J. Phys. I France
Volume 1, Numéro 5, May 1991
Page(s) 721 - 741
DOI: 10.1051/jp1:1991165
J. Phys. I France 1 (1991) 721-741

The crystal structure of RESr $\mathsf{_{2}}$GaCu $\mathsf{_{2}}$O $\mathsf{_{7}}$

G. Roth1, P. Adelmann1, G. Heger2, R. Knitter3 and Th. Wolf4

1  Kernforschungszentrum Karlsruhe, Institut für Nukleare Festkörperphysik, P.O. Box 3640, D-7500 Karlsruhe, Germany
2  Laboratoire Léon Brillouin laboratoire commun CEA-CNRS, CEN Saclay, F-91191 Gif-sur-Yvette, France.
3  Kernforschungszentrum Karlsruhe, Institut für Material- und Festkörperforschung III, P.O. Box 3640, D-7500 Karlsruhe, Germany
4  Kernforschungszentrum Karlsruche, Institut für Technische Physik, P.O. Box 3640, D-7500 Karlsruhe, Germany

(Received 20 December 1990, accepted 5 February 1991)

The crystal structure of RESr 2GaCu 2O 7 (with RE=Er...La and Y) has been studied by powder neutron, powder X-ray and single crystal X-ray diffraction. These compounds crystallize in space group Ima2 with approximate lattice parameters $a\approx 22.8$Å  $\approx 2\cdot c$ (1-2-3), $b\approx 5.5$Å  $\approx\sqrt{2}\cdot b$ (1-2-3) and $c\approx 5.4$Å  $\approx\sqrt{2}\cdot a$ (1-2-3) and 4 formula units per cell. Like the high- $T_{\rm c}$ superconductor YBa 2Cu 3O 7 ("1-2-3") the structure contains double layers of Cu-O 5-pyramids separated by Y or trivalent rare earth ions. The Cu-O4 square planar chains of 1-2-3, however, are replaced by Ga-O4 tetrahedral chains running along the diagonal of the basal plane of the 1-2-3 subcell. The role of Ba to fill the large voids in 1-2-3 is played by Sr in these compounds. The structure forms with Y and all trivalent rare earth ions from Er through La with the exception of Ce. The oxygen stoichiometry is fixed to O 7 and the compounds are semiconducting. They can be p-doped by partially replacing RE by divalent earth alkali ions, thus reducing the resistivity drastically. However, despite their strong similarity to 1-2-3, particularly with respect to the Cu-O 5-pyramidal bilayers, combined with the possibility of p-doping all attempts to make these compounds metallic or even superconducting have failed so far.

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