Local structure around Ce in the Nd2-xCexCuO4 +/-delta superconductor probed by EXAFS

The local structure around the Ce substitutional defects has been investigated by Ce and Nd K-edge EXAFS on Nd2-x Ce (x) CuO4+/-delta samples with different dopant concentrations (x = 0.05, 0.10, 0.15 and 0.20) and oxygen amounts. The lattice distortion around the Ce-Nd(.) can be described as a shrinkage of the oxygen cuboid along the z crystallographic direction. Moreover, the CuO4 planes become corrugated. The lattice distortion is well localized and is largely independent of temperature and Ce and oxygen content. EXAFS measurements made at the Nd-K edge indicate that the local chemical environment of Nd is not distorted. The difference electron density maps, obtained from low temperature single crystal X-ray diffraction data, revealed three strong positive residuals which have been ascribed to cerium (at 0, 0, zcongruent to0.41) and to two non equivalent oxygen apical positions O3 (at 0, 0, zcongruent to0.19) and O4 (at 0, 0 zcongruent to0.24). The electron population of O4 site, which is closer to cerium (d(Ce-O4)=1.96 Angstrom), is 3-4 times the O3 one.