Magazine article Science News

Solid Surprise: High-Pressure Oxygen Takes Unpredicted Form

Magazine article Science News

Solid Surprise: High-Pressure Oxygen Takes Unpredicted Form

Article excerpt

For more than a quarter century, scientists have been trying to determine the structure of a particular form of solid oxygen. X-ray analysis of the substance under high pressure now indicates that oxygen's two-atom molecules aggregate into groups of four in a crystalline structure that's never been seen before and isn't accounted for in current theory.

Oxygen is the third-most-common element in the universe, trailing only hydrogen and helium. At the pressures and temperatures ordinarily found at Earth's surface, molecules of oxygen form a gas. At various combinations of lower temperatures and higher pressures, oxygen becomes a liquid. At very low temperatures or exceedingly high pressures, the substance takes on solid form.

Solid oxygen has six known varieties, each designated by a Greek letter, says Lars F. Lundegaard, a physicist at the University of Edinburgh. Scientists first observed the dark-red epsilon phase, or [epsilon]-oxygen, during high-pressure experiments in 1979.

Despite nearly 3 decades of analyses, scientists hadn't come up with a convincing model of [epsilon]-oxygen's crystalline structure. Some teams had suggested that the form's crystals are groups of eight-atom chains, Lundegaard notes. Others had speculated that the atoms link to form a single ring of eight atoms, as does sulfur, oxygen's chemical relative. "As it turns out, no one was right," Lundegaard says.

When some researchers compressed pure oxygen, their equipment applied shearing stresses that distorted the material. Lundegaard and his colleagues avoided that problem by mixing helium and oxygen. …

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