Phase decomposition and chemical inhomogeneity in Nd 2 − x Ce x CuO 4 ± δ

Extensive x-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd{sub 2-x}Ce{sub x}CuO{sub 4{+-}}{sub {delta}} (NCCO) in a low-oxygen-fugacity environment such as that typically realized during the annealing process required to create a superconducting state. Unlike a typical situation in which a disordered secondary phase results in diffuse powder scattering, a serendipitous match between the in-plane lattice constant of NCCO and the lattice constant of one of the decomposition products (Nd,Ce){sub 2}O{sub 3}, causes the secondary phase to form an oriented, quasi-two-dimensional epitaxial structure. Consequently, diffraction peaks from the secondary phase appear at rational positions (H,K,0) in the reciprocal space of NCCO. Additionally, because of neodymium paramagnetism, the application of a magnetic field increases the low-temperature intensity observed at these positions via neutron scattering. Such effects may mimic the formation of a structural superlattice or the strengthening of antiferromagnetic order of NCCO, but the intrinsic mechanism may be identified through careful and systematic experimentation. For typical reduction conditions, the (Nd,Ce){sub 2}O{sub 3} volume fraction is approximately 1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO{sub 2} sheets and have a typical thickness of approximately 100 A. The presence ofmore » the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.« less