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

Abstract

The crystal structure of the high- T c Bi 2- x (Ca, Sr) 3 Cu 2+ x O 9- y with 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 Bi 4 Ti 3 O 12 . The double perovskite unit interleaved by the double bismuth layers is B-type, i.e. a CuO 6 octahedron is surrounded by eight alkaline earth metal ions. The pyramidal CuO 5 configuration is found in this oxide. The copper valency is estimated at about 2.3 on the basis of the determined chemical composition.