Composition Effects on the Formation and Superconducting Character of c=31 Å and c=37 Å Phases in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O Systems. X-Ray and ESR Analysis

In this paper we examine the effect of starting composition on the formation of the two crystallographic phases characterized by c≈31 Å and c≈37 Å in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems. The superconducting properties have been studied by field-modulated microwave non-resonant absorption. The c≈37 Å phase was completely isolated in the Pb-containing system and shows a quite sharp superconductive transition at 105 K, well evidenced by the microwave absorption profile as a function of temperature. For the c≈31 Å phase, obtained in a wide range of composition, the ESR analysis points out a more complex behaviour, with a superconductive transition taking place in the 75-110 K temperature range. Strong contribution to the 110 K transition has been observed in samples showing no evidence in the X-ray spectra of the typical c≈37 Å reflections, nor of peak broadening due to stacking faults. A possible correlation of this phenomenon with the partial substitution of Bi in the Bi2O2 double layers by lighter Ca, Sr or Cu ions is discussed.