Sebastien Merkel | Searcharxiv

arXiv2026

Iron spin crossover in ferropericlase and its effect on lower-mantle thermal conductivity

Thermal conductivity of Earths lower mantle controls heat transfer across the core-mantle boundary (CMB) and strongly influences mantle convection. We report direct measurements of the thermal conductivity of single-crystal ferropericlase (Mg$_{1-x}$Fe$_x$O, $x = 0.09$-0.13), the second most abundant lower-mantle mineral, using optical laser flash and X-ray free-electron laser heating in diamond-anvil cells up to $\sim2200$~K and 130~GPa. These experiments provide the first conductivity data for ferropericlase at simultaneous lower-mantle pressures and temperatures. A marked reduction in conductivity between 60 and 100~GPa at $\sim1700$~K is consistent with the iron spin crossover. Combined with our previous results for Fe- and Fe,Al-bearing bridgmanite, the data define a lower-mantle conductivity profile that increases with pressure to $\sim10$~W\,m$^{-1}$\,K$^{-1}$ near the CMB, constraining mantle heat flux, plume buoyancy, and long-term geodynamic evolution.

physics.geo-ph↗

arXiv2025

Observation of Body-Centered Cubic Iron above 200 Gigapascals

The crystallographic structure of iron under extreme conditions is a key benchmark for cutting-edge experimental and numerical methods. Moreover, it plays a crucial role in understanding planetary cores, as it significantly influences the interpretation of observational data and, consequently, insights into their internal structure and dynamics. However, even the structure of pure solid iron under the Earth's core conditions remains uncertain, with the commonly expected hexagonal close-packed structure energetically competitive with various cubic lattices. In this study, iron was compressed in a diamond anvil cell to above 200 GPa, and dynamically probed near the melting point using MHz frequency X-ray pulses from the European X-ray Free Electron Laser. The emergence of an additional diffraction line at high temperatures suggests the formation of an entropically stabilized bcc structure. Rapid heating and cooling cycles captured intermediate phases, offering new insights into iron's phase transformation paths. The appearance of the bcc phase near melting at extreme pressures challenges current understanding of the iron phase diagram under Earth's core conditions.

cond-mat.mtrl-sci↗