Ice ice baby: the discovery of ice XXII

Ice phase with orthorhombic symmetry revealed under high pressure.

H₂O ice is of fundamental importance to all fields of the physical sciences. Existing in the most extreme environments of the universe, ice is found in comets traveling through the cold vacuum of space, and deep within the hot, dense interiors of giant planets. Despite being a simple molecule, ice possesses a complex pressure-temperature phase diagram, with 21 distinct solid phases reported to date.

At room temperature, the picture is far simpler, with only three known solid ices observed upon compression: ice VI, ice VII and ice X. It was predicted almost 30 years ago that beyond ice X there would be an eventual transformation from a crystal with cubic symmetry to one with orthorhombic symmetry, however it has remained elusive and thought beyond experimental capabilities.

Now a team from the Centre for Science of Extreme Conditions (CSEC), a multi-disciplinary group involving researchers from the School’s Institute for Condensed Matter and Complex Systems (ICMCS), reports on the discovery of this phase in Nature Materials. Named ice XXII, this phase exists only at pressures around 300 GPa (which is 3 million times atmospheric pressure) and was formed by squeezing water between the tips of two diamond anvils. 

Through a combination of x-ray diffraction, spectroscopy and density functional theory calculations, the team demonstrates that the cubic ice X structure possessing symmetric oxygen-hydrogen bonds continuously distort before the bonds buckle, giving rise to ice XII. 

Given the vast pressure stability regime predicted for ice XXII, current experimental limitations most likely render this the last phase of ice to be discovered at room temperature.

Dr Israel Osmond, first author of the study said: 

Given that water barely scatters x-rays at all, this really is pushing high-pressure crystallography to its absolute limits. It’s a real testament to how far diamond anvil cell techniques have come.

Recalling the moment of the discovery, Dr Ross Howie, co-lead of the study, said:

We ran the sample by chance as we didn’t expect to observe anything – how wrong we were! Exciting discoveries like this don’t happen every day and it is great that so many early career researchers and students involved in the study got to be a part of it.

Dr Miriam Pena-Alvarez, co-lead of the study, said:

Most matter in the universe doesn't exist at the atmospheric conditions we are familiar with but exists under high pressure. This is what drives our lab, and this work in particular: recreating those planetary interiors in house, to provide insight on how one of the universe's most abundant molecule behaves under these extreme pressures.

The project also involved contributions from Professor Andreas Hermann, and the work was funded through the ERC Starting Grant of Dr Ross Howie and Future Leaders Fellowship of Dr Miriam Peña-Alvarez.