Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7-delta

The structural and superconducting properties of (Gd1-x-yCayHfx)Ba2Cu3Oz samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd1-xHfx)Ba2Cu3Oz lowers the oxygen content and decreases T-c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd0.85-yCayHf0.15)Ba2Cu3Oz provides proper matching between the ionic radius and valence of Gd3+ (0.94 Angstrom) and the average ionic radius and valence of Hf4+ (0.78 Angstrom) and Ca2+ (0.99 Angstrom). As the Ca content increases, the T-c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa2Cu3O7-delta due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R1-xHfx)Ba2Cu3Oz (R=Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T-c. (C) 1997 Elsevier Science Ltd.