SYNTHESIS AND STRUCTURE OF A NEW FAMILY OF CUPRATE SUPERCONDUCTORS - LNSR2CU2GAO7

The synthesis of layered cuprates with nonmagnetic and fixed oxidation state cations separating the Cu-O planes has, until now, led to nonsuperconducting materials. A new series of layered copper oxides with the general formula LnSr2Cu2GaO7 (Ln = La-Yb, Y) has been synthesized. The structure has been studied by both powder neutron diffraction and single-crystal X-ray analysis. The typical unit cell is a almost-equal-to 22.8 angstrom, b almost-equal-to 5.5 angstrom, c almost-equal-to 5.4 angstrom, in space group Ima2 (No. 46). The structure is closely related to that of YBa2Cu3O7 and can be described as replacement of the square-planar copper with a tetrahedral gallium. The larger lanthanides are distributed unevenly over the two A-cation sites, whereas the smaller lanthanides occupy the site between the cuprate planes. We have been able to introduce superconductivity in the Y-Sr-Cu-Ga-O system by appropriate doping with calcium and annealing.