Unusually simple crystal structure of an Nd–Ce–Sr–Cu–O superconductor

Abstract

Recently, a new copper oxide superconductor with an onset tem-perature of 28 K was found in the Nd–Ce–Sr–Cu–O system¹. The superconducting phase was identified by X-ray diffraction and electron microscopy², and the metal-atom positions are identical with those in the K₂NiF₄ or Nd₂CuO₄ structure³, except for the order of the cations Nd(Ce) and Sr. Here we report the refinement of the crystal structure using neutron powder diffraction. We find that it comprises alternating slabs of the K₂NiF₄ and Nd₂CuO₄ structures, and that all the copper atoms are crystallographically identical, with oxygen in fivefold (square-pyramidal) coordination. In having only one kind of copper site, this new compound is structurally simpler than the other CuO₅-bearing superconductors such as YBa₂Cu₃O₇, and may therefore be particularly helpful in the effort to understand the origins of high-temperature superconductivity.