Not all water ice is the same. Locked inside, the arrangement of the molecules varies significantly, depending on the pressure and temperature conditions under which it is formed.
We knew of 18 of these different phases of ice, some of which occur naturally, some are only seen in laboratory conditions.
Three years ago, a team of researchers modified one of the existing ice structures, transforming it into a shape they called β-XV ice. Now the members of this team have determined its exact crystal structure, answering questions about how it is formed and giving it the name Ice XIX.
This discovery could help us better understand how ice forms and behaves in very different alien conditions from those found on Earth.
Ice that is seen in the freezer or that falls from the sky like 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, however, is geometrically frustrated, with the hydrogen atoms much more disordered.
When gel I cools in a certain way, the hydrogen atoms can be ordered periodically, in addition to the oxygen atoms. This is how scientists in a laboratory can create different phases of ice that have lattice of crystalline molecules much more ordered than their disordered parental forms.
A team of physical chemists from the University of Innsbruck, Austria, has been working for some time with the ice phase VI. This is one of the forms of ice that can be found in nature, but only at very high pressures 10,000 times higher than atmospheric pressure at sea level (about 1 gigapascal), such as those found in the mantle of the Earth, or surrounded around Titan’s core by Saturn’s moon.
Like ice I, ice VI is relatively messy. Its hydrogen-ordered form, ice XV, was only discovered about a decade ago. It is created by cooling the ice below 130 Kelvin (-143 degrees Celsius, -226 degrees Fahrenheit) at pressures of about 1 gigapascal.
A few years ago, by changing this process, researchers created another phase of ice. They slowed the cooling and lowered it by 103 Kelvin and increased the pressure to 2 gigapascals. This produced a second arrangement of hydrogen molecules that was different from gel XV, which was what they called β-XV gel.
Validating that ice was an independent phase was a separate hurdle, requiring normal water to be replaced by “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 a neutron in its nucleus.
In order to calculate the order of atoms in a crystal lattice, scientists must scatter the neutrons in the nuclei, so that normal hydrogen atoms will not cut it.
“Unfortunately, this also changes the time scales for placing orders in the ice-making process,” said physicist Thomas Loerting of the University of Innsbruck.
“But doctoral student Tobias Gasser then had the crucial idea of adding a few percent normal water to heavy water, which turned out to greatly speed up the order.”
This allowed the team to obtain the neutron data they needed to bind the crystal structure. As it was thought, it was different from Ice XV, which allowed it to obtain an official place as the nineteenth known phase, Ice XIX.
This makes the pairs Germanic phases: the first known to have the same oxygen lattice structure, but with different arrangements of hydrogen atoms.
“This also means that for the first time it will now be possible to make the transition between two ordered ice shapes in experiments,” Loerting said.
The research has been published in Communications on Nature.