Not all ice water is the same. Enclosed inside, the arrangement of the molecules varies significantly, depending on the pressure and temperature conditions in which it is formed.
We knew of 18 of these different phases of ice, some of which occur naturally, others are only seen under laboratory conditions.
Three years ago, a team of researchers modified one of the existing ice structures, transforming it into a form they called β-XV ice. Now members of that team have determined its exact crystal structure, answering questions about how it is formed and giving it the ice designation XIX.
This discovery could help us better understand how ice forms and behaves in extraterrestrial conditions very different from those found on Earth.
The ice you see in the freezer, or that falls from the sky in the form of snowflakes or hail, is the most common natural ice on Earth. This is called ice I and its oxygen atoms are arranged in a hexagonal grid. However, the structure is geometrically thwarted, with the hydrogen atoms much more disordered.
When ice I is cooled in a certain way, the hydrogen atoms can periodically arrange themselves in addition to the oxygen atoms. This is how scientists in a laboratory can create different phases of ice that have networks of crystalline molecules much more ordered than their original disordered shapes.
A team of chemical physicists from the University of Innsbruck in Austria has been working with phase VI for some time. 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 (about 1 gigapascal), such as those found in the mantle of the Earth, or wrapped around the core of Titan, Saturn’s moon.
Like Ice I, Ice VI is relatively messy. Its hydrogen-ordered form, Ice XV, was discovered only a decade ago. It is created by cooling ice below 130 Kelvin (-143 degrees Celsius, -226 degrees Fahrenheit) at pressures of around 1 gigapascal.
A few years ago, by changing this process, researchers created another phase of the ice. They slowed the cooling down, brought it below 103 Kelvin, and increased the pressure to 2 gigapascals. This produced a second arrangement of hydrogen molecules that was distinct from XV ice, which was what they called β-XV ice.
Validating that ice was a separate phase was a separate obstacle, requiring normal water to be replaced by ‘heavy’ water. Normal hydrogen does not have neutrons in the nucleus. Heavy water, on the other hand, is based on deuterium, a form of hydrogen that has a neutron in the nucleus.
To calculate the order of the atoms in a crystal lattice, scientists need to scatter the neutrons from the nuclei so that normal hydrogen atoms do not cut through it.
“Unfortunately, this also changes the timescales for ordering in the ice-making process,” said physical chemist Thomas Loerting from the University of Innsbruck.
“But PhD student Tobias Gasser then had the crucial idea to add a small percentage of normal water to heavy water, which greatly accelerated the order.”
This allowed the team to obtain the neutron data they needed to reconstruct the crystal structure. As they thought, it was different from ice XV, which earned it an official place as the nineteenth known phase, ice XIX.
This makes the pair of sister 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 transition between two ordered ice forms in experiments,” Loerting said.
The research has been published in Communications from nature.