Water’s New Trick is Freezing at Room Temperature

Water is everywhere, but this simple molecule, two hydrogens clinging to an oxygen, still surprises scientists. A team from the Korea Research Institute of Standards and Science (KRISS) and the European XFEL in Germany have found a new form of ice, called ice XXI, that forms at room temperature under extreme pressure.

Ice, as we know it, comes in many flavors. Your freezer has ice I, a hexagonal crystal lattice that floats in your drink. But water can harden into over 20 different forms, each sculpted by precise temperatures and pressures. Ice XXI, the 21st known phase, is a new one with a tetragonal crystal structure that packs 152 water molecules into a single cell. This one would sink in your glass due to its density of 1.413 g/cm3 at 1.6 GPa. What’s amazing is that it forms at 25°C, under 20,000 times the pressure of the Earth’s atmosphere, not in a deep freeze.

To do this, the scientists built a dynamic diamond anvil cell that squeezes water between two diamond tips with incredible precision. Imagine a tiny vise with a pressure of up to 2 GPa, enough to crush most materials into dust. Over 1,000 cycles, the researchers compressed and decompressed water in milliseconds, while the European XFEL’s X-ray laser shot a million times a second. These ultra-fast images recorded water molecules reorganizing into crystals, showing not one, but five different paths to freezing. Some cycles saw water flow directly into ice VI, a phase thought to exist in moons like Titan. Others took detours through ice VII or the new ice XXI, a metastable structure that persists before transitioning to more stable forms.

Geun Woo Lee, the lead researcher, said that by rapidly increasing the pressure of water, you can prevent it from forming ice VI, where it should solidify. This super-compressed state gives a window of opportunity for fleeting forms like ice XXI to appear. Using X-ray pulses and computer simulations, the scientists were able to see water molecules halfway in before they suddenly erupted into very detailed crystals, all because of how fast energy passes through it just before it solidifies.


What makes Ice XXI truly stand out is its one of a kind structure. It’s a body centered tetragonal lattice that looks a lot like a cube that’s been stretched and then settled back into a pattern with symmetrical planes. The scientists used DESY’s particle accelerator to gain a closer look at the crystal and map out its recurring patterns, which they were able to confirm using the machine. All of this adds up to a rather significant issue: water is much more difficult to deal with under high pressure than previously imagined. Water turns to ice in many different ways, each one of those paths is a little different every time and influenced by tiny changes in pressure and timing.

You know the old saying – you can’t improve on perfection? Well, lets just say water is the exception to the rule because, being so darn simple, it still baffles us. Lee believes that there are loads of other ice phases just waiting for those perfect conditions to come along and reveal themselves to us.
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