Crystal Structure of the High T_c Superconductor LnBa₂Cu₃O_7-δ (Ln=Sm, Eu and Gd)

Abstract

The crystal structure of the high T c superconductor LnBa 2 Cu 3 O 7-δ (Ln=Sm, Eu and Gd) has been determined by Rietveld analysis of X-ray powder diffraction data. The compounds crystallize in an oxygen-deficient layered perovskite structure with space group Pmmm. The structure is isomorphic with YBa 2 Cu 3 O 7-δ . The Ln and Ba atoms have an ordered arrangement, oxygen on a z =1/2 plane is missing and the oxygen site on the basal Cu plane is selectively occupied.