Ordinary water, compressed to enormous pressure, has just taken on a form of ice that no one had previously observed in the laboratory.
The ice in our freezers is only one of the many ways water molecules can organize themselves. When pressure increases sharply, their atoms change arrangement and new solid forms appear. Researchers have just observed a new one, called ice XXII. Its formation begins at around 308 gigapascals, more than 3 million times the atmospheric pressure at sea level.
Principle of a high-pressure experiment in a diamond anvil cell, with a laser beam and X-rays passing through the sample.
Public domain, via Wikimedia Commons.
To reach such a value, the team placed a tiny sample between two diamond tips. This device concentrates force on an extremely small surface. The researchers then tracked the arrangement of the atoms using highly intense X-rays produced by a synchrotron. Another technique, based on laser light, allowed them to detect changes in the material.
Before reaching this pressure, the water had long since stopped resembling our familiar ice. Above approximately 70 gigapascals, it adopts a form called ice X. The oxygen atoms follow a highly regular arrangement there. As the pressure increased further, the researchers found that this arrangement gradually began to deform.
Then, at around 308 gigapascals, the measurements changed markedly. The signals produced by the X-rays indicated a new arrangement of the atoms. Ice X was then replaced by ice XXII. The experiments indicate that this new form remains stable at least up to around 340 gigapascals.
Proposed phase diagram for water up to 500 GPa.
This transformation had been expected for nearly thirty years. Calculations had predicted that at very high pressure, the highly symmetrical arrangement of ice X would eventually give way to a less symmetrical structure. The measurements match this prediction. According to the authors, ice XXII is also the densest solid form of water observed experimentally to date.
These pressures seem excessive on Earth, but they may exist inside large planets. Water there is not only subjected to high pressure: temperatures can also reach several thousand degrees. Understanding the possible forms of water therefore helps build more realistic models of the interiors of giant planets and some water-rich worlds.
Ice XXII should not, however, be regarded as simply a very compressed version of an ice cube. At these pressures, the bonds between hydrogen and oxygen have changed profoundly. The arrangement of the atoms determines, among other things, the material’s density and behavior. The experiments thus provide a concrete reference point for testing the calculations used when conditions become impossible to reproduce directly with large quantities of matter.
The next frontier lies much farther away. Calculations predict another modification of ice at around 760 gigapascals, but current static-compression techniques do not yet make it possible to study it adequately. The researchers also want to connect these experiments with those conducted through shock compression, in which water heated to more than 2,000 °C can adopt other states under comparable pressures.