Microstructure, lattice parameters, and superconductivity of YBa₂(Cu_1-xFe_x)₃O_7-δ for 0≤x≤0.33

Iron was substituted for copper in YBa₂(Cu_1-xFe_x)₃O_7-δ for 0≤x≤0.33. Superconducting transition temperatures and lattice parameters have been determined as a function of Fe content. The orthorhombic distortion Δa₀=2(b-a)/(a+b) decreased to zero near x=0.03. Beyond x=0.03 the structure appears tetragonal. For compositions 0≤x≤0.15 the superconducting critical temperature T_c decreased smoothly from T_c=90 K to below 4.2 K. Transmission electron microscopy (TEM) revealed that the spacing between the twin boundaries decreased from ∼200 nm for x=0 to ∼20 nm for x=0.02 before the twins disappeared for x>=0.03. The sharp streaks in TEM diffraction patterns were interpreted as due to the twin boundary layers whose thickness varied from ∼1 nm for the pure to ∼2 nm for the Fe (x=0.02) containing specimen. A tweed structure in the TEM image and associated diffuse streaks along the 〈110〉 directions in electron diffraction pattern appeared for x>0.015. This observation suggests that the structure consists of fine (<5 nm) orthorhombic domains, each domain having the twinlike crystallographic relation with its neighbors. Rietveld refinement of powder neutron diffraction for x=0.10 and 0.167 determined that Fe substituted primarily on the Cu ‘‘chain’’ site, and for every two atoms of Fe substituted, approximately one extra oxygen is incorporated in the Cu-O plane.