Microstructure, lattice parameters, and superconductivity ofYBa2(Cu1−xFex)3O7−δfor0≤x≤0.33

Abstract

Iron was substituted for copper in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for 0\ensuremath{\le}x\ensuremath{\le}0.33. Superconducting transition temperatures and lattice parameters have been determined as a function of Fe content. The orthorhombic distortion \ensuremath{\Delta}${a}_{0}$=2(b-a)/(a+b) decreased to zero near x=0.03. Beyond x=0.03 the structure appears tetragonal. For compositions 0\ensuremath{\le}x\ensuremath{\le}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 \ensuremath{\sim}200 nm for x=0 to \ensuremath{\sim}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 \ensuremath{\sim}1 nm for the pure to \ensuremath{\sim}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.