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 s tructure 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 indi cate a high hole concentration (p approximate to 0.4) in the CuO2 planes, w hich is inconsistent with the previously observed underdoped transport prop erties (p approximate to 0.1). This may reflect extensive hole trapping by the Ru moments. [S0163-1829(99)02834-9].