Thallium, bismuth superconductivity.

Thallium, bismuth superconductivity 

By giving the highest temperatures yet recorded for reproducible bulk superconductivity, thallium and bismuth are generating intense interest among physicists and chemists. Early this year, full superconductivity appeared at a temperature of 106 kelvins in a bismuth compound studied at the University of Arkansas in Fayetteville (SN: 3/5/88, p.148). Now, in a thallium compound, the same researchers have found the onset of superconductivity at 140 K and complete superconductivity at 119 K.

The leader of the Arkansas group, Allen M. Hermann, reported the March 19 findings in New Orleans last week at a meeting of the American Physical Society. At the same meeting Stuart Parkin of the IBM Almaden Research Center in San Jose, Calif., reported that his group had found reproducible bulk superconductivity in a thallium compound at 125 K, according to them the highest temperature yet.

The rush to find compounds in which superconductivity sets in at relatively high temperatures, rather than within a few degrees of absolute zero, began a couple of years ago when J. George Bednorz and K. Alex Muller of the IBM Zurich (Switzerland) Research Center found superconductivity at 30 K in a compound involving the rare-earth element yttrium and copper oxide. Further work with copper oxides of other rare-earth elements quickly raised the superconducting transition temperature to a little more than 90 K. There it stuck for more than a year.

Then came the report from Arkansas. According to Associated Press (AP) Science Editor Paul Raeburn, the news service at first refused to put the story on its national wire, believing that nothing like that could come from such a place. AP soon found out different, as various physics and chemistry departments began exchanging samples of the new materials to try to elucidate their structures and possibly raise the superconducting temperature. So far, a group led by Mas Subramanian of E.I. du Pont de Nemours and Co. in Wilmington, Del., seems to be the only one to claim to have made and solved the structure of single crystals, but a number of others claim to have the structure of unit cells of the crystal.

As Robert Hazen of the Carnegie Institution of Washington, D.C., notes, these are layered structured, sandwiches in which planes of copper and oxygen lie between outer layers composed of bismuth and oxygen. The more copper-oxygen planes there are in the "meat" of the sandwich, the higher the superconducting transition temperature. At the meeting he presented a chart in which one copper-oxygen slice produced a transition temperature of less than 80 K, two gave 105 K and three gave 125 K. A four-plane compound has not been investigated, but Hazen predicted it might go to 200 K. Paul Martin of IBM Almaden points out that the space the sandwich can occupy limits the number of planes, probably to about 10. He suggests the four-plane compound will give 150 K to 160 K, and the 10-plane 200 K.

Work with thallium has not spread as widely as that with the rare-earth compounds. Thallium is extremely toxic; Hermann says ingesting 1.7 grams of it would be fatal. The physicists are generally wary of thallium. Bednorz says he refuses to work with it: "I do not wish to be poisoned." He believes thallium is just a step to something better and safer, although he will not speculate on what element that might be. Most of the physicists prefer to leave the handling of thallium to chemists who are experienced with toxic substances and who use isolating hoods, gloves and other protective clothing. Several participants in the bismuth-thallium session at the meeting stressed that the amateur scientists, teachers and schoolchildren who have experimented with the rare-earth compounds should not meddle with thallium. "I would hate to see anyone get hurt over this," Hermann said. ''D. E. Thomsen