A CHEMICAL APPROACH TO HIGH-TC SUPERCONDUCTIVITY

A concise review is given on a new chemical approach to high T(c) superconductivity recently proposed by the present author, the multivalence resonance-condensation (MVRC) model. The MVRC state of superconductivity is a Bose condensate of the resonance hybrids between two kinds of multivalence states of the chemical species (Cu1,2,3+, Bi3,4,5+, etc.) responsible for the superconductivity, which is realized macroscopically through the self-organization (valence fluctuation-valence ordering-resonance-condensation) process of the electron system as the fundamental chemical bonding force. This is demonstrated to provide a possible unified picture of the superconducting state of various high T(c) systems for both the so-called "hole" and "electron" type superconductors in a symmetrical manner. In this paper, through the analysis of oxygen nonstoichiometry dependence of the superconductivity of YBa2Cu3O7-x, these main features of the model are delineated. emphasizing the importance of the combined defect-chemical and quantum-chemical considerations on the mixed (multi) valence of high T(c) systems.