Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7−δ [M = Mo, Hf ]

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The structural and superconducting properties of (Gd1−x−yCayMx)Ba2Cu3Oz with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd1−xMox)Ba2Cu3Oz changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in Tc at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd1−xHfx)Ba2Cu3Oz, which maintains the orthorhombic structure and decreases Tc very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of Tc by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases Tc with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd1−x−yCayMx)Ba2Cu3Oz indicates that the valence of the dopant M = Mo4+,6+, Hf4+ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.

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