Synthesis, structural and transport properties of (Hg,Fe)-1212 superconducting cuprate

A new series of barium-free 1212-type superconducting mercury-based cuprates involving iron, (Hg1−yFey)Sr2(Y1−xCax)Cu2O7−δ (y = 0.5; 0.0 ⩽ x ⩽ 1.0), has been synthesized through the citrate-gel process and the sealed quartz tube method. Their lattice parameters, electrical resistivity, dc magnetic susceptibility, thermoelectric power, oxygen content and copper valence have been systematically investigated as a function of Ca content, x. X-ray diffraction studies revealed that both Fe and Y are necessary for the stabilization of the 1212-type Hg/Sr cuprates. For the samples with x = 0.3-0.6, Tc increased from 30 K to 82 K with increasing x, while the samples with x = 0.6-0.8 exhibited a Tc decrease from 82 K to 69 K. This result indicates that the doping state of the samples can be extensively varied from underdoped to overdoped states by the Ca substitution. This variation in the doping states was supported by the systematic change in the temperature dependence of electrical resistivity, dc magnetic susceptibility and thermoelectric power. Samples in the underdoped regime clearly exhibit the anomaly in the resistivity and thermoelectric power probably associated with the formation of normal-state gap. Oxygen analysis showed that the copper valence gradually increased from 2.02 to 2.26 with increasing x from 0 to 0.8. The copper valence with the maximum Tc of 82 K in these samples was determined to be 2.18, which is comparable to the value for the optimum Tc in other high-Tc cuprates.