Superconductors and nonsuperconductors in Nd_2-χCe_χCuO₄

Results are presented on the chemical composition and structure of the n-type superconductors Nd_2-χCe_χCuO₄. Electron diffraction and high resolution electron microscopy indicate that there are three slightly different structures, two of which are modulated versions of the first. By matching both the images and diffraction patterns it is shown that the modulations are due to small displacements of the cations, possibly a charge density wave, not simply ordering of the Ce or oxygen vacancies, although oxygen stoichiometry may be the driving force for the modulations. Both types of phases occur in materials which are superconducting, and those which are not. Microanalysis indicates that the Ce content of the superconducting specimens after reduction is slightly smaller at 0.16 rather than 0.19, and the cation stoichiometry is not detectably different between the modulation and unmodulated materials.