Short-range structural properties of
Nd₂CuO₄ and Nd_1.85Ce_0.15CuO₄
studied by multiple-scattering EXAFS data-analysis

Andrea Di Cicco and Francesco Sperandini

INFM, Dipartimento di Matematica e Fisica, Universita' di Camerino, via Madonna delle Carceri, 62032 Camerino (MC), Italy. http://gnxas.unicam.it.
E-mail: dicicco@unixas.unicam.it

Beamlines: D42 (EXAFS 1) and D21, DCI.

An accurate multiple-scattering (MS) Cu K-edge EXAFS study of the average local structure of Nd_1.85Ce_0.15CuO₄ (NCCO) and Nd₂CuO₄ has been performed in the range of temperature 5-150 K using measurements collected at LURE [1]. This work follows previous determinations of the in-plane local Cu-O-Cu structure using EXAFS multiple-scattering analysis [2,3].
Clear differences between experimental EXAFS signals of doped and undoped Nd-based compounds recorded at the same temperature are found.
Shortening of 0.015 Angstroems of the average Cu-Nd/Ce distance is unambiguously detected in the doped superconducting compound. We show that the contraction of the unit cell along the c direction can be assigned to the shift of the Nd/Ce atoms toward the adjacent CuO2 plane.
No evidence for anomalies of the in-plane Cu-Cu and Cu-O distance distribution is found as a function of temperature and doping. The strong multiple-scattering signal associated with the quasi-linear Cu-O-Cu chains in the CuO2 planes is used to evaluate the angle distribution at the oxygen site, using a suitable model describing a realistic three-atom vibrating system. Differences between the doped and the undoped compound are detected showing that the angle distribution is broader below Tc in the doped compound. The increased width of the angle distribution is consistent with the existence of local modes or permanent structural distortions involving the in-plane oxygens.

[1] F. Sperandini, A. Di Cicco, and M. Gazda, ``Short-range structural properties of Nd2CuO4 and Nd1.85Ce0.15CuO4 studied by multiple-scattering EXAFS data-analysis'', Phys. Rev. B 57, 6067 (1998).
[2] A. Di Cicco and F. Sperandini, ``Cu-O-Cu in-plane angle distribution in layered superconductors probed by EXAFS'', Physica C 258, 349 (1996).
[3] A. Di Cicco and M. Berrettoni, ``X-Ray Absorption Multiple-scattering Study of High-Tc Superconductors.'', Phys. Lett. A 176, 375 (1993).