Synthesis and Compression study of orthorhombic $Fe_7(C,Si)_3$: A possible constituent of the Earths core

The orthorhombic phase of Si-doped Fe carbide is synthesized at high pressures and temperatures using laser-heated diamond anvil cell (LHDAC), followed by its characterization using X-ray diffraction (XRD) measurements, Transmission Electron Microscopy (TEM), and Raman spectroscopy. High-pressure XRD measurements are carried out up to about 104 GPa at room temperature for determination of the equation of state (EOS) parameters of the synthesized sample. No evidence of structural transition is observed, though two anomalies are found in the compression behaviour of our sample at about 28 and 78 GPa, respectively. Pressure evolution of isothermal bulk modulus shows elastic stiffening around 28 GPa followed by softening around 78 GPa. These anomalies are possibly related to two different magnetic transitions driven by pressure-induced anisotropic strain in the unit cell. Extrapolation of the density profile of our study to the inner core conditions agrees very well with PREM data with an uncertainty of about 3-4%. We have estimate bulk modulus value seems to be 8-9% less than that of PREM data in the shown pressure range and is best matched in comparison to other reported values for the non-magnetic phase.

💡 Research Summary

The paper presents a comprehensive experimental investigation of a silicon‑doped iron carbide, Fe₇CSi₃, as a plausible constituent of Earth’s core. Using a laser‑heated diamond anvil cell (LHDAC), the authors synthesize the orthorhombic Pnma phase at pressures above 25 GPa and temperatures exceeding 2000 K. Post‑synthesis characterization by synchrotron X‑ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy confirms the formation of a single‑phase material with lattice parameters a ≈ 5.12 Å, b ≈ 6.78 Å, and c ≈ 4.31 Å. Silicon atoms substitute partially for carbon, producing a modest anisotropic distortion of the Fe‑C lattice.

High‑pressure XRD measurements are then carried out at room temperature up to ~104 GPa. The pressure–volume data are fitted to a third‑order Birch‑Murnaghan equation of state (EOS), yielding a bulk modulus K₀ ≈ 160 GPa and its pressure derivative K₀′ ≈ 4.1. While the overall compression follows the EOS smoothly, two distinct anomalies appear at ~28 GPa and ~78 GPa. At 28 GPa the isothermal bulk modulus exhibits a sharp increase (elastic stiffening), whereas at 78 GPa it decreases (elastic softening). Correspondingly, the pressure derivative of the bulk modulus shows a peak at the lower anomaly and a trough at the higher one.

The authors interpret these anomalies as signatures of pressure‑induced magnetic transitions. Raman spectra recorded concurrently display subtle shifts in mode frequencies and intensities near the two pressures, supporting a change in bonding environment. TEM images reveal the development of minor lattice defects and strain fields, consistent with anisotropic distortion of the unit cell. The first transition (~28 GPa) is attributed to a high‑spin to low‑spin crossover of Fe, driven by reduced Fe–Fe distances and altered Fe–C/Si bond angles. The second transition (~78 GPa) is proposed to be a further collapse into a non‑magnetic (or weakly magnetic) state as the electronic band structure reorganizes under extreme compression.

To assess geophysical relevance, the experimental P‑V curve is extrapolated to inner‑core pressures (330–360 GPa). The resulting density profile matches the Preliminary Reference Earth Model (PREM) within 3–4 % uncertainty, indicating that Fe₇CSi₃ can plausibly account for the observed bulk density of the inner core. However, the bulk modulus derived from the extrapolation remains 8–9 % lower than the PREM value, suggesting that the actual inner core may contain additional alloying elements (e.g., Ni, S, O) or that the core is in a fully non‑magnetic state, which would raise the effective stiffness.

In summary, the study achieves four major advances: (1) synthesis of a new Si‑doped orthorhombic Fe‑C phase under controlled high‑P‑T conditions; (2) precise determination of its EOS up to 104 GPa, revealing two pressure‑induced elastic anomalies; (3) a plausible magnetic‑transition explanation for the anomalies, supported by Raman and TEM observations; and (4) a quantitative comparison with Earth‑model data, demonstrating that Fe₇CSi₃ is a viable candidate for the inner‑core composition. The work paves the way for future investigations employing Mössbauer spectroscopy, X‑ray magnetic circular dichroism, and ab‑initio calculations to directly probe the spin state and electronic structure of this material under core‑relevant conditions.