Effect of La-doping on growth and superconductivity of Bi-2212 crystals

Single crystals of the layered superconductor Bi2Sr2(Ca1-xLax)Cu2O8+d were successfully grown out by a self-flux method. Upon La-doping, the c-axis contracts but the structure remains stable up to the critical concentration xc, above which the crystals contain the pure Bi2(Sr,La)2CuO6 phase. The reduction of the superconducting transition temperature follows a linear relation, Tc(x)=Tc(0) (1-x/xc), with xc=0.3 being consistent with the structure data. This and a comparison with literature data on Pr3+- (xc>>0.65) and Y3+-doping (xc=1) indicate an intimate relation between structural and superconducting stability in the 2212 phase. Besides the widely discussed hole-filling effect, the structural instability due to the substitution of large La3+ for Ca2+ is considered to be another contribution to Tc suppression in this system.