Abstract
(Tl
0.5Pb
0.5)Sr
2Ca(Cu
2−
x
M
x
)O
7 (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 (Tl
0.5Pb
0.5)Sr
2Ca(Cu
2−
x
M
x
)O
7 system (M=Co or Ni), the superconducting critical temperature
T
c decreases linearly with both Co and Ni concentrations and the rate of
T
c decrease is around −6.5 and −7.0
K/at%, respectively. For (Tl
0.5Pb
0.5)Sr
2Ca (Cu
2−
x
Zn
x
)O
7 system, the dependence of
T
c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses
T
c much less (−2.5
K/at%) than the Co and Ni substitutions. The suppression in
T
c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of
T
c 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
d
x
2
−
y
2
state due to the full d band of Zn ions.