The role of the A ion in the Fe local structure of the AFe2As2 systems (A=Sr, Eu, Ba): an EXAFS study

D. TOBIA; M. E. SALETA; M. RADAELI; M. M. PIVA; C. B. R. JESUS; R. R. URBANO; E. GRANADO; P. G. PAGLIUSO

Campinas

Encuentro; XV Brazil MRS Meeting; 2016

The recent discovery of high-TC superconductivity on Fe-based intermetallic materials has motivated an extensive research in this area due to the rich phase diagrams that they present. The non-doped AFe2As2 compounds (A122) are oxygen free iron arsenides (with A= Ba, Sr, Ca, Eu). At room temperature they are paramagnetic and crystallize in a tetragonal structure. At low temperature they exhibit a spin-density-wave (SDW) ordering transition (typically 100 K < TSDW < 200 K), connected with a structural phase transition to an orthorhombic structure. These transitions are reflected as anomalies in the electric resistivity, magnetic susceptibility and specific heat data. Although the pure A122 compounds are not superconductors, in these systems, it was observed that the SDW phase can be destabilized through chemical substitution or external applied pressure, leading to the emergence of superconductivity. In particular, the atomic substitution of the alkali atoms with different ionic radii in the A site results in a change of the cell parameters and, consequently, these systems experience effects of chemical pressure. In this context, it is of great interest to investigate the role of the local distortion on the Fe-As bonds for temperatures above and below TSDW, as the Fe-As bond distance controls directly the chemical pressure on Fe and ultimately the orbital differentiation of the Fe 3d bands on the Fermi Surface of these compounds. Extended X-ray absorption fine structure (EXAFS) measurements are ideal to obtain information about the relationship between local structural distortions and the system spin dynamics. In this work we present a study of the As local environment in AFe2As2 (for A=Eu, Sr, Ba) as a function of temperature through EXAFS measurements in the As K absorption edge. We have investigated the effects of each particular A-site ion on the Fe-As bond from a local point of view. In all cases, high quality thin single crystals grown from In-flux were used and the measurements were performed in transmission mode the XAFS-2 beamline at the Brazilian Synchrotron Laboratory (LNLS).