CRYSTAL-CHEMISTRY OF BISMUTHATE-BASED SUPERCONDUCTORS - THE ORIGIN OF(LOCAL) CHARGE DISPROPORTIONATION

The crystal structures of the superconducting BaPb1-xBixO3 and Ba1-xKx BiO3 systems are summarized and the role of lead or potassium substitu tion is discussed. It is pointed out that the metal-insulator transiti on in these systems can readily be explained with a model based on the competition between (conduction) bandwidth and charge disproportionat ion. In the context of this model, the origin of the charge disproport ionation of Bi (whether long-range or local ordering of Bi3+ and Bi5+) can be understood. This qualitative model may also explain the negati ve results of other bismuth-doped systems, providing a useful test for experimental challenges.