Crystallography and Phase Relations of Iron Sulfides at Earth and Planetary Core Conditions

Sulfur is a primary alloying element in Earth and planetary cores. Investigating iron-sulfide phase relations at high pressures and temperatures is integral to constraining core chemistry and dynamics. At ambient conditions, numerous iron sulfides adopt complex structures, but little work has been done to examine the crystallography of high P-T iron-rich sulfides that may influence the thermodynamics of Earth's complex core. Over the course of this work, I combined single-crystal X-ray diffraction with powder X-ray diffraction techniques on multigrain iron-sulfur samples synthesized in a laser heated diamond anvil cell to 200 GPa and 3500 K. The results unveil novel complexities of the Fe-S system under extreme conditions, and several iron-rich sulfides are characterized and discussed: C23 Fe2S*, Cr2P-type Fe2S*, C22 Fe2S, C37 Fe2S, Fe3S, Fe12S7*, and Fe11S4* (* novel phase discovered in this work). As tetragonal Fe3S was previously reported to remain stable in iron-rich systems across the P-T range of this study, the stability of these additional iron sulfides are placed into the context of iron-rich systems to develop an updated iron-sulfide phase diagram relevant to Earth's core. The complexity of the results highlight the analytical power of combining powder-diffraction techniques for examining phase synthesis in-situ with high pressure single-crystal diffraction techniques for analyzing crystal structures at high pressures, and provides critical insight on the material properties of iron-rich sulfides relevant to Earth and terrestrial planetary cores.