Chromium clustering and ordering in Hg1-xCrxSr2CuO4+delta

Solid-solution compounds where Cr, Re, and other metals are substituted for Hg in Hg-Ca-Ba-Cu-O superconductors have been reported to exhibit enhanced flux-pinning behavior. We have determined the structural modifications resulting from the incorporation of Cr in Hg1-xCrxSr2CuO4+delta (for x approximate to 0.4) using neutron powder diffraction, electron diffraction, and lattice imaging. Cr substitutes at the Hg site, but is displaced to allow tetrahedral coordination by oxygen atoms. Additional oxygen is incorporated to provide four oxygen atom neighbors for each Cr atom. These CrO4 units cluster to form a supercell of approximate dimensions 5aX5aX2e in which Cr-rich and Hg-rich regions alternate in all three crystallographic directions. Because the Cu-O apical bond associated with the CrO4 unit is lengthened to 3.13 Angstrom, the superconducting planes are best viewed as consisting of CuO5 pyramids, oriented up or down as dictated by the supercell ordering, rather than CuO6 octahedra. Local structural constraints associated with individual Cr sites require that considerable disorder is present, even in the supercell. Extended defects in this supercell (e.g., columns of CuO6 octahedra associated with Hg-rich regions) may contribute to the enhanced flux pinning.