Crystallography, chemistry and structural disorder in the new high- T_c Bi–Ca–Sr–Cu–O superconductor

Abstract

High-temperature superconductivity with onset temperatures up to ∼120K has been observed recently in the Bi–Ca–Sr–Cu–O system^1,2. The primary high-temperature superconducting phase has been identified by Hazen et al.² as a layer structure, probably related to the 22-K superconducting phase recently described by Michel et al³ and to the family of layered bismuth compounds described by⁴. Hazen et al. described the unit cell, approximate composition, and electrical properties of the Bi–Ca–Sr–Cu–O superconducting phase. Diffraction experiments indicate that the new Bi–Ca–Sr–Cu–O layer-structure superconductor possesses a primitive orthorhombic unit cell with probable space group Pnnn. The material exhibits severe structural disorder, which is primarily related to stacking within the layers. The apparent orthorhombic strucure is an average resulting from orthorhombic material mixed with monoclinic domains in two twinned orientations. We also describe two distinct types of structural disorder that are common in materials synthesized to date. This disorder complicates the crystallographic analysis and suggests that X-ray and neutron diffraction methods may yield only an average structure.