Structural investigations of ACu₃Ru₄O₁₂ (A=Na, Ca, Sr, La, Nd) - A comparison between XRD-Rietveld and EXAFS results

The structures of five perovskite-related oxides with the general composition ACu₃Ru₄O₁₂ with A = Na, Ca, Sr, La and Nd, have been examined both by XRD-Rietveld refinements and Ru-K EXAFS-spectroscopy. In addition, the behavior on reduction was investigated by thermogravimetry (TG). The TG measurements revealed that the composition was almost exactly A₁Cu₃Ru₄O₁₂ for all samples. The inter-atomic distances derived from EXAFS- and XRD-Rietveld fits show an excellent agreement with differences smaller than 8 mÅ even for R > 5 Å. All inter-atomic distances increase in the order Na<Ca< Sr<Nd<La and were found to depend linearly on the product of charge and ionic radius of the A-cation. The experimentally found distances are compared with the corresponding values expected from bond-valence calculations.