Pressure-stabilized structures of water-neon system under high pressure

Neon (Ne), as the fifth most abundant element in the universe, is rare to react with other elements by forming stable solid compounds. It is well known that pressure is a powerful tool to generate the compounds that are inaccessible at ambient pressure. In this work, we performed structure-searching simulations to examine stable compounds of Ne and ${\mathrm{H}}_2\mathrm{O}$ at a wide pressure range of 0–600 GPa. Our simulations identified two phases of ${\mathrm{H}}_2\mathrm{ONe}$ and ${\mathrm{H}}_2{\mathrm{ONe}}_2$ under high pressure. By employing chemical-bonding analysis, interestingly, we found that Ne-O interactions are comparable in strength to that of conventional hydrogen bond. Moreover, our molecular dynamic simulations indicate the diffusion behavior of hydrogen atoms within a fixed Ne-O lattice framework of ${\mathrm{H}}_2{\mathrm{ONe}}_2$ at high pressure and high temperature. These results provide the implications for the possible existence of pressure-stabilized ${\mathrm{H}}_2\mathrm{ONe}$ and ${\mathrm{H}}_2{\mathrm{ONe}}_2$ compounds viable in a variety of astronomical objects.

Figure 1

Thermodynamics of the $\mathrm{Ne}\text{−}{\mathrm{H}}_2\mathrm{O}$ system of the stable compounds. (a) Convex hull for formation enthalpies ($\mathrm{\Delta }H$ with respect to Ne and ${\mathrm{H}}_2\mathrm{O}$) at different pressures. (b) Pressure-composition phase diagram of the $\mathrm{Ne}\text{−}{\mathrm{H}}_2\mathrm{O}$ phases discovered in this work within zero-point energy, together with the previously known phases (green) in the pressure range of 0–50 and 100–600 GPa. (c) Ternary phase diagram of H-O-Ne system at 5 GPa. (d) Ternary phase diagram of H-O-Ne system at 500 GPa.

Figure 2

Crystal structures of the stable compounds. (a) Crystal structures of $I{4}_{1}md\text{−}{\mathrm{H}}_2\mathrm{ONe}$. (b) Crystal structures of $Fd$-3m-${\mathrm{H}}_2{\mathrm{ONe}}_2$. The detailed structural information are shown in Table 3.

Figure 3

Electronic properties of $Fd\text{−}3m$ of ${\mathrm{H}}_2{\mathrm{ONe}}_2$. (a) Electronic band structure of $Fd\text{−}3m\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ at 400 GPa. (b) ELF of $Fd\text{−}3m\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ at 400 GPa.

Figure 4

Pressure-temperature phase diagram of ${\mathrm{H}}_2\mathrm{ONe}$ and ${\mathrm{H}}_2{\mathrm{ONe}}_2$. (a) Stability fields separating ${\mathrm{H}}_2\mathrm{O}+\mathrm{Ne}$ of $I{4}_{1}md\text{−}{\mathrm{H}}_2\mathrm{ONe}$ compounds. (b) Stability fields separating ${\mathrm{H}}_2\mathrm{O}+\mathrm{Ne}$ of $Fm\text{−}3d\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ compound. Solid (white), superionic (green), liquid (blue) phases, and unstable (pink) of ${\mathrm{H}}_2\mathrm{ONe}$ and ${\mathrm{H}}_2{\mathrm{ONe}}_2$ compounds, obtained from AIMD simulations. Temperature and pressure curve inside Earth [41]; Uranus and Neptune [42] are shown as orange, red, and blue lines.

Figure 5

Behavior of H and Ne atoms compared to O atoms in Fd-$3m {\mathrm{H}}_2{\mathrm{ONe}}_2$ from AIMD simulations at 3000 and 4000 K. (a) Averaged MSDs for the H, O, and Ne atoms of Fd-$3m\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ from AIMD simulations at 3000 K and 53 GPa. (b) Averaged MSDs for the H, O, and Ne atoms of Fd-$3m\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ from AIMD simulations at 4000 K and 55 GPa. (c) Atomic trajectories of the Fd-$3m\text{−}{\mathrm{H}}_2{\mathrm{ONe}}_2$ solid phase (3000 K, 53 GPa). (d) Superionic H phase (4000 K, 55 GPa).

Figure 6

Calculated phonon dispersion and density of states (DOS) curves of ${\mathrm{H}}_2{\text{ONe}}_2$ at 300 GPa, 400 GPa and 600 GPa.

Figure 7

The equation of state. (a) The equation of state of ${\mathrm{H}}_2\text{ONe}\text{−}I{4}_1\mathit{md}, {\mathrm{H}}_2\mathrm{O}\text{−}I{4}_1$/amd and Ne-$\mathit{Fm}\text{−}3m$. (b) The equation of state of ${\mathrm{H}}_2{\text{ONe}}_2$-Fd-$3m, {\mathrm{H}}_2\mathrm{O}$-Pbcm and Ne-$\mathit{Fm}\text{−}3m$.