SHORT-RANGE STRUCTURAL-PROPERTIES OF ND1.85CE0.15CUO4 AND ND2CUO4 STUDIED BY MULTIPLE-SCATTERING EXAFS DATA-ANALYSIS

An accurate multiple-scattering Cu K-edge extended x-ray-absorption fi ne-structure (EXAFS) study of the average local structure of Nd1.85Ce0 .15CuO4 and Nd2CuO4 is presented in the range of temperature 5-150 K. Clear differences between experimental EXAFS signals of doped and undo ped compounds recorded at the same temperature are found. Shortening o f 0.015 Angstrom of the average Cu-Nd/Ce distance is unambiguously det ected in the doped-superconducting compound. We show that the contract ion of the unit cell along the c direction can be assigned to the shif t of the Nd/Ce atoms toward the adjacent CuO2 plane. No evidence for a nomalies of the in-plane Cu-Cu and Cu-O distance distribution is found as a function of temperature and doping. The strong multiple-scatteri ng signal associated with the quasilinear Cu-O-Cu chains in the CuO2 p lanes is used to evaluate the angle distribution at the oxygen site, u sing a suitable model describing a realistic three-atom vibrating syst em. Differences between the doped and the undoped compound are detecte d showing that the angle distribution is broader below T-c in the dope d compound. The increased width of the angle distribution is consisten t with the existence of local modes or permanent structural distortion s involving the in-plane oxygens.