STRUCTURAL MODULATION, OXYGEN-CONTENT AND TRANSPORT-PROPERTIES IN BI2.13SR1.87CUO6+GAMMA AND BI2.05SR1.54LA0.41CUO6+GAMMA SUPERCONDUCTORS

A neutron diffraction study, electrical conductivity and thermopower m easurements have been performed on single-crystal superconductors of t he 2201-type-Bi2.13Sr1.87CuO6+y with T(C)=6 K (gamma=0.17) and Bi2.05S r1.54La0.41CuO6+gamma with T(c)=4-11 K (gamma=0.28-0.33). The crystals exhibit an ''oblique'' modulation with wave vectors tau=[0.198, 0, 0. 36] and tau=[0.221, 0, 0.75], respectively. The structure determinatio n of the La-free compound located two extra oxygen sites in the BiO pl ane with periodicity approximately 5a. One is inserted into the Bi-O-B i- ribbons and the second one between the ribbons, similarly as observ ed earlier in Bi2Sr2.4Y0.6Cu2O8.5. The modulation exhibits a phase shi ft between bismuth double layers, which is a consequence of interlayer Bi-O links and oxygen vacancies in the SrO planes. The measurements o f transport properties relate the occurrence of the superconductivity to an interplay of two effects: (1) generation of charge carriers due to oxygen insertion and (2) low-temperature localization due to bismut h cations in the strontium sites.