Aug. 6, 2001 ITHACA, N.Y. -- Two researchers at Cornell University are predicting that high-temperature superconductivity could exist in a class of exotic materials containing silver and fluorine. However, none of the materials has yet been made.
The hope that these materials might be high-temperature superconductors -- that is, capable of conducting electricity with virtually no resistance -- is purely theoretical, warns Cornell chemist Roald Hoffmann. "This is not so much a prediction as a plausibility argument. I quite realize that with this argument, we are walking into a minefield," he says. "Even so, I do feel we have something exciting."
Although several silver fluorides and many complex fluorides -- known as fluoroargentates -- are known, none has yet been shown to be a high-temperature superconductor. A major problem in trying to turn the materials into superconductors is that these fluoroargentates contain silver in unusually high oxidation states (meaning that they are highly deficient in electrons) and thus easily lose elemental fluorine when warmed. Also, since fluorine is the most oxidizing of the elements, it corrodes glass and some other ceramics, probably requiring the use of Teflon or gold containers for large-scale manufacture of the potential superconductors.
Hoffmann, who shared the Nobel Prize for chemistry in 1981 and is the Frank H.T. Rhodes Professor in Humane Letters at Cornell, and his postdoctoral associate Wojciech Grochala, discuss their theory in the latest issue (Aug. 3) of the prestigious German journal, Angewandte Chemie. Grochala currently is carrying out research at the University of Birmingham, England.
Interestingly, fluoroargentates bear a strong similarity to oxocuprates, compounds that in 1993 set the record for high-temperature superconducting (minus 109 degrees Centigrade, or minus 164 degrees Fahrenheit). The compounds studied by Hoffmann and Grochala, with names like sodium tetrafluoroargentate and potassium trifluoroargentate, have been known since the 1970s. Why has the similarity between oxocuprates and fluoroargentates not been noticed until now? "Because fluoroargentates are exotic; they are familiar only to a small community of inorganic fluorine chemists, for whom the synthesis of the materials (which is very hard to do) was the main challenge," Hoffmann says.
The actual temperature at which the highly unstable material could become superconducting simply cannot be predicted, says Hoffmann. "But the analogy to cuprates, supported by detailed calculations, is sufficiently strong to make us think it will be high." Cuprates are compounds containing copper, oxygen and several other elements, such as barium, yttrium or bismuth.
Francis J. DiSalvo, director of the Cornell Center for Materials Research, which helped to support the research, notes that "it would really be a first for a theoretical group to predict a new class of high-temperature materials before experimentalists discovered them." However, he adds, "only time will tell if they have done so." And Neil Bartlett, professor emeritus of chemistry at the University of California -- Berkeley, who adds a commentary to the Angewandte Chemie article, says, "I am excited by their prediction because, as their paper points out, I have observed magnetic behavior in a silver fluoride system that is consistent with the presence of a superconducting phase. Unfortunately, we do not yet know the composition, or structure, of the material that produces this effect."
For much of the last century, it was believed that that superconductivity, which was discovered in 1905, could exist only in metals at extremely low temperatures, with a maximum transition temperature for its appearance of some 23 degrees Kelvin above absolute zero (or about minus 418 degrees Fahrenheit). But in 1987, superconductivity was discovered at substantially higher temperatures -- 93 degrees Kelvin (about minus 292 degrees Fahrenheit) -- in cuprate materials.
Low-temperature superconductivity has been explained by the so-called BCS theory (named for John Bardeen, Leon Cooper and Robert Schrieffer), but the high-temperature phenomenon remains essentially unexplained. Hoffmann says: "Despite the efforts of the best minds in physics over the past 15 years, I would say this remains the outstanding unsolved problem in condensed matter theory."
New high-temperature superconductors continue to be discovered. Last year, a metallic compound, magnesium diboride, which had been known for years, was announced by Japanese researchers to be superconducting at nearly 40 degrees Kelvin (about minus 388 degrees Fahrenheit). It is known that the heart of electrical superconductivity is the formation and flow of pairs of electrons of opposite spin. Grochala and Hoffmann, intrigued by the similarities between oxocuprates and fluoroargentates, such as identical valence electron counts, used different theories to make their prediction, including the BCS theory. (A valence electron takes part in forming chemical bonds.) In addition, the two researchers used their chemical theoretical reasoning, contained in four recently published papers.
Hoffmann warns that it will not be easy to exploit the theoretical predictions by actually making the compounds. Even so, about five laboratories around the world are attempting to produce the proposed superconducting material.
In a biographical note in the joint paper, Hoffmann notes that his interest in chemistry was stimulated by the life of another Polish-born Nobel laureate, Maria Sklodowska Curie. He adds: "And I learned nearly all I know about silver fluorides from another Pole, Wojciech Grochala."
The research also was supported by the National Science Foundation and the Cornell Theory Center.
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