Co and Fe substitution in YBa₂Cu₃O_7−δ

The limiting substitution of Co or Fe for Cu in the orthorhombic superconducting phase YBa₂Cu₃O_7−δ yields the tetragonal, semiconducting compositions YBa₂Cu₂CoO_7.25 and YBa₂Cu_2.5 Fe_0.5O_7.19 after annealing at 400°C and slow cooling in O₂. Single-crystal x-ray structural analyses, conductivity and magnetic-susceptibility measurements, and thermogravimetric analysis (TGA) data provide evidence for Co or Fe substitution at the Cu (1) position and intercalation of peroxide ions (O₂)²⁻ The two compositions decompose for disintercalation of oxygen beyond O_6.7 for Co and O_6.4 for Fe when heated in N₂. The Cu(2) atoms of tetragonal YBa₂Cu₄O₆ appear to be antiferromagnetic below a T_n ∼ 240 K, which is similar to comparable Cu–O planar nets in stoichiometric La₂CuO₄.