm{H}}_2{\mathrm{O}}_2$ at 6 TPa. The O atoms are shown in red and the H atoms in gray.

Figure 4

(a) Convex hull diagram and enthalpies of $(1/2{\mathrm{H}}_2{\mathrm{O}}_2{)}_{1-x}{\mathrm{H}}_x$ structures. The upper (red) and lower (black) broken lines show the convex hulls at 4 and 6 TPa, respectively. The upper (red) and lower (black) solid lines show the enthalpies of selected structures at 4 and 6 TPa, respectively. At 4 TPa the $P{2}_1/c$ phase [lower dot at $x=0.5$ (red)] is more stable than $C2/m$ [upper dot at $x=0.5$ (red)]. At 6 TPa the $P{2}_1/c$ and $C2/m$ phases are almost degenerate [middle dot at $x=0.5$ (black)]. Six values of $\delta$ are labelled on the enthalpy curves, and the enthalpies of the corresponding structures for $\delta \simeq 1/8$ to $\delta \simeq 5/12$ lie on the convex hull at 4 and 6 TPa. (b) Six ${\mathrm{H}}_{2+\delta }\mathrm{O}$ structures at 6 TPa with values of $\delta$ from 0 to $1/2$, with O atoms shown as large balls (red) and H atoms as small balls (gray). These structures have very similar arrangements of O atoms for $\delta =0$ to $1/4$, but between $\delta =1/4$ and $1/2$ the O packing changes to bcc.