ABSTRACT
In this chapter, the authors focus on a study of various extended defects-intergrowths, order-disorder phenomena, cation nonstoichiometry, and oxygen nonstoichiometry-in connection with their influence on superconductivity. In reality, they have represented idealized structures as well as idealized formulas. The main difference between stoichiometric and the thallium-deficient phase deals with the existence of modulations of the structure. This difference between stoichiometric and thallium-deficient samples may explain the disaccord regarding the existence of modulations that arose among the various authors at the beginning of studies of thallium cuprates. The incommensurate satellites are systematically associated with the existence of more or less extended areas where the contrast is modulated. This suggests that the intensity of the satellites, which varies from one crystal to another, is directly correlated with the extent of the modulated zone.
