On electron microscopic studies of structural characteristics of the Tl(Bi, Pb)SrCaCuO high-temperature superconductor

Abstract

The Tl(Bi, Pb)SrCaCuO HTSC system corresponding to nominal compositions Tl2-(x+y)BixPbxSr2Ca2Cu3Oz (ifx=0.75, y=0.25; x=0.5, y=0.5; x=0.25, y=0.75 and x=0.25, y=0.25) is synthesized and has been shown to possess a Tc ranging from 77 K to 115 K. The TEM exploration revealed the changes in the structural characteristics with variation of the compositions of Tl(Bi, Pb). For higher Bi concentration a new type of modulated phase with ten times modulation together with the usual incommensurate modulation of the type Bi:2212 is obtained. As the concentration of Bi decreases and of Pb increases the system tends towards the stabilisation of 1223 phase. The density of 1223 phase is found to be maximum for the material synthesized with nominal composition Tl1.0Bi0.5Pb0.5Sr2Ca2Cu3Oz corresponding to which the maximum Tc of ∼ 115 K is obtained. TEM studies for this composition also revealed the presence of an intergrowth type of structure. For higher Pb content only 1212 type of structure was found to be present. It is thought that the substitution of Bi and Pb at the Tl-site in the TlSrCaCuO system leads to the optimization of carrier concentration and hence enhancement in Tc. © 1992.