EXAFS study of the local atomic structure in AFe₂As₂ (A= Eu, Sr, Ba)

Full text: The recent discovery of superconductivity in new families of Fe-based intermetallics and oxides has opened new fields for the research of the properties of high temperature superconductors. From the structural point of view, these Fe-based superconductors share a local structure, based on planar layers of FeAs. The Fe 3d electrons form multiple bands at the Fermi level and play an important role in superconductivity. The non-doped AFe₂As₂ compounds (named A122) are oxygen free iron arsenides (with A= Ba, Sr, Ca, Eu). At room temperature the A122 systems are paramagnetic and crystallize in a tetragonal structure. At low temperature they exhibit a spin-density-wave (SDW) ordering transition, connected with a structural phase transition to an orthorhombic structure. This phase is formed at a critical temperature TSDW, observed typically in the 100K2As₂ (for A=Eu, Sr, Ba) as a function of temperature. We have investigated the effects of each particular A-site ion on the Fe-As bond from a local point of view. We employed high quality single crystals grown from In-flux and the measurements were performed in transmission mode in the As K absorption edge, at the XAFS-2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). (author)