Local structure around cations in the T'-type Nd(Ce) based superconducting system: an EXAFS study

EXAFS measurements on all the cations in Nd2CuO4, Nd2-xCexCuOy (NCCO) and Nd(1.82-z)A(z)Ce(0.18)CuO(y) (NACCO), A=Sr, Ca or Ba, x = 0.06 or 0.18, have been made using synchrotron radiation. Bond lengths (R), coordination numbers (N) and Debye-Waller factors (sigma(2)) have been obtained from the EXAFS analysis. The curved wave theory in the single-scattering approximation and the small-atom multiple-scattering theory have been applied to experimental EXAFS data and it is found that the latter gives a better fit. The CuO2 plane contains a strong multiple-scattering contribution. Further, all the dopants (Ce, Sr, Ca and Ba) occupy the Nd site without affecting the basic T' structure. Doping affects the local structure around the substituents as reflected by the changes in R and sigma(2) values. The observed CuO2 layer distortion (or large Cu-O-Cu angle deviation from linearity, 180 degrees) is found to have an adverse effect an T-c as observed in Ca or Ba doped NACCO samples. Further, it is realized that critical levels of charge carriers, oxygen vacancies and CuO2 planar effects have a strong influence on the SC behaviour of the T'-type electron doped cuprates.