As water freezes into ice, its molecules rearrange themselves. Freeze it at higher pressure and the molecular rearrangement can be different, enabling ice to form a number of different crystal structures. Ordinary ice has a honeycomb-like structure, but take this ice to a lower temperature or higher pressure and significant changes start to occur.
“At low temperatures the system tries to increase its degree of order,” explains John Finney, a physicist at University College London, who has, together with Christoph Salzmann from Oxford University and Paolo Radaelli from ISIS, been using ISIS neutrons to study ice structures.
In order to increase order, the water molecules need to take on different orientations with respect to each other. By doping their ices with a small amount of hydrochloric acid, the scientists can free-up the molecules so they can re-order. This kick-starts the rearrangement process, which is usually prevented by the temperature being too low: these new ice phases form at around -160°C.
The phase diagram of ice, including
liquidus lines of metastable ices IV and XII
(long dashed lines) and extrapolated equilibrium
lines at low temperatures (short dashed lines).
Using this technique, the team has discovered three new phases of ice, bringing the total number of known ice forms up to sixteen so far. “None of these phases are likely to occur naturally on Earth, but they might be found on some of the icy moons of the outer planets,” says Salzmann.
A major advantage of using ISIS to study the ice was that the scientists could monitor how the structure changed over time. “The pulsed neutron source enabled us to take ‘snapshots’ of the structure and observe the phase transitions as they occurred,” says Finney. The penetrating power of neutrons makes them ideal for experiments requiring thick-walled containers to study samples under pressure. The ability of neutrons to see light elements enables hydrogen bonding in substances such as ice to be studied.
Understanding the moons of outer planets is not the only application of ice research. “If we can understand the different forms and how they transform from one to another then it will help us to understand the water molecule better, which is essential if we are to understand the role of water in biological and chemical systems,” explains Finney.
The crystal structures of two new ice phases, ice XIII and ice XIV. Oxygen atoms are red, hydrogens are white. Each water molecule is bonded in a tetrahedral arrangement to four others. Ice XIII was formed at pressures of around 5000 atmospheres, ice XIV at around 12000 atmospheres.
Research authors: Prof John Finney (University College London), Christop Salzman (Oxford), Paolo Radaelli (ISIS)
Further information: The preparation and structures of hydrogen ordered phases of ice, CG Salzmann et al., Science 311 (2006) 1758.
Notes for Editors:
1. About the London Centre for Nanotechnology
The London Centre for Nanotechnology is an interdisciplinary joint enterprise between University College London and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Research programmes are aligned to three key areas, namely Planet Care, Healthcare and Information Technology and bridge together biomedical, physical and engineering sciences. Website: www.london-nano.com
2. About ISIS
ISIS is a world-leading centre for research in physical and life sciences operated by the Science and Technology Facilities Council at the Rutherford Appleton Laboratory, Oxfordshire, UK.
ISISsupports an international community of around 1600 scientists who use neutrons and muons for research in physics, chemistry, materials science, geology, engineering and biology. It is the most productive pulsed neutron spallation source in the world.