Spin crossover in FeO under shock compression

Abstract

FeO (wüstite), which exhibits complex electronic and structural properties with increasing pressure and temperature, is a key mineralogical phase for understanding deep planetary interiors. However, direct measurements of its spin state at high-pressure and temperature remain challenging in static compression experiments. Here, we employ laser-driven shock compression to extend the FeO principal Hugoniot up to $\sim$900 GPa and perform in situ X-ray diffraction and X-ray emission spectroscopy up to 250 GPa, probing FeO's crystal structure and spin state. We demonstrate a continuous spin crossover of iron in FeO over a broad pressure range, with the high-spin state persisting beyond Earth's core-mantle boundary (CMB) conditions. These observations provide new experimental constraints on iron spin state at extreme conditions essential for geophysical models of (exo)planetary interiors.

Figures (6)

• Figure 1: FeO principal Hugoniot relationship in (left): shock velocity (U$_{S}$)-particle velocity (U$_{P}$) relationship and (right) pressure-density (P-$\rho$) relationship. Experimental data from this work (blue triangle) are fitted together with Jeanloz1980 (red circle), and Yagi1988 (green square) data from U$_{P}$$>$ 1.8 km/s (black line). The red and blue dashed lines are the fit Jeanloz1980's data only and the data from this study only, respectively
• Figure 2: X-ray diffraction images from Q0 (2$\theta$: 15° to 35° and Q-range: 22 to 50 nm$^{-1}$) and Q1 (2$\theta$: 19° to 28° and Q-range: 13 to 40 nm$^{-1}$) from ePix 10k detectors and 2D integrated patterns from Q0, Q1, and Q2 at 17 keV for run 638 - 242 GPa, run 529 - 246 GPa, and run 635 - 258 GPa. The orange dashed line is the FeO liquid structure factor from Morard2022
• Figure 3: Top: Fe K$\beta$ X-ray emission spectra collected from shocked FeO between 82 and 261 GPa. The black curve shows the HS reference spectrum at ambient pressure. Bottom: Difference curves between the reference and shocked spectra. The IAD (Integrated Absolute Difference) value Vanko2006 corresponds to the integral of the absolute difference of the solid gray areas. Detailed thermodynamic conditions and IAD values for each shot are listed in Table S2 of the Supplementary Material. Separated XES spectra of all shots are shown in Fig. S4.
• Figure 4: Estimated IAD values Vanko2006 as a function of pressure for shocked FeO. Estimated LS IAD values are indicated by the darker blue area from K$\beta$$^{\prime}$ peak disappearance. Red areas indicate the estimated pressure where the melting curves Fu2024Fischer2010Morard2020Komabayashi2014 crosses Jeanloz1980's Hugoniot.
• Figure 5: Comparison of the experimental spectra with simulated high-spin (HS, black dashed) and low-spin (LS, blue dashed) Fe emission spectra shows. Shocked spectra at 246–261 GPa closely matches the LS simulation, indicating that Fe in FeO approaches a fully low-spin state under these conditions.