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

<p>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&#8217;s interior and planetary matter. Information on their electronic structure can be obtained by X-ray emission spectroscopy (XES) measurements, where the iron&#8217;s K&#946;_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&#8217;s K-edge reveals even more detailed information about the electronic structure [1].</p><p>We present a setup to investigate the electronic structure of iron-bearing compounds <em>in situ</em> 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 K&#946;_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 <em>in situ</em> XES, including valence-to-core, as well as <em>in situ</em> 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 K&#945; and K&#946; detection are feasible, which provides L-edge and M-edge like information.</p><p>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&#8217;s spin state at <em>in situ</em> conditions of about 75 GPa and 2000 K XES K&#946;_1,3 imaging &#160;as well as RXES measurements for low and high pressure siderite at ambient temperature conditions for K&#945; and K&#946; emission.</p><p>[1] M. L. Baker et al., <em>Coordination Chemistry Reviews </em>345, 182 (2017)</p><p>[2] G. Spiekermann et al.<em>,&#160; Journal of Synchroton Radiation,</em> 27, 414 (2020)</p><p>[3] C. Weis et al., <em>Journal of Analytical Atomic Spectroscopy</em> 34, 384 (2019)</p><p>[4] S. Petitgirard et al., <em>J. Synchrotron Rad</em>. , 24, 276 (2017)</p><p>[5] J. Liu et al., <em>Scientific Reports, </em>5, 7640 (2015)</p><p>[6] M. Merlini et al., <em>American Mineralogist</em>, 100, 2001, (2015)</p><p>[7] V. Cerantola et al., <em>Nature Communications</em> 8, 15960 (2017)</p>