Metal ordering and oxygen displacements in (Nd, Sr, Ce)₂CuO_{4 - y}

The structural parameters of a fully oxidized, superconducting sample and a deoxidized, non-superconducting sample of Nd_1.32Sr_0.41Ce_0.27CuO_{4 - y} were refined by Rietveld analysis of TOF neutron powder diffraction data. No appreciable structural differences were found between the two samples. Its structure consists of K₂NiF₄- and Nd₂CuO₄-type units stacked alternately along the c-axis. Ce⁴⁺ ions, whose oxidation state was confirmed by XPS, occupy an eight-coordinate site together with Nd³⁺ ions in the Nd₂CuO₄ units. On the other hand, a nine-coordinate site in the K₂NiF₄ units is filled with Nd³⁺ and Sr²⁺ ions. Each Cu atom is coordinated to five oxide ions in a distorted square-pyramidal configuration. O(2) atoms at the apical positions of the CuO₅ pyramids are displaced from an ideal 2c site to an 8j site with lower symmetry, which is mainly caused by the occupation of the same nine-coordinate site by Nd³⁺ and Sr²⁺ ions. The occupation factors of site O(2) were refined to be ca. 0.23, giving the 4 - y value of ≈3.92. The formal charge of Cu, +1.98, suggests that this superconductor has an unhomogeneous domain structure in which oxygen-rich superconducting regions and oxygen-poor non-superconducting regions coexist within each particle.