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Keywords:

  • superconductivity;
  • mixed valence;
  • SDW;
  • CDW;
  • vibrational softening

Abstract

A “chemical” model based on the theory of mixed valence systems is successful in explaining a number of properties of superconductors. The model is formulated in configuration space but is consistent with Bardeen–Cooper–Schrieffer theory, and connects directly to chemical properties that may be calculated by quantum chemical methods. Crystal structure is important, in particular the difference in equilibrium geometry between different oxidation states. According to the model, high pressure causes less difference between the oxidation states and hence a lower, or possibly disappearing, activation barrier and increased conductivity. This agrees with the experiments. Mixed valence systems with nonequivalent sites or with a large structural difference between the oxidation states are not superconducting or even conducting. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002