Thesis defense of Nicola Thiering

This study investigates the structural and electronic behavior of iron under high-pressure conditions relevant to the Earth's lower mantle, combining experiments on Fe203 and iron-bearing haplobasaltic glasses. X-ray emission and resonant X-ray emission spectroscopy, X-ray diffraction, and synchrotron Mössbauer spectroscopy are used to constrain changes in spin state, valence, and local coordination. In Fe203, spin-state evolution is closely linked to structural phase transformations. An extended mixed-spin regime occurs at intermediate pressures and is associated with mixed-valence oxides such as Fe507. At higher pressures and temperatures, Fe25032 forms, with electronic behavior governed by coexisting ferrous and ferric iron in distinct coordination environments. Thus, the spin is primarily governed by the crystal structure. In haplobasaltic glasses, iron responds more gradually to compression. Ferrous iron undergoes significant coordination changes, whereas ferric iron remains structurally more stable. These changes reflect local structural redistribution. Overall, the behavior of iron under lower mantle conditions is governed by the interplay of electronic state, structure, and phase relations, with important implications for the physical and chemical properties of deep Earth materials.