In situ X-ray diffraction study of high-pressure phase transitions in iron oxide under shock loading

The high-pressure-temperature (P-T) behavior of iron oxides is a topic of considerable interest both due to their importance in Earth’s interior as well as their role as analogs for understanding ultrahigh-P magnesium silicates phase transitions in super-Earth interiors. Hugoniot data for hematite (Fe₂O₃) and magnetite (Fe₃O₄) both show significant densification across a broad mixed-phase region between 40-80 GPa. However, the structures of the high-P phases along the Hugoniot are unknown. In contrast to the limited shock data, there has been significant interest in understanding iron oxides from diamond cell (DAC) experiments, and recent studies have shown complex polymorphism under static high-P-T conditions. However, these results are discrepant, and it is unclear which static high-P phases, if any, form under shock compression. To address this, we carried out an in situ X-ray diffraction study to investigate phase transformations in hematite and magnetite under laser-shock loading at the Dynamic Compression Sector at the Advanced Photon Source. Our results demonstrate that the high-P phase on the hematite Hugoniot corresponds to the θ-Fe₂O₃ structure while magnetite transforms to a Th₃P₄-phase, both of which are structures that have been reported in recent laser-heated DAC experiments.