Not all water ice is the same. Locked inside, the arrangement of molecules varies significantly, based on the pressure and temperature conditions under which it forms.
We knew of 18 of these distinct phases of ice, some occurring naturally, some only seen in laboratory conditions.
Three years ago, a team of researchers tweaked one of the existing ice structures, transforming it into a form they called ice β-XV. Now members of that team have determined its exact crystal structure, answering questions on how it forms, and giving it the designation ice XIX.
This discovery could help us better understand how ice forms and behaves in alien conditions very different to those found on Earth.
The ice you see in the freezer, or falling from the sky as snowflakes or hailstones, is the most common natural ice on Earth. This is called ice I, and its oxygen atoms are arranged in a hexagonal grid. The structure is, however, geometrically frustrated, with the hydrogen atoms much more disordered.
When ice I is cooled in a certain way, the hydrogen atoms can periodically become ordered, in addition to the oxygen atoms. This is how scientists in a lab can create different phases of ice that have much more ordered crystalline molecule lattices than their disordered parent forms.
A team of physical chemists at the University of Innsbruck in Austria have been working for some time with the phase ice VI. This is one of the forms of ice that can be found in nature, but only under very high pressures 10,000 times higher than atmospheric pressure at sea level (around 1 gigapascal), such as those found in Earth’s mantle, or wrapped around the core of Saturn’s moon Titan.
Like ice I, ice VI is relatively disordered. Its hydrogen-ordered form, ice XV, was only discovered about a decade ago. It’s created by cooling the ice to below 130 Kelvin (-143 degrees Celsius, -226 degrees Fahrenheit) at pressures around 1 gigapascal.
A few years ago, by changing this process, the researchers created another phase of ice. They slowed down the cooling, and took it below 103 Kelvin, and increased the pressure to 2 gigapascals. This produced a second arrangement of hydrogen molecules that was distinct from ice XV, which was what they labelled ice β-XV.
Validating that the ice was a separate phase was a separate hurdle, requiring that the normal water be replaced with ‘heavy’ water. Normal hydrogen has no neutrons in the nucleus. Heavy water, on the other hand, is based on deuterium, a form of hydrogen that has one neutron in the nucleus.
In order to figure the ordering of atoms in a crystal lattice, scientists need to scatter neutrons from nuclei, so normal hydrogen atoms won’t cut it.
“Unfortunately, this also changes the time scales for ordering in the ice manufacturing process,” said physical chemist Thomas Loerting of the University of Innsbruck.
“But Ph.D. student Tobias Gasser then had the crucial idea of adding a few percent of normal water to the heavy water – which turned out to speed up the ordering immensely.”
This allowed the team to obtain the neutron data they needed to piece together the crystal structure. As they thought, it was distinct from ice XV, earning it an official place as the nineteenth known phase, ice XIX.
This makes the pair sibling phases – the first known that have the same oxygen lattice structure, but with differing arrangements of hydrogen atoms.
“This also means that for the first time it will now be possible to realise the transition between two ordered ice forms in experiments,” Loerting said.
The research has been published in Nature Communications.