Sébastien Merkel

Publication 100 is out today! Observation of body-centered cubic iron above 200 gigapascals.

Under Earth’s core-like pressures, iron’s expected hexagonal structure energetically competes with other cubic forms. At the heart of this question is then whether or not crystalline phases other than the hexagonal form of iron are stable at inner core conditions. With a team of nearly 40 co-authors, we decided to probe the state of iron at pressures exceeding 200 GPa (over 2 millions atmospheres) using femtosecond X-ray pulses at the European XFEL. Between 120–160 GPa, a stable hexagonal phase is confirmed, with brief transient disordered or cubic phases appearing upon super-fast heating a cooling cycles of 220 nanoseconds. Above 200 GPa, following this uncommon kinetic and pressure-temperature path, we were able to stabilize a cubic structure for iron.

This results came as a big surprise for myself and the whole high pressure and deep Earth communities. Whether this form of iron, which we form over timescales of billionths of a second using intense X-rays, actually exists within the Earth's core remains an open question. Nevertheless, our experiments show that, in fact, one can form a cubic phase of iron under Earth's core pressures and temperatures.

This work was published in Physical Review B in August, with a paper led by Zuzana Konôpková and Eric Edmund. It is a result of a community experiment at the European XFEL which I led nearly four years ago, supported by my project HotCores, funded by the European Research Council (ERC).

I am glad this 100 publications mark falls on this one!

First publication from a PhD student from the HotCores project ! In this publication in the Journal of Applied Physics, Hélène Ginestet uses the European XFEL in Hamburg to explore the phase diagram of pure iron, up to pressures of 150 GPa. The instrument is tuned to send intense X-ray pulses of less that 50 fs duration, repeated every 220 ns on samples placed inside diamond anvil cells to reach temperatures of several thousands of kelvins within microseconds. Experimental challenges arise from temperature gradients within the sample, changes in temperature at the 100 ns timescale, the difficulty of direct temperature estimates, the effect of thermal pressure, and the presence of metastable crystallites due to rapid cycles of heating and cooling. In this work, we develop the metrology for such experiments, how to process data, evaluate pressure and temperature, and validate her technique based on previous publications for iron under pressure.

New paper in Applied Physics Letters with our colleagues from Stanford University: Effect of pressure cycling and compression rate on the bcc-hcp transition in an FeNi alloy. In this work, we use a D-DAC up to over 800 GPa/s to monitor how iron-nickel (FeNi) alloys transition between bcc and hcp structures at about 13 GPa. The transition mechanism changes with compression rates; the phase transformation occurs at a lower pressure; and orientations in the hcp phase after the transition are different from those obtained at low strain rates. The paper was selected as a science highlight by the American Institute of Physics (AIP) journal editors.

I was in Orléans for the week, attending the Rayons X et Matière conference and discussing advanced X-ray experiments for the HotCores project.  Then moved for a seminar at the Institut des Sciences de la Terre d'Orléans, discussing the results of the TIMEleSS project and visiting this nice facility for experimental geosciences!

Yet another tutorial for MAUD: I prepared a new page with a 2025 tutorial for MAUD! MAUD is a Rietveld code which is heavily used for powder diffraction data processing, fitting textures, strains, etc. This tutorial is an updated version of the Berkeley ones which I use in my classes in Lille, based on feedback and failures from my students. I hope you'll find it useful.

I'm in Japan! Attending the Onsen Workshop: Formation and Chemical Composition of Cores in the coming days and presenting some of our work on the ERC HotCores project. Nice to be back in Tokyo, it had been a while. 

The 2025 edition of the EHPRG Meeting was held September 1 to 5 in Manchester, United Kingdom, and I was offered to present a 1-hour keynote lecture on the iron phase diagram at multiple timescales. This was a nice opportunity to highlight our latest results on iron from the ERC HotCores project, obtained using advanced X-ray sources such as the European XFEL and the ESRF synchrotron.  

The 2025 SEES-ISRD Joint Meeting was held at the University of Chicago, United States, August 11-13, 2025, to discuss opportunities for using synchrotron radiation in Earth and Environmental Science as well as for In-situ Studies of Rock Deformation. I gave a keynote presentation on Multi-Grain Diffraction and Deformation Experiments in Diamond Anvil Cells and how the method could be useful for both these fields.

Sunshine on the French northern coast! The Earth and Planetary Materials group of the UMET lab in Lille met in the Univ. Lille Marine Station in Wimereux, on the Nortern Sea coastline, to discuss opportunities for the future of the team.

This 2025 EGU General Assembly was held in Vienna, Austria, 27 April–2 May 2025. I and ERC HotCores members organized the session GD 8.4, Earth and Planetary cores, with several presentations from the project. See you next year!