Crystal Structure of the High Tc Superconductor TmBa₂Cu₃O7-δ

The crystal structure of the high T c superconductor TmBa 2 Cu 3 O 7-δ has been determined by Rietveld analysis of X-ray powder diffraction data. The crystal system is orthorhombic (space group Pmmm) with lattice parameters a =3.8746(2) Å, b =3.8087(1) Å and c =11.6655(6) Å. The structure is isomorphic with YBa 2 Cu 3 O 7-δ . The Tm and Ba atoms have an ordered arrangement, O in the z =1/2 plane is completely missing and O on the basal Cu plane is deficient. The compound shows superconductivity at 90.5 K (midpoint) with the transition width of 3.0 K.