X-ray Raman Scattering at Extreme Conditions: Insights to Local Structure, Oxidation and Spin state

Abstract:

In the last decades, X-ray spectroscopic techniques using very intense synchrotron radiation (SR) have become indispensable tools for studying geomaterials. Due to the rather low absorption of hard X-rays, SR opens up the possibility to perform measurements in high-pressure, high temperature cells. The range of elements accessible by X-ray absorption spectroscopy (XAFS) techniques in these cells is limited by the absorption of X-rays due to the sample environment, i.e. the diamond windows. The indirect measurement of XAFS spectra by inelastic X-ray Raman scattering (XRS) provides a workaround to access absorption edges at low energies (e.g. low Z elements). Therefore, XRS enables measurements that are similar to electron energy loss spectroscopy but offer to measure at in-situ conditions and not just in vacuum.

Measurements of the O K-edge of H₂O from ambient to supercritical PT-conditions (up to 600°C @ 134 MPa; 400°C @ 371 MPa) were used to trace structural changes of the hydrogen-bonded network, which controls many physical and chemical properties of H₂O [1]. The Fe M_3,2-edge measured by XRS were used to characterize the oxidation state and local structure in crystalline compounds and glasses [2]. Furthermore, the M_3,2 yields detailed insight to the crystal-field splitting and electronic spin state. In a reconnaissance study, the pressure-induced high-spin to low-spin transition of Fe in FeS between 6 and 8 GPa was measured. By multiplet calculations of the spectra for octahedral Fe²⁺, a difference in crystal field splitting between the two states of ca. 1.7 eV was estimated [3]. Finally, we successfully assessed the electronic structure of Fe in siderite by measurements of M and L-edge up to 50 GPa, covering the spin transition between 40 and 45 GPa.

[1] Sahle et al. (2013) PNAS, doi: 10.1073/pnas.1220301110.. [2] Nyrow et al. (2014) Contrib Mineral Petrol 167, 1012. [3] Nyrow et al. (2014) Appl Phys Lett 104, 262408.