RELATIONSHIPS BETWEEN COMPOSITION, OXYGEN NONSTOICHIOMETRY, STRUCTURE MODULATION AND SUPERCONDUCTIVITY IN THE 2212 BISMUTH CUPRATES

The correlations between critical temperature, modulation vector q and oxygen non-stoichiometry have been studied for three series of compositions belonging to the 2212 bismuth cuprates. For this exploration three samples were prepared in different conditions: the pure phases Bi2Sr2CaCu2O8 (synthesized in argon), Bi2.1Sr1.9CaCu2O8+delta (synthesized in air), and the mixture Bi2Sr2CaCu2O8+delta (synthesized in air). Oxygen intercalation and disintercalation were performed and the q vector and critical temperature were measured. In each series of oxides, the q vector decreases as the oxygen content delta increases. The comparison of the different series also shows that the Bi/Sr ratio affects q. It is also demonstrated that, if the oxygen non-stoichiometry can dramatically influence T(c)s, it is not the only factor, the cationic composition also playing a role. The importance of the purity of the phases for the physical interpretation is discussed, taking into account the fact that the air synthesized "Bi2Sr2CaCu2O8+delta" composition is not a single phase and may correspond to various members of the non-stoichiometric phase Bi2+xSr2-xCaCu2O8+delta.