Structural, electrical and magnetic properties of (Y1-xCax)Ba2Cu3-yMyO7-d superconducting compounds, where M = Ni, Al, Fe or Cr

Polycrystalline samples of (Y1-xCax)Ba2(Cu3-yMy)O7-d with M Ni, Al, Fe or Cr, where x ≅ 0.30 and y = 0.25, were prepared by solid state reaction. X-ray powder diffraction has been used to check the crystal structure and to find the lattice parameters of the samples. Electrical resistivity, dc and ac magnetic susceptibility measurement showed that the Tc values decrease for the samples with x = 0, as the level of doping, y, increases from 0.0 to 0.25. As the Ca content increases, the Tc increases. For the Ni doped samples, the conduction mechanism of electrons appears to follow a crossover from the purely metallic regime to the localization regime. The results are explained on the basis of hole localization and magnetic pair-breaking mechanism. For the Al doped samples the compensation effect is not so evident possibly, due to the impurities in the system. The disorder created in the system lowers the critical current. For the Cr doped samples, as the Ca content increases, the Tc increases, and a strong decrease of transition width was evidenced. For the Fe doped samples, as the Ca content increases, the Tc increases, and also and increase of the transition width was evidenced. The co-substitution of Ca tends to counteract the effect induced by Ni, Al, Fe or Cr by doping holes and create disorder into CuO2 planes. (authors)