Ice, ice, maybe: water physics thrown into doubt
An attempt to produce a disordered form of ice is a surprise failure, raising awkward questions. Phil Dooley reports.
Scientists may have smashed a theory about ice – throwing doubt on the idea that under the right conditions it can morph into a strange disordered solid form similar to glass.
The ice used to chill drinks is a crystal in which molecules bond together in hexagonal patterns. Scientists expected that when cooled and compressed to unearthly temperatures and pressures, the water molecules would lose their regular structure and lapse into disorder, a structure known as an amorphous solid. Glass and plastics are examples of amorphous solids.
“This type of amorphous ice is thought to be related to liquid water and understanding that link was the original purpose of this study,” says Chris Tulk, from Oak Ridge National Laboratory in the US.
Instead, as Tulk and his colleagues report in the journal Nature, their attempt to create it failed, resulting instead in three unusual crystalline variations.
Ice has 17 known different crystal structures, which form in different pressure and temperature conditions. In this experiment, as the team increased the pressure, the ice went from normal ice, classified as Ih, through three successively denser structures: ice IX, XV and finally XIII.
These forms of ice have been observed before, but what confounded the scientists was that the amorphous form, which has also been observed before, never materialised.
The team included Dennis Klug from the National Research Council of Canada, the lab that originally discovered the pressure-induced amorphisation of ice in 1984.
“I've never previously seen this pressure-temperature path result in a series of crystalline forms like this,” Klug said.
In the experiment, a three-millimetre sphere of water was cooled to minus-173 degrees Celsius then subjected to incrementally increasing pressure up to 28,000 times a single Earth atmosphere. At each new pressure, the ice drop was probed with a neutron beam to reveal its structure.
It was only after they analysed the results for the succession of pressures that Tulk and colleagues realised they could not study the transitions of amorphous ice, because it had never formed.
The team initially thought their experiment was contaminated, but three subsequent repetitions produced identical results.
“If the data from our experiment was true, it would mean that amorphous ice is not related to liquid water but is rather an interrupted transformation between two crystalline phases, a major departure from widely accepted theory,” Klug said.
The team propose that the slow rate of pressure increase and collection of data at a lower pressure allowed the ice structure to relax and form ice IX. Previous experiments had quickly passed over the ice IX conditions without allowing relaxation, resulting in the amorphous phase.
The surprise result challenges a 35-year quest in the study of water.
When supercooled, water does not behave like other crystals. Some theories that attempt to explain this anomalous behaviour predicted a sudden jump from low density to high density at extremely low temperature and high pressures temperature, a so-called second critical point.
Scientists have searched in vain among the different crystal forms for this density jump. The new results question its very existence.
“The relationship between pressure-induced amorphous ice and water is now in doubt, and the second critical point may not even exist,” Tulk says.