EFFECT OF M AND M-CA SUBSTITUTION ON THE STRUCTURE AND SUPERCONDUCTIVITY OF GDBA2CU3O7-DELTA [M = MO, HF]

The structural and superconducting properties of (Gd1-x-yCayMx)Ba2Cu3O z with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasin g the Mo concentration in (Gd1-xMox)Ba2Cu3Oz changes the structure fro m orthorhombic to tetragonal accompanied by a large increase in resist ivity and a fast decrease in T-c,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 T-c,very slowly at the rate o f 0.6 K per atm.% of Hf with nearly no change in resistivity. The supp ression of T-c, by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases T-c,with increasing Ca content showing maximum com pensation for Mo. A comparative study of M = Mo, Hf doped samples in ( Gd1-x-yCayMx)Ba2Cu3Oz indicates that the valence of the dopant M = Mo- 4+,Mo-6+ Hf4+ and its ionic radius play an important role in controlli ng the structural and superconducting properties of the systems.