A superconducting banquet from the periodical table.

A Superconducting Banquet From the Periodic Table

Chemical intuition and an intimate knowledge of the periodic table of chemical elements have again led to the discovery of what may be another new class of superconducting ceramics. By substituting the toxic element thallium for a rare earth metal such as lanthanum and by adding a pinch of calcium, researchers at the University of Arkansas in Fayetteville have synthesized a material that begins to lose its resistance to electrical current at 123 kelvins, or --238[deg.]F, and becomes fully superconducting at 106 K. This surpasses the best results yet achieved with bismuth-containing superconductors (SN: 2/20/88, p.116).

The new thallium superconductors are "extremely easy to prepare," says Allen M. Hermann, who along with Z.Z. Sheng made the discovery. "It takes only about five minutes to make one of these materials." The simplicity of the process allowed researchers in Japan and at the University of Houston and Du Pont to confirm the discovery promptly. The Arkansas results will appear in the March 10 NATURE.

The recipe involves compressing a mixture of finely powdered barium copper oxide, thallium oxide and calcium oxide into a pellet. Heating the pellet for five minutes in an oxygen-supplied furnace at 900[deg.]C melts the mixture. Upon cooling in air, the material shows the properties characteristic of a superconductor.

Whether the thallium compound is a new type of superconductor isn't clear yet. Paul C.W. Chu of the University of Houston suspects that the thallium compounds are similar to the recently discovered bismuth superconductors. However, Hermann and Sheng say they have identified two different superconducting phases within their material. One of the phases seems to have an atomic structure with a pattern of three adjacent copper-oxygen planes. This contrasts with the bismuth structure (see illustration), which contains pairs of copper-oxygen planes.

The bismuth structure was recently worked out by a team of researchers at the Du Pont Experimental Station in Wilmington, Del., and appears in the Feb. 26 SCIENCE. Chu and his colleagues, who fabricated a similar bismuth superconductor, also determined the compound's structure, and their results match the Du Pont findings.

The recent discoveries show that there's plenty of room within the periodic table for more variations on the basic superconductor recipes now known. So far, says Chu, raising the temperature at which superconductivity begins has meant building increasingly complicated, layered atomic structures.

Says Hermann, "My feeling is that there's a whole, rich class of materials out there, all based on copper-oxygen planes."