Simultaneous X-ray Absorption Spectroscopy Research Articles

Emergent high-spin state above 7 GPa in superconducting FeSe

The local electronic and magnetic properties of superconducting FeSe have been investigated by $K\ensuremath{\beta}$ x-ray emission and simultaneous x-ray absorption spectroscopy (XAS) at the Fe $K$ edge at high pressure and low temperature. Our results indicate a sluggish decrease of the local Fe spin moment under pressure up to 7 GPa, in line with previous reports, followed by a sudden increase at higher pressure. The magnetic surge is preceded by an abrupt change of the Fe local structure as observed by the decrease of the XAS preedge region intensity and corroborated by ab initio simulations. This pressure corresponds to a structural transition from the $Cmma$ form to the denser $Pbnm$ form with octahedral coordination of iron. Finally, the near-edge region of the XAS spectra shows a change before this transition at 5 GPa, corresponding well with the onset pressure of the sudden enhancement of ${T}_{c}$. Our results emphasize the delicate interplay between structural, magnetic, and superconducting properties in FeSe under pressure.

Iron oxidation dynamics vs. temperature of synthetic potassic-ferro-richterite: a XANES investigation.

We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 °C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at ∼335 °C accompanied by an anomalous increase in the monoclinic cell angle β. From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at ∼315 °C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mössbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.

New insights on the crystallization process in Ge15Sb85 phase change material: A simultaneous calorimetric and quick-EXAFS measurement

A simultaneous X-ray absorption spectroscopy and calorimetric experiment was performed on the binary Ge15Sb85 phase change material in order to obtain a deeper understanding of the two-step crystallization mechanism. The combination of these techniques allows monitoring the structural and thermal changes related to the crystallization process with a time resolution of the order of 1s. Both observed crystallization transitions go along with significant atomic rearrangements affecting not only the radial distribution function, but also the amplitude of thermal vibrations. From these data, the stoichiometry of the crystallized phases is also extracted.