The crystal structure of RESr₂GaCu₂O₇

The crystal structure of RESr2GaCu2O7 (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≈22.8Å ≈2.c (1-2-3), b≈5.5Å $\approx\sqrt{2}\cdot b$ (1-2-3) and c≈5.4Å $\approx\sqrt{2}\cdot a$ (1-2-3) and 4 formula units per cell. Like the high-Tc superconductor YBa2Cu3O7 (“1-2-3”) the structure contains double layers of Cu-O5-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 O7 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-O5-pyramidal bilayers, combined with the possibility of p-doping all attempts to make these compounds metallic or even superconducting have failed so far