By Tyler Irving
Posted April 2012
Many undergraduates are surprised to learn that oxygen, which they think of as a colourless gas, is blue and magnetic when condensed into a liquid. Now, a group of researchers studying solid oxygen have learned that its properties at extremely high pressures are just as surprising.
Dennis Klug is a researcher at the National Research Council’s Steacie Institute for Molecular Sciences in Ottawa. Along with an international team of collaborators, he’s been running detailed computer simulations of what happens to simple gases when they are subjected to pressures on the order of megabars — a million times higher than atmospheric pressure. The idea is that under these conditions, simple materials might gain interesting properties like superconductivity. Some could even be quench-recovered, meaning they could be returned to normal pressures without losing their new characteristics.
Previous simulations have shown that there are at least five distinct types of solid oxygen, each with its own molecular structure. At about one megabar,oxygen turns into a superconducting, metallic solid. Klug and his team went beyond this, reaching the kinds of pressures one would find at the centre of the earth. “Oxygen did two unexpected things,” says Klug. “First, it maintained its molecular form at pressures much higher than other simple gases, up to almost 20 megabars. Second, at pressures of about 20 megabars, it converts to a square spiral-like polymeric structure, very similar to solid sulphur. This structure has semiconducting properties.”
Other materials such as sodium can change their electrical conductivity at high pressures, but Klug says the finding was surprising because it was so different from other simple gases. “Nitrogen, carbon dioxide and hydrogen are all predicted to go to nice metallic structures, but we found that oxygen is a real oddball,” he says. Klug and his team are currently collaborating with other groups who could run high-pressure shockwave experiments to synthesize the structures they have predicted. The work is published in Physical Review Letters.
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