Structural and physical properties of the metal (M) substituted YBa2Cu3-xMxO7-y perovskite

The mixed YBa2Cu3-xMxO7-y (M=Ni, Zn, Fe, Co, and Al) phases have been characterized for their structural, magnetic, and superconducting properties. The oxygen content in these phases is dependent on the nature and the amount of doping, especially for Co and Fe. The material remains orthorhombic when Cu is replaced by Ni or Zn whereas it evolves to tetragonal symmetry for the Fe-, Co-, and Al-doped compounds when x exceeds 0.05. Evidence for the major substitution of Co in the Cu-O chains only is obtained by means of thermogravimetric analysis and neutron diffraction measurements. The room-temperature Mössbauer spectra of the Fe-doped compounds consist of three doublets; their site assignments are proposed. dc resistance and ac susceptibility have shown that both magnetic and diamagnetic ions destroy Tc in a similar manner. At x=0.2 the Fe and Co compounds are tetragonal, superconduct at 50 K, and show a Curie-type magnetic behavior associated with a magnetic moment of 3.4B per doping atom. The origin of the orthorhombic-tetragonal transition and the importance of the Cu-O chains for superconductivity is discussed. The behavior of these materials with respect to magnetic impurities is apparently different from conventional Bardeen-Cooper-Schrieffer-type superconductors, and we believe that any new mechanism proposed must be mostly sensitive to local structural disorder.