Chances are that all your encounters with frozen water—while trudging through slushy winter streets, perhaps, or treating yourself to cool summer lemonades—have been confined to one structural form of ice, dubbed Ih, with the h referring to its crystal lattice’s hexagonal nature. But there is so much more to ice than that.
For more than a century scientists have been striving to push ice into extreme conditions, creating progressively more exotic structures—they’ve made more than 20 crystalline forms to date, in fact, none of which we are likely to experience in our lifetimes.
“Water is a beautiful, elegant system that consistently shows new, remarkable behavior,” says Ashkan Salamat, a physical chemist at the University of Nevada, Las Vegas. “For something so simple, it has beautiful complexity.”
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At the heart of all these exotic ices—and our more mundane ice, as well as water and steam—is the same molecule: H2O, an oxygen atom flanked by hydrogen atoms forming an angle of 104.5 degrees. In every variety of ice, H2O molecules interact, with weak connections called hydrogen bonds forming between one oxygen and one hydrogen atom in separate molecules. Different arrangements of these hydrogen bonds can shape ice’s crystalline structure into various configurations, from a hexagonal prism to a cubic lattice to less familiar lattice systems such as rhombohedral and tetragonal.
The hydrogen bonds between water molecules are extremely sensitive to changes in temperature and pressure, Salamat says, giving water what he calls “quantumlike behavior.” Molecules are forced into dramatically different relations with one another at certain thresholds of these conditions. So he and other scientists conjure arcane recipes—smashing water with 3,000 times atmospheric pressure, for example, or cooling it (with a dash of potassium hydroxide) to –330 degrees Fahrenheit (–200 degrees Celsius) for a week—all in hot pursuit of new forms of ice.
The newest frozen discovery is ice XXI, announced in Nature Materials. (Salamat wasn’t involved in that work, although his team published the discovery of a new transitional phase dubbed ice VIIt in 2022.) Ice XXI is a fleeting, blocky crystal structure that develops from supercompressed water: the scientists could see it only by using an extremely powerful x-ray free-electron laser that functions essentially like a high-speed camera.
“Looking at things at a very, very fast rate allows us to observe weird and wonderful phenomena,” Salamat says, calling the laser “an incredibly exciting new toy.” The laser enables researchers to spot exotic ices that exist only briefly, introducing time as a variable along with temperature and pressure.
Although they do not exist naturally on Earth, some of these strange forms of ice may form on other worlds—deep inside Neptune, trapped inside a distant moon or at some even more alien location. But for Salamat, the laboratory can prove just as exotic. “There are still new and exciting things that we can discover,” he says.
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