Phase Decomposition and Chemical Inhomogeneity in Nd2-xCexCuO4

TL;DR

This study reveals that Nd2-xCexCuO4 undergoes partial decomposition during annealing, forming an oriented secondary phase that affects experimental observations and must be considered in analyses of its properties.

Contribution

It provides detailed insights into the phase decomposition process and the formation of an epitaxial secondary phase in NCCO, which was previously not well characterized.

Findings

Secondary phase (Nd,Ce)2O3 forms at ~1% volume fraction.

The secondary phase exhibits long-range order and is 50-100 Å thick.

Magnetic fields increase neutron scattering intensity at secondary phase positions.

Abstract

Extensive X-ray and neutron scattering experiments and additional transmission electron microscopy results reveal the partial decomposition of Nd2-xCexCuO4 (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)2O3, 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)2O3 volume fraction is ~1%, and the secondary-phase layers exhibit long-range order parallel to the NCCO CuO2 sheets and are 50-100 angstromsthick. The presence of the secondary phase should also be taken into account in the analysis of other experiments on NCCO, such as transport measurements.