Local structure around cations in the Nd(Ce) based superconducting system: an EXAFS study

Abstract

EXAFS measurements on all the cations in Nd 2 CuO 4 , Nd 2−x Ce x CuO y (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 (σ 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 CuO 2 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 σ 2 values. The observed CuO 2 layer distortion (or large Cu-O-Cu angle deviation from linearity, 180 • ) is found to have an adverse effect on T c as observed in Ca or Ba doped NACCO samples.Further, it is realized that critical levels of charge carriers, oxygen vacancies and CuO 2 planar effects have a strong influence on the SC behaviour of the T -type electron doped cuprates.

X-ray absorption near edge structure study of -type revived superconducting compounds, (NACCO), A = Ca, Sr or Ba,

“…This is as expected. It is also seen from our EXAFS results [19] that Nd(Ce) environment is unchanged (and only the Cu-O environment is affected) by changing the alkaline-earth ion in NACCO. The Nd L 3 -spectral features of all the samples (spectra not reproduced here) remain nearly the same and match well with those of NCCO samples [21].…”

“…In the case of Ba substituted samples (z = 0.18) wherein the WL intensity is only marginally affected, this argument is somehow not extendable. One of the possible reasons may be the exceptionally large ionic radius of Ba (r = 1.36 Å, coordination number = 8) causing local structural distortions as revealed by EXAFS [19].…”

“…The analytical scanning electron microscopic (ASEM) study [18] on some of the NACCO samples has provided direct evidence for the creation of oxygen vacancies. Moreover, analysis of the EXAFS data [19] has revealed that co-doping of Ca, Sr or Ba with Ce in Nd 2 CuO 4 affects the local structure around the cations and the number of coordinating oxygen around Cu is reduced to less than four, indicating creation of oxygen vacancies. It is, therefore, expected that oxygen vacancies should result in increased carrier (electron) density that more than compensates the depletion of electron carriers due to partial substitution of A 2+ ion for Nd 3+ .…”

Synthesis, structural parameters and superconducting properties of 1201-type (Hg,M)Sr2CuO4+ (M = Cr, Mo or Re): an overview

“…6 Present address: Superconducting Materials Center, New Materials Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan. majority carriers and Cu-O layers are the common structural features [8].…”

“…In Hg/Ba-cuprates, the chemical/phase instability and the preparation problems have arisen mainly from high volatility of the toxic mercury (decomposition of precursor HgO) at reaction temperatures, besides sensitivity of the intermediate Ba-Cu-O precursor (forming BaCO 3 under normal or moist laboratory conditions) [10,13,14]. Such difficulties associated with the preparation of single-phase Hg/Ba-cuprates have hampered the accurate determination of many important physico-chemical properties [5][6][7][8][9][10][11][12]; at the same time, this has also prompted the researchers, besides having natural curiosity, to synthesize the isostructural Ba-free Hg/Sr-based cuprates of generic formula HgSr 2 Ca n−1 Cu n O 2n+2+δ (n = 1-3) [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].…”