For something so common in our daily lives, there’s a surprisingly long list of questions scientists have yet to answer about ice: the solid form of water, seen in iced coffee, icy dustings on mountaintops, and, of course, alien surfaces. It’s fascinating that scientists continue to discover entirely new types of ice, adding to the mystery surrounding how ice forms in different environments.
In a recent Nature Materials article, a team of scientists led by the Korea Research Institute of Standards and Sciences (KRISS) reported the discovery of XXI ice, an entirely new phase of ice that forms when water is rapidly compressed at room temperature. What is particularly intriguing about this form of ice is that it emerges in the pressure range of Ice VI, a previously known form of ice believed to exist inside icy moons such as Titan and Ganymede.
Icy trails
Chemically speaking, water, although containing two elements, hydrogen and oxygen, can form a remarkably diverse range of crystal structures as a solid. So far, scientists have discovered 20 different arrangements of pure ice; as the Roman numeral indicates, ice XXI is the 21st.
“There are many questions about how such a simple material produces many different crystal phases,” said Geun Woo Lee, lead author of the study and a researcher at KRISS, in an interview with European XFEL. The researchers therefore want to “understand the detailed pathways from water to ice crystallization”.
To find these forms of ice, researchers experiment with different temperatures and pressures, identifying various “pathways” by which water molecules organize into ice. For the experiment, the team discovered a “hidden” path in the pressure region that forms ice VI.
“The rapid compression of water allows it to remain liquid until higher pressures, where it should have already crystallized into VI ice,” Lee explained in a DESY statement.
On this path to VI, ice XXI forms as a metastable structure, meaning it is caught in a precarious balance between one phase and another, existing for some time “even though another form of ice would be more stable under these conditions,” the researchers explained.
Enter the giant x-rays
For the experiment, the researchers made two diamond anvil cells, a contraption akin to a “nutcracker on steroids,” capable of producing extreme pressures of up to two gigapascals, or 20,000 times more than normal atmospheric pressure. As the team repeatedly compressed and decompressed the water molecules in the cells, giant X-ray lasers at Europe’s XFEL captured every microsecond of change in the ice.
Next, they used PETRA III, a particle accelerator at DESY, to determine the exact structure of ice XXI: a tetragonal crystal composed of large repeating units that is unlike any other ice phase seen before.
“Our results suggest that there may be a larger number of high-temperature metastable ice phases and their associated transition pathways,” added Rachel Husband, study co-author and physicist at the DESY center in Germany. Further investigation could offer “new insights into the composition of icy moons,” she said.
“Water is one of the most mysterious materials in the universe,” Lee added. “Why do two such simple elements create many different types of phases? We think there are still different (unknown) ice crystal phases – when we say unknown, we mean not yet discovered, but it may exist.”
