How new forms of ice can help us to better understand climate change

March 2020

Scientists have created pure samples of a rare form of ice on Earth – cubic ice. Unlike the ice we put in our summer drinks, ice with a cubic structure can only be formed under special conditions. Understanding its properties will bring us closer to answering some of the fundamental questions of ice physics, helping scientists to better model the role ice plays in global challenges, including climate change and global warming. 

H20, the key molecule of life, is closely studied across the world. From the water we drink to hydrate our bodies, to those trapped in glaciers to maintain global temperatures, water is prevalent in every form of our daily lives. For example, the melting of Arctic ice due to climate change has affected weather patterns all around the globe. Yet, there are many mysteries we are still to uncover.

One of these mysteries is ice. 

Ice is made up of water molecules closely packed in three dimensions to form a crystal, and it has long been thought that only ice with a “hexagonal” structural form naturally occurs on Earth. A rarer, “cubic” structure can only be obtained under the special conditions of extremely low temperatures, like those in upper layers of the atmosphere. Recent studies have also revealed that samples of cubic ice are in fact a hybrid, made up of stacked layers of both cubic and hexagonal structure, and pure cubic ice has, in fact, never been found.

In a new study published in Nature Materials, an international group of researchers was able to synthesise a substantial quantity of highly pure cubic ice by decomposing powdered ice XVII – a synthetic form of ice that takes up a cage-like structure called a clathrate. In the process of decomposition, ice XVII is maintained with liquid nitrogen at under -143°C and then warmed up slowly. This results in a nearly perfect cubic structure, with over 95% of the sample being composed of cubically stacked layers.

These findings not only contribute to a better understanding of ice structure and the existence of the two natural ice forms, but also addresses wider questions about the Earth.

“Obtaining purer and purer forms of cubic ice will eventually allow us to derive its specific heat – which is a measure of how much energy is needed to heat an object up. If conditions allow, water vapour in the atmosphere may condensate to cubic ice, and to understand the processes occurring at higher layers of the atmosphere, it is important to know the heat capacities of different particles that may be present there, such as cubic and hexagonal ice. With this understanding, scientists can quantify more precisely the greenhouse effect, which contributes mainly to global warming” said ILL instrument scientist, Thomas Hansen.

To confirm the purity of the ice, the team used a neutron diffractometer to examine the structural evolution of cubic ice as it is formed through the decomposition of ice XVII. The researchers also observed how cubic ice further decomposes to hexagonal ice as it warms up to higher temperatures. Using neutrons allows researchers to see both the positions of hydrogen and of oxygen atoms in the sample and map out its structure by examining the diffraction pattern.

There is more to learn in the field of ice physics. Researchers are keen to explore how these techniques and results can help us to examine other unique forms of ice, in order to help us understand more about how our climate is changing, the planet we live in, as well as the conditions on other celestial bodies.

Re.:  Cubic ice Ic without stacking defects obtained from ice XVII. del Rosso et al. (2020)
doi: 10.1038/s41563-020-0606-y

Contact: Dr Thomas HANSEN, ILL

Instrument :  ILL’s diffractometer D20 was used to look at the structural evolution of sample during decomposition.