New aluminium hydroxide at multimegabar pressures: Implications for water reservoirs in deep planetary interiors

Hydrogen is the most abundant element in the universe and has played an important role in the structure, dynamics, and evolution of the planets. In the Earth, hydrogen is transported into deep mantle regions in the form of hydrous minerals via the subduction of oceanic plates. However, in contrast to the extensive studies on the behaviours of hydrous phases in the deep interior of the Earth, little is known about those in larger planets, such as ice giants and super-Earths. Based on in-situ X-ray diffraction experiments in a laser-heated diamond anvil cell as well as first principles calculations, we found that delta-AlOOH transforms into a new phase with orthorhombic symmetry, named epsilon-AlOOH, at similar to 190 GPa. We also found that FeOOH and AlOOH form solid solutions with a pyrite-type structure at similar to 150 GPa. These findings clarified that some hydrous phases remain stable beyond pressures found in the Earth's core-mantle boundary. Hydrogen may thus be stored in these hydroxides or their potential high-pressure forms in the deep interiors of terrestrial super-Earths, and the rocky cores of some ice giants.