Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028–3.8040 Å and c=12.0748–12.1558 Å. In (Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 system (M=Co or Ni), the superconducting critical temperature Tc decreases linearly with both Co and Ni concentrations and the rate of Tc decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl0.5Pb0.5)Sr2Ca (Cu2−xZnx)O7 system, the dependence of Tc on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses Tc much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in Tc in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of Tc in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu dx2−y2 state due to the full d band of Zn ions.