Abstract
A new series of Sr-based Hg-1212 superconducting cuprate (Hg
1−
y
Se
y
)Sr
2(Y
1−
x
Ca
x
)Cu
2O
6+
δ
(
y=0.25; 0.0⩽
x⩽0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr
2(Y
1−
x
Ca
x
)Cu
2O
z
prepared by the citrate sol–gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg
1−
y
Se
y
)Sr
2YCu
2O
6+
δ
;
y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg
0.75Se
0.25)Sr
2(Y
1−
x
Ca
x
)Cu
2O
6+
δ
samples. Superconductivity is observed only for samples with
x⩾0.3 and
T
c increases with increasing Ca content as well as O
2-annealing. A maximum
T
c(onset) of 85
K is found in the (Hg
0.75Se
0.25)Sr
2(Y
0.3Ca
0.7)Cu
2O
6.84 sample annealed in an oxygen atmosphere. The structure of O
2-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal
(
1
2
,
1
2
,
0
)
site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of
x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them.
A series of Sr-based Hg-1212 superconducting cuprate (Hg
1−
y
Se
y
)Sr
2(Y
1−
x
Ca
x
)Cu
2O
6+δ have been synthesized using highly homogenous and reactive precursor Sr
2(Y
1−
x
Ca
x
)Cu
2O
z
prepared by the citrate sol–gel process. The superconductor ceramics obtained from the sol–gel precursor was found to be so homogenous that the superconducting transition was steeper than the ceramic sample prepared by the solid-state reaction (SSR) method.