High-efficiency X-ray Emission Spectroscopy for In Situ Studies of Iron-Bearing Compounds at Extreme Conditions

A team of scientists at the Technische Universität Dortmund in Germany has developed a new setup for resonant and non-resonant X-ray emission spectroscopy that allows for in situ studies of iron-bearing compounds at high pressure and high temperature (1). The study of iron in iron-bearing compounds at extreme conditions with spectroscopic methods is of great importance because iron is the most abundant transition metal in the Earth's deep mantle. The new setup can help scientists understand geological processes in the deep Earth.

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The sample environment using a diamond anvil cell complicates spectroscopic measurements and leads to long data acquisition times. However, the new setup has measurement times within seconds for robust spin-state analysis results. The team showcased the setup's capabilities by imaging laser-heated FeCO₃ at 75 GPa via Kβ1,3 emission spectroscopy, demonstrating the great potential of this setup.

The results of Kβ1,3 emission spectroscopy of cold-compressed Fe₂O₃ reveal a two-step spin transition with the ζ-phase between 57 GPa and 64 GPa, having iron in different spin states at the different iron sites. The phase transition via ζ- to Θ-phase causes a delocalization of the electronic states, which is supported by 1s2p resonant X-ray emission spectroscopy.

Iron is present in different oxidation states as ferrous (Fe²⁺) and ferric (Fe³⁺) iron and can undergo a spin transition from high spin (HS) to low spin (LS) or vice versa, depending on its local environment and the thermodynamic conditions. The pressure-induced spin transition in iron-bearing compounds can influence many physical and chemical properties, such as sound velocity, conductivity, compressibility, material transport, or element partitioning. Therefore, a detailed understanding of the electronic structure of iron-bearing compounds in situ at extreme conditions is crucial to understand and interpret the material's properties and chemistry of the interior of the Earth and other terrestrial planets.

X-ray emission spectroscopy is widely used to determine spin state, covalency, oxidation state, electronic structure, and structural changes. The new setup for X-ray emission spectroscopy of iron-bearing compounds at extreme conditions offers a powerful tool for scientists to investigate the properties of these materials in situ. This research was published in the Journal of Analytical Atomic Spectrometry (1).

X-ray emission spectroscopy (XES) is a powerful technique to investigate the electronic structure of materials, especially transition metal complexes and compounds. It provides information on the valence and spin state of the material under study. Resonant X-ray emission spectroscopy (RXES) is a variation of XES that allows the detection of specific electronic transitions in the X-ray absorption spectrum. Non-resonant X-ray emission spectroscopy (NEXS) uses X-rays that are not in resonance with any specific transition in the absorption spectrum. It can still provide information on the valence and spin state of the material, but with lower spectral resolution compared to RXES. Both RXES and NEXS are commonly used in in situ studies at high pressure and high temperature to investigate the electronic and structural changes of materials under extreme conditions.

Reference

(1) Albers, C.; Sakrowski, R.; Thiering, N.; Libon, L.; Spiekermann, G.; Kaa, J. K.; Gretarsson, H.; Sundermann, M.; Tolan, M.; Wilke, M.; Sternemann, C. High-efficiency X-ray emission spectroscopy of cold-compressed Fe₂O₃ and laser-heated pressurized FeCO₃ using a von Hámos spectrometer. J. Anal. At. Spectrom. 2023, ASAP. DOI: 10.1039/D3JA00014A