Published online 13 September 2006 | Nature | doi:10.1038/news060911-7

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Solid red oxygen: useless but delightful.

Some crystals of solid red oxygen, encased in helium, captured on camera through a microscope. Serge Desgreniers

Scientists have revealed the crystal structure of a dark red form of solid oxygen that forms at immense pressures. The results are surprising, elegant — and entirely useless. But the high-pressure techniques used to make the crystal could soon find applications.

As a gas, oxygen molecules (O 2 ) normally float around with only passing attraction to each other. But increasing pressure forces the molecules together, turning oxygen into first a magnetic, pale blue liquid, then a pale blue solid at 54,000 times atmospheric pressure (5.4 GPa).

In 1979, chemists discovered that, at pressures above 10 GPa, oxygen becomes a red solid1. At 96 GPa, oxygen molecules are so close that electrons flow freely between them, in a metallic phase seen in 1990.

How the oxygen molecules arrange themselves in the red solid has been a mystery. Now two teams have revealed the crystal's structure.

Diamonds and butter

Physicist Malcolm McMahon, of the University of Edinburgh, UK, and his colleagues grew a single crystal of solid red oxygen between two diamond anvils. To stop it shattering, the crystal was cushioned in solid helium, which at these pressures would be soft like butter, says co-author Paul Loubeyre, of the French Atomic Energy Commission in Bruyéres-le-Châtel. The team then shot X-rays through the crystal to determine its structure2.

Meanwhile, Hiroshi Fujihisa from the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, and his colleagues have fired X-rays at red oxygen powder, again crushed between diamonds3. This is easier to make than the crystal form, but harder to analyse.

Both studies gave surprising results. Usually, the chemical bonds in a solid break under high pressure, leaving individual atoms or long chains of molecules. Elements also often behave like heavier members of their group in the periodic table when under pressure, so one might expect pressurised oxygen to group up into eight-membered rings, as sulphur does.

Clumps of four oxygen molecules take the shape of squashed cubes in the crystal.

Instead, it appears that oxygen gangs up under pressure into groups of four pairs, in a shape like a squashed cube. These clumps of four O 2 molecules could also be called a single O 8 molecule — but they aren't in a ring. The result demands a rethink of theoretical calculations about the behaviour of dense oxygen.

The new structure adds to the molecules O 2 (oxygen), O 3 (ozone), and O 4 (seen at small concentrations in oxygen fluid).

"I'm really happy the structure looks correctly solved," says Roberto Bini, who has worked on high-pressure oxygen phases at the University of Florence, Italy. "You can't imagine how difficult the technology is."

What's the use?

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Exciting as it is, solid red oxygen seems, for the moment, useless. It is made in tiny amounts and vaporizes as soon as the pressure lifts. Nor will it be found in nature: despite the high pressures found in places such as inside the Earth, non-gaseous oxygen almost always joins to other elements, as an oxide or in water. "There is no astrophysical or geophysical situation where you would observe these solid phases — only in the laboratory," says Loubeyre.

But high-pressure techniques have already been used to create ultrahard materials such as diamond. Other chemicals, such as nitrogen and carbon monoxide, form solid polymers under pressure that store a lot of energy. If similar structures could be retained at atmospheric pressure, they might make excellent rocket fuel, suggests McMahon.

The real prize of high-pressure research, says Loubeyre, is metallic hydrogen, now predicted to be formed at some 450 GPa. That material would have remarkable properties, including exotic superconductivity and superfluidity, and might be found in the core of Jupiter.

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University of Florence, Italy