Neutron and electron diffraction study of the electron-doped superconductor Nd1.845Ce0.155CuO4 − y

Abstract

The crystal structure and non-stoichiometry of Nd1.845Ce0.155CuO4 − y were investigated by the combined use of neutron powder diffraction and selected-area electron diffraction. It is isomorphous with Nd2CuO4 (tetragonal, I4/mmm), but a minor portion of a superconducting sample had a pseudo-tetragonal superlattice with A = 2√2 a. This superstructure results from the partial ordering of oxygen vacancies and/or the two lanthanoid metals. The distance of an equatorial Cu-O bond, 0.1973 nm, in Nd1.845Ce0.155CuO4 − y is somewhat longer than those in hole-doped superconductors containing Cu, which reflects higher electron densities along the Cu-O bond in the former, compared with the latter. An oxygen site between two Nd(Ce) sheets was 3–4% deficient. Furthermore, another oxygen position on a two-dimensional [CuO2]∞ sheet may also become very slightly vacant on thermal treatment under a low partial pressure of O2. Electrons are, therefore, doped into the [CuO2]∞ sheets by not only substitution of Ce4+ ions for Nd3+ ions but creation of the oxygen vacancies.