E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

1 2 8 H I G H L I G H T S 2 0 2 3 I

X-ray experiments reveal elastic behaviour of iron that likely makes up the Earth s core Single crystals of a high-pressure phase of iron were synthesised using a novel pathway, and X-ray diffraction and scattering techniques were used to characterise its elastic properties. The results shed new light on how seismic waves propagate in the type 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 [1]. 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, it has not been possible to synthesise ε-Fe crystals until now. 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 precise analysis [2].

In this work, a novel alternative pathway was used to synthesise ε-Fe single crystals. A single crystal of α-Fe was first compressed in a diamond anvil cell to pressures of up to 7 GPa and resistively 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 103a). 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 103b).

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 103c). 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 (Figure 104a). Not only did the calculations accurately reproduce the experimental findings (Figure 104a), they also predicted

Fig. 103: 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) IXS signal scattered close to a

(01-1-1) X-ray diffraction spot of ε-Fe. 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.