Multiple Bi2Sr2−xBaxCuOy microstructures and the effect of element doping (Ba,La,Pb) on the 2:2:0:1 phase

Abstract

The microstructure of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{x}}$ ${\mathrm{CuO}}_{\mathit{y}}$ (0≤x≤0.5) was characterized by transmission-electron microscopy and x-ray diffraction. Electron-diffraction analyses reveal that in the Bi-Sr-Cu-O system there also exists a c=12.5 Å insulating phase with commensurate superstructure with the exception of the c=24 Å 2:2:0:1 phase. However, with the partial substitution of Ba for Sr in this material, the c∼24 Å 2:2:0:1 single phase is obtained, but Ba intercalation reduces the wavelength of modulation of the 2:2:0:1 phase along the b and c directions. Measurement of the superconductivity of the ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ba}}_{\mathit{x}}$ ${\mathrm{CuO}}_{\mathit{y}}$ samples shows that the appropriate substitution of Ba for Sr (x≤0.2) can raise the ${\mathit{T}}_{\mathit{c}}$ of the 2:2:0:1 phase. ${\mathit{T}}_{\mathit{c}}$ reaches 20 K for the x=0.2 sample, however, with excessive Ba substitution (x≥0.2) the metal-to-insulator transition for the 2:2:0:1 phase occurs and the superconductivity of the 2:2:0:1 phase disappears as x exceeds 0.3. In addition, the influence of La and Pb doping on the modulation structure of the 2:2:0:1 phase is also investigated in this article. We discuss the modulation structures of the three substituted systems and the origin of the incommensurate-modulation structure of the Bi-Sr-Cu-O system.