Structural effect of bismuth substitution in Bi2-xMxSr2Ca2Cu3Oz (M = Pb, Cd, Sn, Ge, Sb) systems

Samples belonging to Bi2-xMxSr-2Ca-2Cu-3Oz systems were initially obtained as glasses by quickly undercooling from 1200 deg. C to room temperature. Bi was partially substituted by Pb, Cd, Sn, Ge or Sb. In these glass samples crystalline phases were developed by heat treatments applied between 840 deg.C and 850 deg. C for different times from 24 hours to 170 hours in several stages. The heat treated samples Bi1.8Pb0-.2Sr2Ca2CunOz and Bi1.9Sn0.1Sr2Ca2Cu3Oz exhibit superconducting behaviour at 85 K and 115 K, respectively. X-ray diffraction patterns show that the Bi1.8Pb-0-.2Sr2Ca2CunOz sample is a 2221 phase and Bi1.9Sn0.1Sr2Ca2Cu3Oz is a 2223 phase. As the Sn content is increased to 0.2 (Bi1.-8Sn0.2Sr-2Ca2Cu3Oz sample) the superconducting properties disappear, at least above the liquid nitrogen temperature. An interesting structural effect obtained for this sample consists in the splitting of certain reflections denoting the transition tendency from the tetragonal system to the orthorhombic one. This tendency may be explained by the ionic radius of Sn5+ ion (0.71 A) which is very different from that of Bi3+ ion (1.2 A). (authors)