Not all water ice is the same. The molecule arranges on the inside, and it varies considerably depending on the pressure and temperature conditions under which it is formed.
We knew of 18 of these different ice phases, some occurring naturally, some being seen only in laboratory conditions.
Three years ago, a team of researchers adapted one of the existing ice structures and transformed it into a shape they called ice β-XV. Now members of the team determined the exact crystal structure and answered questions about how it formed and gave the name ice XIX.
This discovery can help us better understand how ice forms and acts in alien conditions that are very different from those on Earth.
The ice you see in the freezer, or when snowflakes or hailstones fall from the sky, is the most common natural ice on earth. It is called ice I, and its oxygen atoms are arranged in a hexagonal grid. However, the structure is geometrically frustrated, with the hydrogen atoms much more disordered.
If ice I is cooled in a certain way, the hydrogen atoms can be ordered regularly, in addition to the oxygen atoms. This is how scientists in a laboratory can create different phases of ice that have much more ordered crystal molecule grids than their disordered parent forms.
A team of physical chemists at the University of Innsbruck in Austria has been working on phase ice VI for some time. It is one of the forms of ice that occurs in nature, but only at very high pressures 10,000 times higher than atmospheric pressure at sea level (about 1 gigapascal), as in the Earth’s mantle or around the core of Saturn’s moon Titan .
Like ice I, ice VI is relatively disordered. The hydrogen order form, Ice XV, was first discovered about a decade ago. It is created by cooling the ice to below 130 Kelvin (-143 degrees Celsius, -226 degrees Fahrenheit) at pressures about 1 gigapascal.
By changing this process, the researchers created ice a few years ago. They slow down the cooling and take it below 103 Kelvin and increase the pressure to 2 gigapascals. This yielded a second arrangement of hydrogen molecules different from ice XV, which they termed y β-XV.
To confirm that the ice was a separate phase was a separate obstacle, which required the normal water to 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 its nucleus.
To determine the arrangement of atoms in a crystal lattice, scientists must scatter neutrons from the nuclei so that normal hydrogen atoms will not cut it.
“Unfortunately, it also changes the time scales for ordering in the ice making 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 normal water to the heavy water – which apparently speeds up the order very quickly.”
This enabled the team to obtain the neutron data they needed to compose the crystal structure. As they thought, it was different from Ice XV and deserves an official place as the nineteenth known phase, Ice XIX.
This makes the sibling phases of the pair – the first known with the same structure for oxygen lattice, but with different rank order of hydrogen atoms.
“It also means that it will now be possible for the first time to realize the transition between two ordered ice forms in experiments,” Loerting said.
The research was published in Nature communication.