Thesis defense of Christian Albers

Iron is the most abundant transition metal in the deep Earth. Due to its complex electronic structure, it can appear in two different oxidation states (Fe2+ and Fe3+) and can also undergo a spin transition. Hence, it plays an important role in the physical and chemical properties of the deep Earth assemblage. Within this thesis, three eologically relevant sample systems were investigated regarding their electronic structure at pressure and temperature conditions that are relevant to the Earth’s lower mantle. Therefore, (nonresonant) Kβ1,3 and valence-to-core X-ray emission spectroscopy as well as resonant 1s2p X-ray emission spectroscopy were utilized. First, the study of laser-heated FeCO3 at about 80 GPa results in the synthesis of Fe4C3O12 and Fe4C4O13. The iron in both of these phases was verified to be in high-spin state. Furthermore, the emergence of low-wavenumber Raman bands in a wavenumber range between 100 cm−1 and 350 cm−1 could be assigned to Fe4C4O13. Second, the  study on cold compressed Fe2O3 verifies a two-step spin transition from α- via ζ- to Θ-Fe2O3. Moreover, resonant X-ray emission spectroscopy measurements support a possible delocalization of the electronic states in the high pressure phases. Third, the influence of pressure on the electronic structure of FeO shows significant influence on the line shape of the Kβ1,3 emission, although no spin transition occurred. The line-shape changes could be connected to a distortion of the crystal structure. This will have a substantial influence on the interpretation of Kβ1,3-spectra in the future. Additionally, the setup for spectroscopic measurements was significantly improved with reduced data acquisition times of Kβ1,3 X-ray emission spectroscopy within seconds, high-quality valence-to-core emission spectroscopy within minutes and resonant X-ray emission spectroscopy measurements in less than one hour, offering unique spectroscopic opportunities at extreme conditions.