Structural Study on High-T_cSuperconductor Bi_2-x(Ca, Sr)₃Cu_2+xO_9-y

The crystal structure of the high-T_cBi_2-x(Ca, Sr)₃Cu_2+xO_9-ywith x=0.2 and y=0.78 was studied by means of neutron and X-ray powder diffraction. Since the unit cell parameters were relatively large for powder diffraction analysis, an averaged structure was refined, assuming an orthorhombic sub-cell (a=5.39 Å, b=5.39 Å and c=30.37 Å) which corresponds to 1/5 of the unit cell: This is a derivative of Bi₄Ti₃O₁₂The double perovskite unit interleaved by the double bismuthlayers is B-type, i.e. a CuO₆octahedron is surrounded by eight alkaline earth metal ions. The pyramidal CuO₅configuration is found in this oxide. The copper valency is estimated at about 2.3 on the basis of the determined chemical composition.