For something so common to 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, the frosty sprinkle on mountaintops, and, of course, extraterrestrial surfaces. Fascinatingly, scientists keep finding entirely new types of ice, adding to the mystery surrounding how ice forms in different environments.
In a recent Nature Materials paper, a team of scientists led by the Korea Research Institute of Standards and Science (KRISS) reported the discovery of ice XXI, an entirely new phase of ice that forms when water is rapidly compressed at room temperature. What’s particularly intriguing about this ice form is that it emerges within the pressure range of ice VI—a previously known form of ice thought to exist inside icy moons such as Titan and Ganymede.
Icy pathways
Chemically speaking, water—despite containing two elements, hydrogen and oxygen—can form a remarkably diverse range of crystalline 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 [as to] how such a simple material makes a lot of different crystal phases,” said Geun Woo Lee, study senior author and a KRISS researcher, in an interview with European XFEL. So researchers want to “understand the detailed paths of the crystallization from water to ice.”
To find these ice forms, researchers experiment with different temperatures and pressures, identifying various “pathways” through which water molecules arrange themselves into ice. For the experiment, the team uncovered a “hidden” path within the pressure region that forms ice VI.
“Rapid compression of water allows it to remain liquid up to higher pressures, where it should have already crystallized to ice VI,” Lee explained in a DESY statement.
On this pathway to VI, ice XXI forms as a metastable structure—meaning it’s caught in a precarious balance between one phase and another, existing for some time “even though another form of ice would be more stable at those conditions,” the researchers explained.
Enter the giant X-rays
For the experiment, the researchers fabricated 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 air pressure. As the team repeatedly compressed and decompressed water molecules within the cells, giant X-ray lasers at the European XFEL captured every microsecond of change within the ice.
Then, they used PETRA III, a particle accelerator at DESY, to determine the exact structure of ice XXI: a tetragonal crystal composed of large, repetitive units unlike any other ice phase seen before.
“Our findings suggest that a greater number of high-temperature metastable ice phases and their associated transition pathways may exist,” added Rachel Husband, study co-author and a physicist with the DESY Center in Germany. Further investigations 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 such simple two elements make lots of different types of phases? We think there are still different, [unknown] ice crystal phases—when we say unknown, we mean not discovered yet, but it can exist.”