Classification and analysis of the non stoichiometry features in the layered lead and thallium cuprates

Abstract

The numerous and varied non stoichiometry features occuring in the high Tc super- conductors are directly generated by the own nature of the building principles of these structures i.e. the intergrowth of rock salt-type layers and oxygen deficient perovskite slabs. The coexistence of sev- eral cations (sometimes up to six) in the matrix, the possibilities of mixed valencies for some of these cations (Cu-Bi-Tl-Pb-Pr) and the possible occupancy of the crystallographic sites by several cations simultaneously are undoubtly factors susceptible to favor their appearance. Is the acquaintance of these numerous phenomena only a microscopist hobby ? The importance of the microstructural char- acterization has been widely proved in different stages of the superconductor studies: the existence of extended defects can indeed hinder an accurate interpretation of the physical properties, the survey of the structural mechanisms can open the road to the discovery of new phases and can be an essential help in the optimization of these materials. This paper deals with the classification of the phenomena which can be observed and the analysis of the structural mechanisms which are involved in the lead and thallium cuprates; owing to their particular behavior, bismuth cuprates will be only considered as a comparison.