X-ray experiments reveal elastic behaviour of iron that likely makes up the Earth’s core

Our understanding of Earth's internal structure is based on seismological studies, which analyse how elastic waves propagate through the planet. Nearly a century ago, seismology revealed that the Earth's interior consists of a solid inner core and a liquid outer core. The solid inner core is thought to be primarily composed of a form of iron known as epsilon iron (ϵ-Fe), which can only exist under extremely high pressures. Later, it was discovered that the solid inner core behaves anisotropically (i.e., seismic waves travel faster in the polar direction than in the equatorial direction). To understand why, a deeper understanding of the crystalline elastic properties of ϵ-Fe, is required. However, this has not been possible so far. When crystals of the form of iron stable at atmospheric pressure (alpha iron, or α-Fe) are compressed and heated in diamond anvil cells to core pressures and temperatures in an attempt to obtain ϵ-Fe, the α-Fe crystals fracture into smaller crystals that undergo plastic deformation, making them unsuitable for a precise analysis of the elastic properties.
 

Click image to enlarge

Fig. 1: a) XRD-monitored synthesis: equilibrium phase diagram of iron, which shows stability fields for three different solid phases in the 0-15 GPa, 300-1000 K range: α-Fe, γ-Fe and ϵ-Fe. The Pressure-Temperature path used for the synthesis (red arrows) avoids direct α-Fe→ ϵ-Fe transformation. b) Inelastic X-ray scattering (IXS) signal scattered close to a (01-1-1) X-ray diffraction spot of ϵ-Fe; L and T2 designate the IXS signal from longitudinal and transverse phonons, respectively. The insert shows the iron single crystal in the Ne pressure transmitting medium in the diamond anvil cell. c) Elastic anisotropy: representation of elastic velocities in an ϵ-Fe single crystal at 33 GPa, vs. direction of propagation (blue: longitudinal waves; red and green: transverse waves). Below: Schematic representation of an ϵ-Fe hexagonal single crystal.  

In this work, a novel alternative pathway was used to synthesise ϵ-Fe single crystals. A single crystal of α-Fe was first compressed to pressures of up to 7 GPa and heated to 800 K, to transform it into gamma iron (γ-Fe), an intermediate phase of iron that occurs at high temperatures. γ-Fe crystals more readily transform into ϵ-Fe under further compression, with no decrease in crystal quality. The synthesis was monitored with X-ray diffraction (XRD) at beamline ID27 (Figure 1a). Suitable ϵ-Fe single crystals were then transferred to beamline ID28 for inelastic X-ray scattering (IXS) measurements of ϵ-Fe elasticity under pressures reaching 33 GPa (Figure 1b).

The IXS measurements showed that longitudinal waves travel 4.4% faster in the perpendicular plane than in the basal plane of an ϵ-Fe hexagonal single crystal (Figure 1c). However, these findings were observed at a pressure of 33 GPa and temperature of 300 K, while the Earth’s inner core sustains ~350 GPa and 5500 K. Theoretical ab initio calculations for ϵ-Fe were therefore carried out up to inner core pressures. Not only did the calculations accurately reproduce the experimental findings, they also predicted that this anisotropy persists at the pressure and temperature conditions of the Earth’s core, which is consistent with observations of how seismic waves propagate through the planet.

By combining novel experimental and theoretical methods, this study advances our understanding of the composition and behaviour of the materials at the centre of our planet.

Principal publication and authors
Synthesis of Single Crystals of ϵ-Iron and Direct Measurements of Its Elastic Constants, A. Dewaele (a,b), B. Amadon (a,b), A. Bosak (c), V. Svitlyk (c,d), F. Occelli (a,b), Phys. Rev. Lett. 131, 034101 (2023); https://doi.org/10.1103/PhysRevLett.131.034101
(a) CEA DAM-DIF, Arpajon (France)
(b) Université Paris-Saclay, CEA, Bruyères-le-Châtel (France)
(c) ESRF
(d) Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, Dresden (Germany)