Experimental evidence of a body centered cubic iron at the Earths core condition

The crystal structure of iron in the Earth’s inner core remains debated. Most recent experiments suggest a hexagonal-close-packed (hcp) phase. In simulations, it has been generally agreed that the hcp-Fe is stable at inner core pressures and relatively low temperatures. At high temperatures, however, several studies suggest a body-centered-cubic (bcc) phase at the inner core condition. We have examined the crystal structure of iron at high pressures over 2 million atmospheres (>200GPa) and at high temperatures over 5000 kelvin in a laser-heated diamond cell using microstructure analysis combined with $\textit{in-situ}$ x-ray diffraction. Experimental evidence shows a bcc-Fe appearing at core pressures and high temperatures, with an hcp-bcc transition line in pressure-temperature space from about 95$\pm$2GPa and 2986$\pm$79K to at least 222$\pm$6GPa and 4192$\pm$104K. The trend of the stability field implies a stable bcc-Fe at the Earth’s inner core condition, with implications including a strong candidate for explaining the seismic anisotropy of the Earth’s inner core.

💡 Research Summary

The authors address the long‑standing debate over the crystal structure of iron in Earth’s inner core. While most recent high‑pressure experiments and many theoretical studies have identified the hexagonal close‑packed (hcp) phase as the stable form at core pressures, several simulations predict that at the extremely high temperatures of the inner core a body‑centered cubic (bcc) phase may become stable. To resolve this, the researchers performed laser‑heated diamond‑anvil cell (LH‑DAC) experiments at pressures exceeding 200 GPa (≈2 million atmospheres) and temperatures above 5000 K, combining in‑situ X‑ray diffraction (XRD) with detailed post‑quench microstructural analysis and spatially resolved XRD microscopy.

Samples consisted of a thin Fe foil sandwiched between two single‑crystal MgO (100) plates. The MgO surfaces serve as potential epitaxial templates. The Fe was rapidly heated with square‑modulated laser pulses (1–5 ms) and then quenched. Diffraction patterns were recorded before, during, and after each heating event, and the recovered samples were examined by scanning XRD to map grain orientations and defect structures.

Key observations include: (1) Above ≈95 GPa, when the temperature exceeds a threshold of roughly 3000 K, the quenched Fe exhibits a strongly bi‑axial texture relative to the MgO substrate, with hcp‑Fe grains showing preferred azimuthal alignment and characteristic diffuse streaks indicative of stacking faults. (2) The authors propose that at high temperature a bcc‑Fe layer nucleates epitaxially on MgO(100) with the orientation bcc‑Fe(001)