DOI: 10.1051/jp1:1991165
J. Phys. I France 1 (1991) 721-741

The crystal structure of RESr GaCu O

G. Roth¹, P. Adelmann¹, G. Heger², R. Knitter³ and Th. Wolf<sup>4

1</sup>  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)

Abstract
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 Å  (1-2-3), Å  (1-2-3) and Å  (1-2-3) and 4 formula units per cell. Like the high- 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.

© Les Editions de Physique 1991