High pressure ices

Abstract

H(2)O will be more resistant to metallization than previously thought. From computational evolutionary structure searches, we find a sequence of new stable and meta-stable structures for the ground state of ice in the 1-5 TPa (10 to 50 Mbar) regime, in the static approximation. The previously proposed Pbcm structure is superseded by a Pmc2(1) phase at p = 930 GPa, followed by a predicted transition to a P2(1) crystal structure at p = 1.3 TPa. This phase, featuring higher coordination at O and H, is stable over a wide pressure range, reaching 4.8 TPa. We analyze carefully the geometrical changes in the calculated structures, especially the buckling at the H in O-H-O motifs. All structures are insulating--chemistry burns a deep and (with pressure increase) lasting hole in the density of states near the highest occupied electronic levels of what might be component metallic lattices. Metallization of ice in our calculations occurs only near 4.8 TPa, where the metallic C2/m phase becomes most stable. In this regime, zero-point energies much larger than typical enthalpy differences suggest possible melting of the H sublattice, or even the entire crystal.

Fig. 1.

Relative ground state enthalpies of known and new ice crystal phases. Zero-point motion is not included. Upper horizontal axis shows the volume compression of Pmc2₁ phase relative to ice XI (the H-ordered ground-state phase of ice) at P = 1 atm (25).

Fig. 2.

From left to right: crystal structures of ice X (), Pbcm, and Pmc2₁, seen along the a axis. Unit cells are indicated ( cell for ice X). All structures are at p = 700 GPa (and ground-states).

Fig. 3.

Various views of the P2₁ structure. Left: along c axis; middle: two sublattices (see text), along a axis; right: superposition of sublattices, view close to c axis.

Fig. 4.

Various static lattice views of the structure. Left: along the c axis; middle: the two sublattices, along the a axis; right: one of the two interpenetrating networks.

Fig. 5.

Ground state enthalpies of new ice structures up to p = 5 TPa, relative to the P2₁ structure. Upper x axis gives the volume compression of P2₁ structure relative to ice XI at 1 atm.

Fig. 6.

Static crystal structure of C2/m phase, at p = 2 TPa. Different levels of brightness of atoms indicate separate sublattices.

Fig. 7.

Top: DFT band gaps for various static ice structures, up to p = 5 TPa; the gray band indicates the global minimum (GM) band gap curve. Bottom: electronic DOS (in states per electron per eV) for metallic C2/m phase and its O^2- sublattice, at p = 4.8 TPa. The dashed line in the DOS plots indicates a free-electron DOS per electron with a bandwidth of 60 eV (left) and 59 eV (right).

Fig. 8.

Vibrational frequencies for H atom in the center-position of the O-H-O bridging bond (see text).