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 Bi₂Sr₂CuO _~6 composition corresponding to the n=1 phase in the Bi₂Sr₂Ca_n-1Cu_nO_2n+4 system are described. The samples Bi_2.1Sr_1.9CuO_y and Bi_2.2Sr_1.8CuO_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 Bi₂Sr_2-xLa_xCuO_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-xPb_xSr_2-xR_xCuO_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-xPb_xSr₂CuO _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.