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A very special snowball

The long-predicted ice XV has been spotted in the lab

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12:57pm, September 11, 2009
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Scientists have created the final predicted form of stable ice, called ice XV, in the lab. But don’t worry — Kurt Vonnegut had nothing to do with it, and the exotic new form of ice can’t destroy civilization.

Types of ice are classified by how close the water molecules pack together and the structure the molecules arrange themselves in. With the new discovery, researchers have identified 16 forms of ice (including two types of ice I) named in order of discovery. Most of the ice on Earth is type Ih (h for hexagonal, hence the six-sided symmetry of all snowflakes). Researchers had long predicted the existence of ice XV, but had never seen it before.

“We have removed the question mark from the phase diagram of water,” says Christoph Salzmann of the University of Oxford in England, coauthor of a paper published online September 2 in Physical Review Letters. A phase diagram maps how molecules will behave at certain pressures and temperatures.

To create the elusive ice, Salzmann and his colleagues dropped the temperature on another kind of ice, ice VI, in which water molecules are bonded to each other willy-nilly. As the researchers lowered the temperature to 130 kelvins (around -143º Celsius) and held the pressure around one gigapascal (almost 10,000 atmospheres), disordered hydrogen bonds in ice VI snapped into a highly ordered, tight conformation and created ice XV. Ice on Earth is downright fluffy in comparison with the newly discovered ice XV.

Earlier predictions guessed that ice XV might be ferroelectric, possessing the ability to carry a charge. Ice with that property could have had interesting effects on geological events on planets, Salzmann says. Instead, water molecules in ice XV pack in such a way that the charges all cancel out.

Ice XV’s stability at high pressures and low temperatures may allow it to exist somewhere out in the cosmos—maybe in deep interiors of icy planets or moons, Salzmann says. The only places on Earth with high enough pressure to sustain ice XV are also extremely hot, so ice XV can’t form there, he says.

Ice IX, made fictionally famous by Vonnegut in Cat’s Cradle, also exists only under high pressure.

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