Superconductivity of Bi-2201 (n=1) as influenced by the substitution of Pb and/or rare-earths (R=La, Nd and Pr)

Abstract

Preparation conditions for stabilizing the bulk superconductivity (T c=9.5 K ) in the nominal Bi 2 Sr 2 CuO ∼ 6 composition corresponding to the n=1 phase in the Bi 2 Sr 2 Ca n−1 Cu n O 2n+4 system are described. The samples Bi 2.1 Sr 1 .9 CuO y and Bi 2.2 Sr 1.8 CuO y become superconducting at 10 K and 12 K, respectively. Single-phase compounds (space group Amaa) could be synthesized for the range 0⩽x⩽1.0 in the series Bi 2 Sr 2−x La x CuO 6+y but the orthorhombicity decreased gradual ly with increase in x and the structure became tetragonal at x=1.0. The T c (R=0) value increased to a maximum of 19 K for x=0.4, corresponding to a value of ∼2.2 for the formal valence of copper. For x⩾0.5 a sudden loss of superconductivity is observed. Simultaneous substitution of Pb for Bi, and, La for Sr yields further improvement in superconducting characteristics, hitting a peak of 26 K (R=0) for x=0.4 in the series Bi 2−x Pb x Sr 2−x R x CuO y (R=La). The R=Nd- and and Pr-s ubstituting compounds showed lower values of T c for the same extent of doping (x=0.4). Interestingly, doping of Pb alone (i.e., Bi 2−x Pb x Sr 2 CuO y ) results in suppression of superconductivity altogether. The electrical resistivity, magnetic susceptibility and the cell parameter data are presented for varying levels of different substituents and discussed.