Iron is one of the main elements found in the Earth’s inner core, which is characterized by extremely high temperatures and pressures. Determining how iron behaves in these extreme conditions could thus help to advance the current understanding of our home planet’s structure and geodynamics.

Researchers at the European Synchrotron Radiation Facility in Grenoble, the Polytechnic Institute of Paris and other institutes worldwide carried out a study examining the melting temperature and phase stability of shock-compressed iron at high temperatures and pressures using ultrafast X-ray absorption spectroscopy.

Their findings, published in Physical Review Letters, shed new light on the melting curve and structural phase of iron under extreme conditions.

“The goal of this study was to explore the microscopic behavior of iron under extreme pressure and temperature conditions, up to the multi-Mbar and thousands of Kelvin ranges, using ultrafast synchrotron X-ray absorption spectroscopy,” Sofia Balugani, first author of the paper, told Phys.org.

“This research is crucial for understanding the properties of the Earth’s core, which is primarily composed of iron with small amounts of lighter elements.”

As iron is the primary component of the Earth’s core, its properties (e.g., its melting temperature at the pressures found in the proximity of the Earth’s inner core boundary) set an upper limit for the melting temperature at this specific boundary, which separates the inner and outer core.

Determining the melting temperature at this pressure can in turn aid the study of geodynamics, offering insight into the process via which the outer core, which is liquid, crystallizes to form the solid inner core.

“There is also significant debate regarding the structural phase of iron under these extreme conditions,” Balugani said.

“We set [out] to gather both structural and electronic data of iron at these conditions. The team is still working on interpreting the electronic structure information of iron under these extreme conditions, as this area of research is entirely new.”

Balugani and her colleagues carried out their experiment at the European Synchrotron Radiation Facility in France, specifically within its new High-Power Laser Facility. This research site hosts various advanced technologies, combining high-power lasers with an energy above 40J with an energy-dispersive ID24-ED beamline, optimized for ultra-fast (≈100 ps) X-ray absorption spectroscopy.

“The high-power laser is focused onto a multi-layered target, ablating the first layer (typically a polymer) to create a hot plasma,” explained Balugani.

“This plasma expands and generates a shock wave, propagating at supersonic speeds through the iron sample. The shock wave induces extreme pressure and temperature conditions in the iron. Simultaneously, the X-rays are synchronized to capture the XAS spectrum of iron at the moment the shock wave exits the sample corresponding to the peak pressure and temperature in iron.”

The ultra-fast (≈100 ps) X-ray absorption spectroscopy measurements collected by the researchers yielded detailed information about the structural phase of iron at extremely high pressures and temperatures.

In addition to measuring the bulk temperature of iron near its melting curve at 240 GPa, the researchers were able to determine the structural changes that this element undergoes in conditions that mirror those found in the Earth’s core.

“Temperature is a particularly difficult parameter to measure in both shock and static compression experiments,” said Balugani.

“In shock compression, the thermal self-emission from the heated sample is typically captured and fitted to the Planck black body model to estimate the temperature. However, this method has limitations, particularly for opaque samples like metals, where only the surface temperature can be measured.”

Notably, shock compression measurements collected using conventional approaches are also only reliable above 3,000 K. In contrast, the methods used by the researchers allowed them to measure the phase diagram of iron under conditions mimicking those at the depths of the Earth, which they could then use to extrapolate its melting temperature at the inner core boundary, where the pressure is known to be 330 GPa.

“I believe this work has paved the way for a new method to determine reliable bulk temperatures of metals using XAS, which could be applied to experimentally constrain the melting curves of various metals,” said Balugani.

“Additionally, we determined that the phase of pure iron at 240 GPa and 5,345 K, just before melting, is hexagonal close-packed (hcp), rather than the body-centered cubic (bcc) structure predicted by many theoretical studies.”

This study by Balugani and her colleagues could have important implications for the future study of the Earth’s geodynamics. The measurements collected by the researchers could ultimately advance the understanding of our planet’s inner structure and its thermal history.

“Seismological data has observed shear softening under Earth’s core conditions, which has been attributed to the bcc (body-centered cubic) phase of pure iron by some theoretical studies,” said Balugani. “In our study, we ruled out the bcc phase of iron at 240 GPa and 5,345 K, near the melting curve.”

The researchers’ findings set new constraints on the melting curve of iron under extreme conditions, disproving some earlier theoretical predictions.

Nonetheless, their measurements do not exclude the possibility that a phase other than the bcc phase may become more favorable when iron is alloyed with lighter elements, both in the Earth’s core or in other pressure-temperature regions of the iron phase diagram.

“It would now be fascinating to explore iron alloys under these extreme conditions and conduct similar experiments,” added Balugani.

“There is already significant progress being made in this area, and hopefully, we will soon gain a better understanding of the Earth’s core. With technologic advancements in high-power laser facilities, it will also be possible to explore even more extreme regions of the iron and iron alloys phase diagram.”

By examining iron and iron alloys at even more extreme temperatures and pressures, the researchers could better understand the structure of telluric exoplanets (e.g., super-Earths).

In addition, their future works could contribute to nuclear fusion research, for which iron plays a key role, as it is the main component of the stainless steel used to conduct inertial confinement fusion studies.

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