THE CRYSTAL-STRUCTURE OF THE SUPERCONDUCTING COPPER-OXIDE CARBONATE (BA1-XSRX)2CU1+YO2+2Y+Z(CO3)1-Y - DEFECTS IN CHARGE RESERVOIRS AND SUPERCONDUCTIVITY

The crystal structure of a new superconductor in the Ba-Sr-Cu-C-O system has been determined by neutron powder diffraction with the auxiliary use of electron diffraction and high-resolution transmission electron microscopy. This compound is a member of a solid solution with the general formula (Ba1-xSrx)2Cu1+yO2+2y+z(CO3)1-y, where x = 4/9, y = 1/9, and z = 0.10. It is tetragonal with a space group P42(1)2 or P42(1)mBAR and lattice parameters of a = 5.56405(18) angstrom and c = 7.8570(4) angstrom. It is closely related in structure to Sr2CuO2CO3, which is an end member of the solid solution: x = 1, y = 0, and z = 0. Two-dimensional CuO2 sheets and carbonate slabs, (Ba1-xSrx)2CuyO2y+z(CO3)1-y, alternate with each other along the [001] direction. As the formula of the slab shows, about 11% of C atoms are substituted by excess Cu atoms coordinated to O atoms which amount to 2y + z in the formula unit. These substitutional and interstitial defects enable hole carriers to move from the carbonate slab to the CuO2 conduction sheet.