Solid Surprise: High-pressure oxygen takes unpredicted form

For more than a quarter century, scientists have been trying to determine the structure of a particular form of solid oxygen.

TWO BY FOURS. Under high pressure, oxygen molecules (labeled O1-O2 and O3-O3) aggregate into groups of four, each of which is surrounded by four neighbors. Such a grouping of molecules had never been observed. Lundegaard, et al./Nature

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 e-oxygen, during high-pressure experiments in 1979.

Despite nearly 3 decades of analyses, scientists hadn’t come up with a convincing model of e-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. As the researchers added pressure, the gas mixture liquefied and then separated into the two components. Adding even more pressure caused the oxygen and helium to solidify, resulting in 100-micrometer-long e-oxygen crystals that were surrounded by solid helium and therefore protected from shear stresses.

Passing X rays through the e-oxygen crystals revealed that the two-atom molecules were arranged in rhombus-shaped groups of four. The distance between molecules within a group is 0.218 nanometer, and the distance between groups is at least 0.256 nm, the researchers report in the Sept. 14 Nature.

This “pressure-induced association of molecules was unanticipated,” says team member Paul Loubeyre, a physicist at France’s Atomic Energy Commission in Bruyères-le-Châtel. Although there seems to be some type of bonding among the molecules in each group, it’s not yet clear what that is, he notes.

Scientists usually apply quantum mechanics equations to come up with approximations of crystal structure. That didn’t work, however, in the case of e-oxygen, indicating that the approximations don’t account for all possible molecular interactions.

“The standard methods failed for this problem,” says Burkhard Militzer, a physicist at the Carnegie Institution of Washington (D.C.). The new findings “open up a fresh dimension of chemistry that we are only just getting to know,” he comments in Nature.

“This structure shows [that] we have a limited understanding of even simple materials,” says Lundegaard.

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