INFLUENCE OF HOLE FILLING BY HF AND HOLE DOPING BY CA ON THE SUPERCONDUCTIVITY AND FLUX-PINNING OF YBA2CU3O7-DELTA

The structural and superconducting properties of (Y1-x-yCayHfx)Ba2Cu3O z samples are investigated using resistivity, ac susceptibility, and d c magnetization measurements. The effect of increasing the Hf concentr ation in Y1-xHfxBa2Cu3Oz lowers the oxygen content and decreases T(c). The substitution of Ca for Y in (Y0.85-yCayHf0.15)Ba2Cu3Oz provides p roper matching between the ionic radius and valence of Y3+ (0.89 angst rom) 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) increa ses from y = 0 (74 K) to y = 0.15-0.20 (compensated oxides) having T(c ) of 84.5-85.5 K. Magnetization measurements on powdered Hf-doped samp les exhibit pronounced M(H) hysteresis loops indicative of extensive f lux pinning and enhanced critical currents.