Local structure around Ce in the Nd $\mathsf{_{2-x}}$ Ce $\mathsf{_{x}}$ CuO $\mathsf{_{4\pm \delta}}$ superconductor probed by EXAFS

The local structure around the Ce substitutional defects has been investigated by Ce and Nd K-edge EXAFS on Nd 2-x Ce x CuO $_{4\pm \delta}$ samples with different dopant concentrations (x=0.05, 0.10, 0.15 and 0.20) and oxygen amounts. The lattice distortion around the Ce $_{\rm Nd}^\bullet$ can be described as a shrinkage of the oxygen cuboid along the z crystallographic direction. Moreover, the CuO 4 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, $z\cong 0.41$ ) and to two non equivalent oxygen apical positions O3 (at 0, 0, $z\cong 0.19$ ) and O4 (at 0, 0 $z\cong 0.24$ ). The electron population of O4 site, which is closer to cerium ( $d_{\rm Ce-O4}=1.96$ Å), is 3-4 times the O3 one. Copyright Springer-Verlag Berlin/Heidelberg 2004