A MONOCLINIC FORM OF THE 7TH MEMBER OF THE TUBULAR OXIDES (BI2+XSR2-XCUO6)NSR8-XCU6O16+Y

A new monoclinic phase of the BiSrCuO system has been observed by X-ray and electron diffraction techniques in samples of the n = 7 tubular phases; the cell parameters are close to a ≈ap √2 ≈ 5.4Å, b ≈ 24.7Å, c = 24.65Å, and α ≈ 96°, B centered. The high resolution electron microscopy study allowed a model to be proposed for this new phase. The structure is directly related to the orthorhombic tubular phases by a mechanism of shifting of the [(Bi, Sr)O] layers; it results in a double intergrowth along b and c of orthorhombic and monoclinic members. The mechanism is compared to those observed in the 2201 superconductor oxides. © 1992.