The chemistry of superconductivity.

The chemistry of superconductivity 

The unprecedented rate of evolution--some would say revolution--in high-temperature superconductivity has catalyzed the physical sciences as few topics have since the World War II effort to harness the atom. In recent weeks, researchers have been abandoning sleep and home life to collaborate across disciplines and among laboratories in an all-out rush to understand why certain new copper oxides lose all electrical resistance at the "high' (above liquid-nitrogen) temperatures of 77 to 90 kelvins and beyond (SN: 3/28/87, p.196).

With only 10 days' planning, researcherspresented a tutorial on the chemistry of these materials at the American Chemical Society's spring meeting in Denver last week. It was an important forum, according to Arthur Sleight of E.I. duPont de Nemours in Wilmington, Del., since the properties of and the production techniques for high-temperature superconducting materials "have largely been discovered through solid-state chemistry.'

Sleight reports finding a new attributeassociated with oxide superconductors. Previously, only one or two electrons were found occupying the essential ingredient's --say niobium's--d-orbital, or outermost subshell of electrons. In the new 90 K materials, where copper appears to be the essential element, there are nine. This suggests a new range of materials that may qualify for substitution, he says--those whose d-orbital is "nearly filled' instead of "nearly empty.'

The first announcement of a 90 Ksuperconductor--the highest-temperature material at which electrical resistance has been demonstrated to cease-- was made by physicist Paul Chu and his colleagues (SN: 3/14/87, p.164). But no sooner had Chu begun circulating preprints on the work than chemists joined the fray. The Chu preprint arrived on a Friday, recalls Edward M. Engler of IBM Almaden Research Center in San Jose, Calif., and by Monday "at least a dozen laboratories that we knew of [had] made it [the material].'

All they had to go on was a one-sentencedescription of the Chu material's ingredients. But within a week of playing with the material, Engler says, "we [at IBM] found that the major phase in the superconducting component of the yttrium/barium/copper-oxide material is a variation of a well-known class of inorganic structures called perovskites [see diagram].' After identifying the chemical structure of the superconducting component out of the complex mix of crystals that formed, they developed a recipe for producing it in pure form.

His group got the highest "critical'(superconducting) temperatures from recipes whose starting ingredients had been "cooked' in pure oxygen instead of air, then cooled slowly. "This sensitivity to [production] conditions appears to be related to the amount and ordering of oxygen in the material,' he says.

Thomas Mason, a solid-state chemistfrom Northwestern University's Materials Research Center in Evanston, Ill., has been developing some of the first "phase diagrams'--a mapping of which recipes, based on the same staring ingredients, produce stable materials--for the constituents used in the new 90 K superconductors. From these diagrams, materials designers can make more informed variations in their recipes for the next generation of superconductors.

Since Chu's first announcement, researchershave developed more than 20 new 77 K superconductors, which are being fashioned into crude but usable shapes--such as wires for magnets, and thin films for electronics applications.

Photo: Left: One of first "usable,' current-carrying superconducting perovskite wires, undergoing tests. Right: Basic structure of superconducting perovskite.