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Rearranging oxygen for superconductivity.

Rearranging oxygen for superconductivity

The superconducting ceramic yttrium-barium-copper oxide behaves like an oxygen sponge, readily sopping up or releasing oxygen atoms. A team of researchers has taken a close, careful look at how oxygen content and arrangement influence the temperature at which a material starts to lose its resistance to electrical current. The findings, reported in the Aug. 24 NATURE, overturn certain commonly held views about the effect of oxygen arrangements on the superconducting transition temperature.

Researchers often refer to yttrium-barium-copper oxide as the 1-2-3 compound because those numbers represent the relative number of yttrium, barium and copper atoms in the material. Its corresponding oxygen proportion can vary from 6.4 to 7. For certain ranges of oxygen content, its superconducting transition temperature stays relatively constant: about 90 kelvins (-183 [degrees] C) for the range from 6.8 to 7, and 60 kelvins between 6.6 and 6.7. When plotted on a graph of transition temperature versus oxygen content, those regions where the graph levels off appear as plateaus. Many scientists assume these two plateaus correspond to two different arrangements of oxygen atoms within the ceramic.

To check this idea, Robert B. Beyers of the IBM Almaden Research Center in San Jose, Calif., and his colleagues developed a special apparatus allowing them to control exactly how much oxygen gets into their samples. They used electron diffraction techniques to study the crystal structures of the resulting materials.

"We see a progression of ordered structures," Beyers says. "The only places where we see more than one type of electron diffraction pattern are right in the middle of the plateaus." That's the opposite of what many researchers had expected. EAch plateau seems to represent not a single phase, or type of crystalline arrangement, but a mixture of two phases differing only slightly in oxygen content. At the same time, the researchers find no evidence for mixtures of phases that have very different oxygen contents.

"This is the kind of scientific underpinning you need for the development of any consistent, reproducible technology," says Robert A. Huggins of Stanford University. "The oxygen [content] is a critical feature of the properties of these materials. It may even turn out to be what determines who ends up getting the basic patents on the 1-2-3 compounds."
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Author:Peterson, I.
Publication:Science News
Date:Aug 26, 1989
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