EFFECT OF HF AND HF-CA SUBSTITUTION ON THE SUPERCONDUCTIVITY OF ERBA2CU3O7-DELTA

The structural and superconducting properties of (Er1-x-yCayHfx)Ba2Cu3 Oz samples are investigated using X-ray diffraction, resistivity, AC s usceptibility and oxygen-content measurements. The effect of increasin g the Hf concentration in Er1-xHfxBa2Cu3Oz lowers the oxygen content a nd decreases T-c which is attributed to hole filling by Hf. The substi tution of Ca for Er in (Er0.85-yCayHf0.15)Ba2Cu3Oz provides a proper m atching between the ionic radius and valence of Er3+ (0.88 Angstrom) a nd the average ionic radius and valence of Hf4+ (0.78 Angstrom) and Ca 2+ (0.99 Angstrom). As the Ca content increases, the T-c increases fro m 70 K (y = 0.05) to 86 K (y = 0.25, compensated oxide), closer to the value of 91 K for pure ErBa2Cu3O7-delta due to the balance between th e hole filling by Hf and hole doping by Ca.