  (Pb0.5Cu0.5)(Sr0.5La0.5)2CuOz (abbreviated as (Pb,Cu)-"1-2-0-1") with
superconducting transition temperature (Tc) of 25 K is a member (n = 1) of one
of the homologous series of cuprate superconductors,
(Pb4+,Cu2+)(Sr2+,Ln3+)2(Y3+,Ca2+)n-1Cu2+nO2-2n+3 (n = 1-4; Ln: lanthanoid
elements). For the (Pb,Cu)-"1-2-0-1", substitution effects of 3d transition
metal elements M (M = Fe, Co, and Zn) for the Cu site in the (Pb,Cu)-O
charge-reservoir layer (labelled as Cu(1)) are systematically investigated.
Because Fe, Co and Ni ions exist as divalent or trivalent in ionic crystals,
the Sr2+/La3+ ratio in the (Sr,Ln) site is adjusted to satisfy charge
neutrality, assuming that they are in a trivalent state. This results in the
successful synthesis of new materials with nominal compositions of
(Pb0.5M0.5)(Sr0.75La0.25)2CuOz (M = Fe and Co). This observation suggests that
Fe and Co are trivalent in "1-2-0-1". For M = Zn, the nominal composition of
(Pb0.5Zn0.5)(Sr0.5La0.5)2CuOz was found to yield a nearly single "1-2-0-1"
phase. Temperature dependence of electrical resistivity and magnetization were
measured, and superconductivity was confirmed only for the case of M = Zn with
a Tc of 19.7 K. For these three materials, the distribution of Fe, Co and Zn
between Cu(1) and another Cu site in the Cu-O2 plane labelled as Cu(2) was
investigated employing transmission electron microscopy, which showed that Fe,
Co, and Zn occupy both the Cu(1) and Cu(2) sites.
