Inside Ice Under High Pressure
New insights from neutron diffraction reveal changes to atomic structure.
The Science
Researchers used neutron scattering to measure the location of hydrogen atoms in dense ice at high pressure. They found that a substantial fraction of hydrogen dissociates from the oxygen atoms, indicating a radically new kind of bonding and disruption of the familiar water molecule.
The Impact
The discovery of this new and completely unpredicted phenomenon will have a broad impact on our understanding of how a very commonplace substance, ice, behaves under pressure. In addition these results have potential implications for our understanding and modeling of ice-rich planets, such as Uranus and Neptune.
Summary
Although pressure has long been expected to dissociate hydrogen from the water molecule, all present theoretical models for water predict that this occurs smoothly, with the hydrogen atoms lying close to their sites in the original molecules. However, researchers using neutron-diffraction measurements have revealed an unexpected and more dramatic mechanism for pressure-induced dissociation of so-called heavy water (D2O), and by inference regular water (H2O). What the researchers working at the Spallation Neutron Source (SNS) discovered is that approximately 25% of the protons were dissociated from the molecule, and moved to holes (octahedral voids) in the oxygen lattice. The existence of these displaced protons in this high pressure form of ice (called ice VII) has important implications for the structures of other phases of ice that occur at even higher pressures. In particular, a phenomenon called “superionicity” could occur in ice under more extreme conditions and has been proposed as a source of magnetic field generation in Neptune and Uranus. The key to unraveling the unexpected ice structure lay in the high sensitivity of neutron scattering to hydrogen (as compared with X-rays used previously) combined with the high intensity of the SNS neutron source and the unique capabilities of an instrument called SNAP (Spallation Neutrons and Pressure). This instrument is used to study materials under extreme pressures, and by using a new type of diamond anvil pressure cell, scientists have been able to conduct experiments at pressures above 90 GPa, which is a record for neutron diffraction.
Contact
Malcolm Guthrie
European Spallation Source, Edinburgh, UK
+44 (0) 131 6517220
[email protected]
Funding
Work performed at the Oak Ridge National Laboratory Spallation Neutron Source’s Spallation Neutrons and Pressure (SNAP) instrument was supported by Scientific User Facilities Division, Office of Basic Energy Sciences (BES), Department of Energy (DOE) Office of Science. This work is supported by EFree, an Energy Frontier Research Center funded by BES, DOE.
Publications
R. Boehler, M. Guthrie, J. J. Molaison, A. M. dos Santos, S. Singogeikin, S. Machida, N. Pradhan, and C. A. Tulk. “Large-Volume Diamond Cells for Neutron Diffraction Above 90 GPa.” High Pressure Research 33:3 (2013): 546-554. [DOI: 10.1080/08957959.2013.823197]
M. Guthrie, R. Boehler, C. A. Tulk, J. J. Molaison, A. M. dos Santos, K. Li, and R. J. Hemley. “Neutron diffraction observations of interstitial protons in dense ice.” Proceedings of the National Academy of Sciences of the United States of America 110: 26 (2013): 10552-10556. [DOI: 10.1073/pnas.1309277110]
Related Links
Oak Ridge National Laboratory Press Release
Carnegie Institution for Science Press Release
Highlight Categories
Performer: University , DOE Laboratory , SC User Facilities , BES User Facilities , SNS
