Strong reduction of the electron-doped superconductor Nd 2 − xCe xCuO 4 − δ: Correlation between crystal and electronic structure

Abstract

The microstructure, microchemistry and electronic structure of the electron-doped superconductor Nd 2 − x Ce x CuO 4 − δ were studied as functions of cerium and oxygen concentration by means of X-ray diffraction and transmission electron microscopy including selected area electron diffraction, energy dispersive X-ray microanalysis and electron energy loss spectroscopy. In strongly reduced Nd 2 − x Ce x CuO 4 − δ samples at low Ce concentrations ( x < 0.1) we observed some small crystals, about 1 μm to 5 μm in size, with twin lamellae on {110} which had an orthorhombic distorted structure different from the bulk material with the tetragonal 14/mmm structure. The twinning is due to the phase transition 14/mmm to Bmmm during the reduction process which is connected with loss of point group symmetry. The twinned crystals share the orientation of the twin plane and the crystallographic c-axis with {110} and [001] of the tetragonal matrix. During in situ heating experiments on single grains with the electron beam in the high vacuum of the microscope several structural transformations were observed, whereby this orthorhombic modification could be induced by heating the undoped parent compound Nd 2CuO 4 − δ . The concomitant change of the electronic structure, especially the transitions O 1s → 2p and Cu 2p → 3d were determined with EELS. The EELS data indicate that there exist mixed. valences of Cu 1+ and Cu 2+ in the orthorhombic distorted grains.