Setup to study the electronic structure of iron-bearing compounds in situ at conditions of the Earth’s lower mantle

The determination of the electronic structure of iron-bearing compounds at high pressure and high temperature (HPHT) conditions is of crucial importance for the understanding of the Earth’s interior and planetary matter. Information on their electronic structure can be obtained by X-ray emission spectroscopy (XES) measurements, where the iron’s Kβ_1,3 emission provides information about the spin state and the valence-to-core region focusses on the coordination chemistry around the iron and its electronic state. Furthermore, resonant XES (RXES) at the iron’s K-edge reveals even more detailed information about the electronic structure [1].

We present a setup to investigate the electronic structure of iron-bearing compounds in situ at HPHT conditions using XES and RXES. The HPHT conditions are accomplished by diamond anvil cells (DACs) in combination with a portable double-sided Yb:YAG-laser heating setup [2]. The spectroscopy setup contains a wavelength dispersive von Hamos spectrometer in combination with a Pilatus 100K area detector [3]. This setup provides a full Kβ_1,3 emission spectrum including valence-to-core emission in a single shot fashion. In combination with a dedicated sample preparation and use of highly intense synchrotron radiation of beamline P01 at PETRA III, the duration of the measurements is shortened to an extend that in situ XES, including valence-to-core, as well as in situ spin state imaging becomes feasible. The use of miniature diamonds [4] enables RXES measurements at the Fe-K edge. By using different analyzer crystals for the von Hamos spectrometer, simultaneous Kα and Kβ detection are feasible, which provides L-edge and M-edge like information.

The presented sample is siderite (FeCO₃), which is in focus of recent research as it is a candidate for the carbon storage in the deep Earth. Siderite exhibits a complex chemistry at pressures above 50 GPa and temperatures above 1400 K resulting in the formation of carbonates featuring tetrahedrally coordinated CO₄-groups instead of the typical triangular-planar CO₃-coordination. These carbonates are well understood on a structural level but information on their electronic structure is scarce [5-7]. We present information on the sample’s spin state at in situ conditions of about 75 GPa and 2000 K XES Kβ_1,3 imaging  as well as RXES measurements for low and high pressure siderite at ambient temperature conditions for Kα and Kβ emission.

[1] M. L. Baker et al., Coordination Chemistry Reviews 345, 182 (2017)

[2] G. Spiekermann et al.,  Journal of Synchroton Radiation, 27, 414 (2020)

[3] C. Weis et al., Journal of Analytical Atomic Spectroscopy 34, 384 (2019)

[4] S. Petitgirard et al., J. Synchrotron Rad. , 24, 276 (2017)

[5] J. Liu et al., Scientific Reports, 5, 7640 (2015)

[6] M. Merlini et al., American Mineralogist, 100, 2001, (2015)

[7] V. Cerantola et al., Nature Communications 8, 15960 (2017)