CHROMIUM CLUSTERING AND ORDERING IN HG1-XCRXSR2CUO4+DELTA

Solid-solution compounds where Cr, Re, and other metals are substitute d for Hg in Hg-Ca-Ba-Cu-O superconductors have been reported to exhibi t enhanced flux-pinning behavior. We have determined the structural mo difications resulting from the incorporation of Cr in Hg1-xCrxSr2CuO4delta (for x approximate to 0.4) using neutron powder diffraction, ele ctron diffraction, and lattice imaging. Cr substitutes at the Hg site, but is displaced to allow tetrahedral coordination by oxygen atoms. A dditional oxygen is incorporated to provide four oxygen atom neighbors for each Cr atom. These CrO4 units cluster to form a supercell of app roximate dimensions 5aX5aX2e in which Cr-rich and Hg-rich regions alte rnate in all three crystallographic directions. Because the Cu-O apica l bond associated with the CrO4 unit is lengthened to 3.13 Angstrom, t he superconducting planes are best viewed as consisting of CuO5 pyrami ds, oriented up or down as dictated by the supercell ordering, rather than CuO6 octahedra. Local structural constraints associated with indi vidual Cr sites require that considerable disorder is present, even in the supercell. Extended defects in this supercell (e.g., columns of C uO6 octahedra associated with Hg-rich regions) may contribute to the e nhanced flux pinning.