A fragment of the crystal structure of the new ice is shown where the oxygen atoms are blue and the molecular hydrogen atoms pink. Hydrogen atoms that have been pulled off the water molecules are colored gold. These appear to locate in polyhedral voids in the oxygen lattice (one of which is shaded light grey). Previously, these voids were believed to remain even after the water molecule breaks up at enormous pressures. (Credit: ORNL)
A collaboration between Oak Ridge National Laboratory researchers and a team led by the Carnegie Institution for Science’s Malcolm Guthrie has led to discoveries about how ice behaves under pressure, changing ideas that date back almost 50 years. The findings could alter scientists’ understanding of how the water molecule responds to conditions found deep within planets and could have implications for energy science. The team’s work is published in the Proceedings of the National Academy of Sciences.
When water freezes into ice, its molecules are bound together in a crystalline lattice held together by hydrogen bonds. Hydrogen bonds are highly versatile and, as a result, crystalline ice reveals a striking diversity of at least 16 different structures.
By designing a new class of tools optimized to exploit the unrivalled flux of neutrons at ORNL’s Spallation Neutron Scource, Guthrie and his team — Carnegie’s Russell Hemley, Reinhard Boehler, and Kuo Li, as well as Chris Tulk, Jamie Molaison, and António dos Santos of Oak Ridge National Laboratory — have obtained the first glimpse of the hydrogen atoms themselves in ice at unprecedented pressures of over 500,000 times atmospheric pressure.
“The neutrons tell us a story that the other techniques could not,” said Hemley, director of Carnegie’s Geophysical Laboratory. “The results indicate that dissociation of water molecules follows two different mechanisms. Some of the molecules begin to dissociate at much lower pressures and via a different path than was predicted in the classic 1964 paper.”
“Our data paint an altogether new picture of ice,” Guthrie said. “Not only do the results have broad consequences for understanding bonding in H2O, the observations may also support a previously proposed theory that the protons in ice in planetary interiors can be mobile even while the ice remains solid.”
And this discovery may prove to be just the beginning. “Being able to ‘see’ hydrogen with neutrons isn’t just important for studies of ice,” ORNL’s Tulk said. “This is a game-changing technical breakthrough. The applications could extend to systems that are critical to societal challenges, such as energy. For example, the technique can yield greater understanding of methane-containing clathrate hydrates and even hydrogen storage materials that could one day power automobiles.”
Guthrie, M., Boehler, R., Tulk, C., Molaison, J., dos Santos, A., Li, K., & Hemley, R. (2013). Neutron diffraction observations of interstitial protons in dense ice Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1309277110
