Water is squeezed between two diamonds. The pressure rises to 20.000 times atmospheric pressure in ten milliseconds. An X-ray laser captures a million images per second. And something strange happens: the water becomes solid. At room temperature. It shouldn't, but it does.
It's called ice XXI, Ice XXI and it is, precisely, the twenty-first phase of ice that we know. A tetragonal crystalline structure with 152 water molecules per repeating unit. Nothing like it had ever been observed before. The researchers at Korea Research Institute of Standards and Science they discovered it using theEuropean XFEL in Germany, the most powerful X-ray laser in the world. It's ice, but not as we know it.
When pressure replaces cold
Forget the freezer. To create Ice XXI, you need a diamond anvil cell, a device that uses the hardness of diamonds to generate monstrous pressures. The study published in Nature Materials It describes a mind-blowing process: water is compressed to 2 gigapascals (about 20.000 times atmospheric pressure) in just 10 milliseconds. Then the pressure is slowly released over the course of a second. The cycle repeats over a thousand times.
Meanwhile, the X-ray beam from theEuropean XFEL It works like a high-speed camera: it captures images every microsecond, creating a film of the molecular transformation. It's like watching water decide what it wants to be when it grows up, only the decision happens in a fraction of a second under pressure that would pulverize anything short of a diamond.
A structure that looks like nothing else
The interesting thing about ice XXI isn't just that it forms at room temperature. It's that its molecular architecture doesn't resemble any of the other twenty known ice phases. Its structure is tetragonal (like a squashed cube) with enormous repeating units: each cell contains 152 water molecules. To give you an idea, regular ice (ice I, the kind we put in drinks) has a much simpler hexagonal structure.
How do you explain Geun Woo Lee, physicist of the Korea Research Institute and lead author of the study: “Rapidly compressing water allows it to remain liquid at pressures where it should already have crystallized into ice VI. The structure in which liquid H₂O crystallizes depends on the degree of supercompression of the liquid.”
Practically, ice XXI is a transition phase, a controlled accident on the road that carries water to ice VI, an exotic form of ice that likely fills the depths of icy moons like Titan e GanymedeWater needs to be compressed at room temperature so quickly that it doesn't have time to organize itself into the "intended" shape. The result is this intermediate configuration that persists just long enough to be photographed.
Why study alien ice?
The discovery of ice XXI isn't just an academic exercise. Understanding how water behaves under extreme pressure helps us imagine what happens inside icy planets and moons. According to Rachel Husband, co-author of the study and physicist of the DESY Center in Germany,
“Our results suggest that a larger number of high-temperature metastable ice phases and their transition pathways may exist, potentially offering new insights into the composition of icy moons.”
Titan, Saturn's largest moon, has a subsurface ocean that may hold more water than all of Earth's oceans combined. GanymedeJupiter's moon, Ice VI, likely conceals a 100-kilometer-deep ocean beneath its icy crust. In these environments, the pressure is such that water could assume exotic configurations such as Ice VI or, indeed, undergo intermediate phases such as Ice XXI.
The laser that sees the invisible
THEEuropean XFEL (X-Ray Free-Electron Laser) in Hamburg is no ordinary laser. It's a 3,4-kilometer-long machine that generates X-ray pulses a billion times brighter than those produced by conventional sources. Each pulse lasts a few femtoseconds (a millionth of a billionth of a second) and can capture processes occurring at the atomic scale.
In the case of ice XXI, this speed was crucial. The transformation of water under extreme pressure occurs in a matter of milliseconds. Without an instrument capable of "freezing" the process frame by frame, it would have been impossible to document the formation of this intermediate phase. As we have reported in the past for other exotic states of matter, advanced imaging technologies are revealing phenomena that until a few years ago were only theoretical hypotheses.
Sakura Pascarelli, scientific director ofEuropean XFEL, comments: “It's fantastic to see another great result from our Water Call, an initiative that invites scientists to submit innovative studies on water. We look forward to seeing many more exciting discoveries.”
Water is still a mystery
It may seem absurd, but water (H₂O, two hydrogen atoms and one oxygen atom) is one of the most complex substances in the universe. Despite its simple chemical formula, it can assume over twenty different solid configurations depending on temperature and pressure. Each phase has unique properties: density, crystalline structure, and optical behavior.
The discovery of ice XXI suggests that there may be other, as yet unknown phases hidden in corners of the pressure-temperature diagram that we have not yet explored. As Geun Woo Lee says:
Water is one of the most mysterious materials in the universe. Why do two such simple elements create so many different phases? We think there are still unknown crystalline phases of ice. When we say unknown, we mean undiscovered, but possibly existing.
Research continues. The next step will be to understand whether there are other metastable phases along the crystallization pathways of water, and whether these can tell us something new about how the icy worlds of the solar system function.
As long as we have diamonds hard enough and lasers fast enough, we'll keep squeezing water, even at room temperatures, to see what else it can do.
