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Iron melting temperature and phase stability under nuclear-like conditions shed light on Earth’s geodynamics

Laser-driven impact phenomena in iron at high-power laser facilities (HPLF). X-ray absorption spectroscopy (XAS) was used to determine the melting temperature and microstructure of iron at pressures relevant to geosciences. Credit: Jean-Alexis Hernandez, European Synchrotron Radiation Facility.

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

Researchers at the European Synchrotron Radiation Facility in Grenoble, the Ecole Polytechnique de Paris, and other research institutions around the world are using ultrafast X-ray absorption spectroscopy to determine the melting temperature and phases of impact-compacted iron at high temperatures and pressures. A study was conducted to investigate the stability.

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

“The goal of this study is to use ultrafast synchrotron X-ray absorption spectroscopy to investigate the microscopic behavior of iron under extreme pressure and temperature conditions up to a few Mbar and thousands of Kelvin range. ,” said Sofia Bargani, lead author of the paper. paper, he told Phys.org.

“This research is critical to understanding the nature of Earth’s core, which is composed primarily of iron and small amounts of lighter elements.”

Since iron is a major component of the Earth’s core, its properties (for example, the melting temperature of iron at the pressures found near the Earth’s inner core boundary) set an upper limit for the melting temperature at this particular boundary separating the inner core. will be done. and the outer core.

Determining the melting temperature at this pressure is useful in geodynamic studies, providing insight into the process by which a liquid outer core crystallizes to form a solid inner core.

“There is also important debate about the structural facies of iron under such extreme conditions,” Bargani said.

“We set out to collect both structural and electronic structure data for iron under these conditions. Because this area of ​​research is completely new, the team has We are still working on interpreting the electronic structure information.”

Balugani and her colleagues conducted their experiments at the European Synchrotron Radiation Facility in France, specifically within its new high-power laser facility. The research site features a variety of advanced technologies, combining high-power lasers with energies exceeding 40 J and an energy-dispersive ID24-ED beamline optimized for ultrafast (approximately 100 ps) X-ray absorption spectroscopy. It has been introduced.

“A high-power laser focuses on a multilayer target and ablate the first layer (usually a polymer) to create a hot plasma,” Bargani explained.

“This plasma expands to generate a shock wave that propagates at supersonic speeds through the iron sample. The shock wave causes extreme pressure and temperature conditions within the iron. At the same time, the X-rays are synchronized and The XAS spectrum of the iron is captured.” A shock wave leaves the sample corresponding to the peak pressure and temperature of the iron. ”

Ultrafast (about 100 ps) X-ray absorption spectroscopy measurements collected by the researchers provided detailed information about the structural phases of iron at extremely high pressures and temperatures.

In addition to measuring the bulk temperature of iron near its melting curve of 240 GPa, the researchers were able to determine the structural changes this element undergoes under 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,” Bargani said.

“In shock compression, the thermal self-emission from a heated sample is typically captured and fitted to a Planck blackbody model to estimate temperature. However, this method has limitations, especially for opaque materials such as metals. For samples, only the surface temperature is known.

Notably, shock compression measurements collected using conventional approaches are also reliable only at temperatures above 3,000 K. In contrast, the method the researchers used allowed them to measure the phase diagram of iron under conditions that mimic conditions deep in the Earth. Use this to estimate the melting temperature at the inner core boundary, where the pressure is known to be 330 GPa.

“We believe this work paves the way for a new method to determine reliable bulk temperatures of metals using XAS. This method can be applied to experimentally constrain the melting curves of various metals. “We can,” Bargani said.

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

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

“Seismological data have observed shear softening under Earth’s core conditions, which some theoretical studies attribute to the BCC (body-centered cubic) phase of pure iron.” Bargani said. “In our study, we excluded the bcc phase of iron at 240 GPa and 5,345 K, which is close to the melting curve.”

The researchers’ findings set new constraints on the melting curve of iron under extreme conditions and disproved some previous theoretical predictions.

Nevertheless, their measurements indicate that phases other than the bcc phase become more favorable when iron is alloyed with lighter elements, both within the Earth’s core or in other pressure-temperature regions of the iron phase diagram. This does not exclude the possibility that this may occur.

“It would be very interesting to investigate iron alloys under such extreme conditions and perform similar experiments,” Bargani added.

“Significant progress has already been made in this field, and we hope that we will soon have a better understanding of the Earth’s core. Technological advances in high-power laser facilities will also allow us to explore even more extreme regions of the planet. Phase diagram of iron and iron alloys. ”

By examining iron and iron alloys at more extreme temperatures and pressures, researchers may be able to better understand the structure of exoplanets such as super-Earths.

Additionally, their future research could contribute to fusion research, where iron, the main component of stainless steel used to conduct inertial confinement fusion research, plays a key role.

Further information: S. Balugani et al, New constraints on melting temperature and phase stability of impact iron up to 270 GPa by ultrafast X-ray absorption spectroscopy, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.133.254101

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Citation: Iron melting temperature and phase stability under core-like conditions shed light on Earth’s geodynamics (January 21, 2025) https://phys.org/news/2025-01-temperur Retrieved from -phase-stability- on January 21, 2025 iron core.html

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