Crystal Structure of the High Tc Superconductor YbBa₂Cu₃O7-δ

The crystal structure of the high T c superconductor YbBa 2 Cu 3 O 7-δ is determined by Rietveld analysis of X-ray powder diffraction data. The crystal system is orthorhombic (space group Pmmm) with lattice parameters a =3.8710(2) Å, b =3.8018(2) Å and c =11.6576(5) Å. The structure is isomorphic with YBa 2 Cu 3 O 7-δ . The layered perovskite structure with the tripled c -axis exhibits ordering of Yb and Ba, an oxygen-deficient Yb layer at z =1/2 and preferential oxygen occupation on the basal Cu plane. The compound shows superconductivity at 90 K (midpoint) with the transition width of 3.0 K.