New Phases of Water Ice Predicted at Megabar Pressures

Based on density functional calculations we predict water ice to attain two new crystal structures with $Pbca$ and $Cmcm$ symmetry at 7.6 and 15.5 Mbar, respectively. The known high-pressure ice phases VII, VIII, X, and $Pbcm$ as well as the $Pbca$ phase are all insulating and composed of two interpenetrating hydrogen bonded networks, but the $Cmcm$ structure is metallic and consists of corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral positions between next-nearest O atoms while they occupy tetrahedral positions between nearest O atoms in the ice X, $Pbcm$, and $Pbca$ phases.

Figure 1

$Cmcm$ ice structures where the large and small spheres denote the O and H atoms, respectively.

Figure 2

Structures of the ice X, $Pbcm$, and new $Pbca$ and $Cmcm$ phases are shown from top to bottom. The large and small spheres denote the O and H atoms, respectively, while the thin lines denote the unit cells. The ice X to $Pbcm$ transition is a displacement of atomic layers. In $Pbca$, the H atoms are squeezed out of midpoint between nearest O atoms. In $Cmcm$, the H atoms occupy midpoints between next-nearest O atoms.

Figure 3

Dispersion curve of the $Pbcm$ phonon band with the lowest energy along the $a$ direction, exhibiting an instability at 7.6 Mbar.

Figure 4

Left: Electronic band structure for the $Cmcm$ phase in a monoclinic unit cell at 22 Mbar. The arrows indicate where bands cross the Fermi level. Right: $Pnma$ and $Cmcm$ band structures at 12.5 Mbar in the orthorhombic unit cell illustrate the Peierls instability.

Figure 5

Enthalpy differences of the ice X, $Pbca$, $Pnma$, and $Cmcm$ structures from the $Pbcm$ structure. PBE [24] was used except where LDA is indicated. Results are without zero point motion.