Structural properties of Ba sub 2 RECu sub 3 O sub 7 High-T sub c superconductors.

The crystal chemistry of Rare-Earth (RE) substituted high T sub c superconductors of the Ba sub 2 RECu sub 3 O sub (7- ) type structure has been investigated using X-ray diffraction on twinned crystals (RE=Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm). Little change in the lattice constants is observed and, with the exception of the Pr compound, all substituted phases show superconductivity above 90K. Although structural studies in this system are best done using neutron powder diffraction because of the twinning, a substantial amount of information can be obtained even from twinned crystals by X-ray diffraction. From the systematic substitution of different RE atoms with different radii, we infer the presence of a [CuO sub 2-BaO- CuO sup square - BaO-CuO sub 2] structural building unit containing chains and planes of distorted square planar copper-oxygen units.