Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2001, 40 (15), 2742-2781
Unusual silver-fluoride compounds
The phenomenon of superconductivity was discovered nearly one hundred years ago - and has not diminished in its allure since. Roald Hoffmann (winner of the 1981 Nobel Prize in Chemistry, together with K. Fukui) and Wojciech Grochala have also fallen under its spell. On the basis of theoretical observations, the two chemists are predicting that an unusual class of compounds known as the fluoroargentates should be high-temperature superconductors.
The discovery of materials that lose their electrical resistance at temperatures near absolute zero ( -273 °C) and thus transport current nearly without loss, raised high hopes. Just the thing for making such things as cables and batteries! However, the energy demands and expense of the required cooling rendered conversion to such a system unprofitable. A new euphoria thus arose in the 1980’s, when high-temperature superconductors were developed. High-temperature being a relative term in this case, materials were considered to be high-temperature superconductors if they became superconductors above -196 °C, the temperature of liquid nitrogen. The hope for a material that is superconducting at room temperature was never fulfilled, however. The world-record temperature has thus far been held by ceramic copper-oxygen compounds called oxocuprates, which become superconducting at -109 °C.
Hoffmann and Grochala are now betting on true exotics: fluoroargentates. The unusual compounds of fluoride and silver (argentum in Latin) which they think are good candidates for high temperature superconductors do not contain common silver ions (of the type one finds in photographic material), but unusual and highliy reactive silver ions, more deficient in electrons. How have they arrived at these compounds in particular? "The electronic properties of fluroargentates closely resemble those of the oxocuprates," explain the researchers. "their crystal lattices can be made to vibrate with equal ease." Vibrations of the lattice play an important role in superconductivity. An electron wandering within the lattice can slightly distort its position. This deformation attracts a second electron. The second electron again causes a deformation of the lattice, thus in turn attracting the first electron. The electron pairs that are formed in this way are the secret to superconductivity, according one of the mechanisms of how the phenomenon functions.
"Unfortunately, verifying our prediction may not happen quickly, because fluoroargentates are distinctly difficult to synthesize," the two chemists caution against exaggerated expectations. But who knows, perhaps the knowledge gained through this will bring us one step closer to room-temperature superconductors?