[Paper Review] Phase transitions and spin-state of iron in FeO at the conditions of Earth's deep interior

Eran Greenberg, Roman Nazarov+9 more|arXiv (Cornell University)|Apr 1, 2020

High-pressure geophysics and materials44 references7 citations

TL;DR

This study combines x-ray diffraction, x-ray emission spectroscopy, and DFT+DMFT calculations to investigate iron's spin-state and structural transitions in FeO under deep Earth conditions (up to 140 GPa and 2100 K). It identifies a high-pressure, high-temperature phase of FeO with metallic high-spin iron near the core-mantle boundary, which significantly alters elastic and transport properties and may explain seismic anomalies in Earth's lower mantle.

ABSTRACT

Iron-bearing oxides undergo a series of pressure-induced electronic, spin and structural transitions that can cause seismic anomalies and dynamic instabilities in Earth's mantle and outer core. We employ x-ray diffraction and x-ray emission spectroscopy along with state-of-the-art density functional plus dynamical mean-field theory (DFT+DMFT) to characterize the electronic structure and spin states, and crystal-structural properties of wüstite (Fe$_{1-x}$O) -- a basic oxide component of Earth's interior -- at high pressure-temperature conditions up to 140 GPa and 2100 K. We find that FeO exhibits complex polymorphism under pressure, with abnormal compression behavior associated with electron-spin and crystallographic phase transitions, and resulting in a substantial change of bulk modulus. Our results reveal the existence of a high-pressure phase characterized by a metallic high-spin state of iron at about the pressure-temperature conditions of Earth's core-mantle boundary. The presence of high-spin metallic iron near the base of the mantle can significantly influence the geophysical and geochemical properties of Earth's deep interior.

Motivation & Objective

• To resolve long-standing controversies about the high-pressure phase diagram and electronic structure of FeO, a key component of Earth's deep interior.
• To determine the interplay between crystal structure, spin-state transitions, and electronic correlations in FeO under extreme conditions.
• To clarify whether iron in FeO undergoes a high-spin to low-spin transition under pressure and how this affects bulk modulus and seismic properties.
• To assess the geophysical implications of metallic high-spin FeO in the D" region near the core-mantle boundary.
• To reconcile conflicting prior experimental and theoretical results on FeO's spin state and phase transitions.

Proposed method

• X-ray diffraction (XRD) was used to identify crystal structures of FeO at pressures up to 140 GPa and temperatures up to 2100 K.
• X-ray emission spectroscopy (XES) measured the electronic configuration and spin state of iron, with particular focus on the Kβ shoulder as a signature of low-spin state.
• Density functional theory combined with dynamical mean-field theory (DFT+DMFT) was employed to model electron correlation, spin-state transitions, and electronic structure changes.
• Simultaneous high-pressure and high-temperature experiments were conducted using a large-volume press at beamline facilities (GeoSoilEnviroCARS and HPCAT) at the Advanced Photon Source.
• Theoretical calculations were validated against experimental data to distinguish between high-spin and low-spin states and to assess structural phase transitions.
• Data analysis focused on identifying phase boundaries, spin-state evolution, and changes in bulk modulus across transitions.

Experimental results

Research questions

• RQ1 Does FeO undergo a high-spin to low-spin transition under high pressure and temperature, and if so, at what conditions?
• RQ2 What is the crystal structure of FeO at pressures and temperatures relevant to Earth's core-mantle boundary?
• RQ3 How do spin-state transitions in FeO affect its bulk modulus and elastic properties?
• RQ4 Is the previously reported low-spin B1 phase of FeO above 70 GPa and 1300 K experimentally confirmed?
• RQ5 To what extent do electronic correlations and structural distortions decouple in FeO under extreme conditions?

Key findings

• FeO exhibits a novel high-spin B1-type phase under conditions relevant to Earth's core-mantle boundary, with iron remaining in a metallic high-spin state up to 140 GPa and 2050 K.
• The rB1-to-B1 phase transition occurs at high temperature (above 1800 K), with iron fully in the high-spin state, contradicting prior reports of low-spin B1 at high pressure and temperature.
• A significant change in bulk modulus is observed due to coupled structural and spin-state transitions, with a ~10% increase in sound velocity across the high-spin to low-spin transition and a ~5% decrease across the B8-to-B2 transition.
• XES measurements show no disappearance of the Kβ shoulder, indicating the absence of a low-spin state at high pressure and temperature, resolving discrepancies with earlier studies.
• The spin state of iron is sensitive to crystal-field splitting changes during the rB1-to-B1 transformation, indicating decoupling between structural and spin-state transitions.
• DFT+DMFT calculations confirm that the high-spin metallic phase is stable under core-mantle boundary conditions, supporting its potential role in seismic anomalies and heat flow variations.