Oxygen vacancy ordering in superconducting Nd_2-xCe_xCuO_4-y

We report transmission electron microscopy results which show that an appreciable fraction of superconducting ceramic and single-crystal samples of the Nd_2-xCe_xCuO_4-y T' phase shows evidence for oxygen vacancies in the [Cu-O] planes which in some cases are ordered, either commensurately or incommensurately with the cation sublattice. As T' phase samples displaying a 100% Meissner signal are not yet available we cannot determine unambigously whether or not the oxygen loss is associated with superconduction but our results strongly suggest that supercell formation accompanying vacancy ordering occurs in over-reduced material which is no longer superconducting.