Pure Fe and Fe-Ni-Si alloys under high-pressure and high temperature at the European XFEL

The Earth’s inner core presents interesting properties such as seismic velocity anisotropy and a complex internal structure that is still under investigation. Establishing the phase diagram of the relevant iron alloys and, first, of pure iron itself is necessary to improve our understanding of planetary cores. The iron phase diagram at high pressure and temperature is still discussed despite numerous experimental and simulation studies. The addition of other elements even complexifies the issue, and, to this day, phase diagrams and melting temperatures of Fe alloys under Earth's core conditions remain to be established.In this work, we explore the phase diagram of Fe and and Fe-Ni-Si alloy up to over 200 GPa and up to melting through a different thermodynamical pathway from conventional laser-heated diamond anvil cell experiments. The experiments rely on new facilities at the European X-Ray Free-Electron Laser, which provides extremely intense X-ray flashes repeated up to every 220 ns. The facility, coupled with the High Energy Density (HED) instrument, allows heating, melting, and crystallizing iron samples repeatedly and probe for its crystal structure as the sample cools from its previous state.The first step of the work was to establish the data processing technique and metrology for working on such dataset, which has now been published very recently (Ginestet et al, J Appl Phys, 2026). In this presentation, I will show our latest results on pure Fe and FeNiSi alloys up to pressures on the order of 200 GPa.