Thomas Lee Elifritz

http://arxiv.org/abs/1209.3448

H₄O and other hydrogen-oxygen compounds at giant-planet core pressures, Shuai Zhang, Hugh Wilson, Kevin Driver and Burkhard Militzer

Water and hydrogen at high pressure make up a substantial fraction of the interiors of giant planets. Using ab initio random structure search methods we investigate the ground-state crystal structures of water, hydrogen, and hydrogen-oxygen compounds. We find that, at pressures beyond 14 Mbar, excess hydrogen is incorporated into the ice phase to form a novel structure with H₄O stoichiometry. We also predict two new ground state structures, P2₁/m and I₄/mmm, for post-C2/m water ice.

As one of the most abundant substances in the solar system, water ice at high pressure is of fundamental importance in planetary science. Over the last two years, a significant effort has been devoted to finding the ground-state structures of ice at the multi-megabar pressures corresponding to the cores of gas giant planets (15-20 Mbar for Saturn and 40-60 Mbar for Jupiter).

In nature, however, high pressure water phases are rarely found in isolation, and, in gas giants, an icy core may be surrounded by a vast reservoir of hydrogen-rich fluid or exist in a mixture with the other planetary ices, such as ammonia and methane. Recent works have emphasized the fact that counter intuitive stoichiometries can occur in post perovskite materials at extreme pressures. This raises the question of whether H₂O is indeed the ground-state stoichiometry for water at high pressure in hydrogen rich environments, or whether pressure effects are likely to result in the incorporation of hydrogen into the ice lattice to form novel ice-like phases with non-H₂O stoichiometry.

In this Letter, we apply ab initio random structure searching (AIRSS) methods to determine the ground state structure of hydrogen-oxygen compounds at extreme pressure to explore whether the H₂O stoichiometry of water is maintained at high pressure. The AIRSS relies on the generation of a large number of random geometries whose structures are then optimized using density functional theory. In the AIRSS process, randomly generated unit cells are filled with randomly positioned atoms, and the structures are geometrically relaxed to the targeted pressure. Structures with competitive enthalpy are picked out and reevaluated with more accurate thermodynamic calculations, from which the most stable structure can be determined. Although the AIRSS is not guaranteed to find the most stable phase, it has achieved remarkable success in discovering structures across a wide range of materials.

In this work we have replicated and extended the phase diagram of solid hydrogen to 180 Mbar and that of water ice to 500 Mbar. We also predict novel, hydrogen enriched meta-ice structures to become more stable than hydrogen-water mixtures at high pressure. Our results imply that water ice under conditions in excess of 14 Mbar at low temperature will absorb excess hydrogen from its environment to form a hydrogen-rich H₄O phase. 14 Mbar pressure in question is comparable to the CMB of Saturn and far below the core pressure of Jupiter, and may potentially also be reached inside super-Neptune ice giants. This result stands in contrast to the tendency of methane to preferentially expel hydrogen from its own structure under pressure to form hydrocarbons and eventually diamond, and in ice mixtures methane could potentially provide the excess hydrogen to incorporate into H₄O. These results underline the fact that chemistry at high pressure may deliver substantially counter intuitive results, and that consideration of structures likely to form at giant planet core conditions requires looking beyond traditional ambient pressure chemistry to explore unfamiliar stoichiometries and combinations of elements.

Some of these hydrogen enriched water ice phases appear to be metallic as well. Cool. Or hot!

Or not.

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