NEWS OF THE WEEK
NEW SUPERCONDUCTOR: Groups race to decipher structure A new round of frenetic competition has begun as researchers around the world scramble to decipher the structure and determine the properties of a new class of high-temperature copper oxide superconductors that was announced about three weeks ago by groups in Japan and at the University of Houston (C&EN, Feb. 1, page 5). Du Pont joined the fray last week when it announced that the new type of superconductor, which consists of bismuth, calcium, strontium, copper, and oxygen, lacks a key structural feature that many scientists had thought to be essential for superconductivity near 100 K. That feature is linear chains of alternating copper and oxygen atoms. Such chains are present in the wellknown "1-2-3" class of superconductors typified by yttrium-bariumcopper oxide (YBa2Cu307). According to Du Pont scientists, the absence of these copper-oxygen chains in the new bismuth-containing material may force a revision of current theories that try to explain why these oxides lose all resistance to electricity at such relatively high temperatures. Last week, the Du Pont group, led by Arthur W. Sleight and Uma Chowdhry, whisked a manuscript detailing their results to Science, which plans to publish it in a few weeks. Seven days earlier, Physical Review Letters received a manuscript from a group led by Ching-Wu (Paul) Chu of the University of Houston. Chu's paper identifies the superconducting phase and contains some structural information. But, unlike Du Pont's, it doesn't give a detailed atomic description of the superconductor's crystal structure, according to Chowdhry, who received a preprint from Chu. Nevertheless, Chu tells C&EN, "We 4
February 15, 1988 C&EN
Sleight: no oxygen annealing needed were the first to get the structure solved/ 7 The two rival groups agree on some points, disagree on others. The material they focused on is basically the same, having the approximate composition Bi2CaSr2Cu20x, where x is between 8 and 9. Its electrical resistance begins dropping between 115 and 120 K, according to both groups. Zero resistivity is reached at about 90 K—similar to what's found in the 1-2-3 materials. The two groups agree that the new material consists of alternating double copper-oxygen sheets and double bismuth-oxygen sheets, interspersed with strontium and calcium ions. But Chu collaborator Robert M. Hazen, a crystallographer at the Geophysical Laboratory of the Carnegie Institution of Washington (D.C.), says they cannot yet rule out the possibility that the structure may contain copper-oxygen chains. In any case, he points out, "It's a much more complicated structure than
most [previously studied] layered structures/' Hazen and his Carnegie coworkers presently are analyzing x-ray data obtained on single crystals of the bismuth superconductor. In certain respects, the new material may prove to be superior to the intensely studied 1-2-3 materials. For example, the 1-2-3 compounds deteriorate on prolonged exposure to moisture and carbon di oxide in air. But with the bismutr superconductor, "we've seen no evidence of that kind of degradation at all," says Du Pont's Sleight. Also, researchers say that some of the processing conditions necessary to make the bismuth superconductor apparently are less exacting than those required by the 1-2-3 materials. The latter require careful annealing in an oxygen atmosphere to produce good superconducting properties, Sleight notes. But with the bismuth material, he says, "we're getting very good superconducting properties without doing any oxygen annealing, even on the single crystals we've grown." Moreover, the new superconductor may be easier to fabricate into useful shapes than the brittle 1-2-3 materials. Hazen says that the bismuth superconductor "behaves more like a clay than a ceramic." Clays can be pressed into flat, layerlike forms and rolled into flexible wires, he notes. According to Hazen and Sleight, the new superconductor also resembles mica, a mineral that separates into very thin plates. Because the superconductor's plates have considerable flexibility, Sleight says, it may be possible to form it into superconducting ribbons. "The materials properties of this superconductor could be very exciting," Hazen remarks. Ron Dagani, Washington
