Structure and microstructure of the ferromagnetic superconductor RuSr₂GdCu₂O₈

Two samples of RuSr2GdCu2O8, in which ferromagnetism and superconductivity coexist at low temperatures, have been studied by high-resolution electron microscopy and synchrotron x-ray-diffraction methods. The average crystal structure is tetragonal at both 10 and 295 K, but a superstructure resulting from coherent rotations of the RuO6 octahedra within subdomains of 50-200 Angstrom are observed by electron diffraction. The Cu-O bond distances indicate a high hole concentration (p approximate to 0.4) in the CuO2 planes, which is inconsistent with the previously observed underdoped transport properties (p approximate to 0.1). This may reflect extensive hole trapping by the Ru moments. [S0163-1829(99)02834-9].