Abstract
The archetypal 3d Mott insulator hematite, Fe2O3, is one of the basic oxide components playing an important role in mineralogy of Earth’s lower mantle. Its high pressure–temperature behavior, such as the electronic properties, equation of state, and phase stability is of fundamental importance for understanding the properties and evolution of the Earth’s interior. Here, we study the electronic structure, magnetic state, and lattice stability of Fe2O3 at ultra-high pressures using the density functional plus dynamical mean-field theory (DFT + DMFT) approach. In the vicinity of a Mott transition, Fe2O3 is found to exhibit a series of complex electronic, magnetic, and structural transformations. In particular, it makes a phase transition to a metal with a post-perovskite crystal structure and site-selective local moments upon compression above 75 GPa. We show that the site-selective phase transition is accompanied by a charge disproportionation of Fe ions, with Fe3±δ and δ ~ 0.05–0.09, implying a complex interplay between electronic correlations and the lattice. Our results suggest that site-selective local moments in Fe2O3 persist up to ultra-high pressures of ~200–250 GPa, i.e., sufficiently above the core–mantle boundary. The latter can have important consequences for understanding of the velocity and density anomalies in the Earth’s lower mantle.
Highlights
Being model objects for studying the Mott transition phenomenon, the iron-bearing oxides play an important role in the mineralogy of Earth’s lower mantle and outer core.[1–8]. Because of their complex electronic, magnetic, and crystal structure behavior under high pressure–temperature conditions, these compounds have been of considerable recent interest.[6–12]
Our results reveal that above 75 GPa Fe2O3 adopts a post-perovskite crystal structure, which is characterized by site-selective local moments, with local moments on half of the Fe sites collapsed into the LS state
The calculated electronic the phase stability of Fe2O3 near the Mott transition, we use the atomic positions and the crystal structure parameters taken from the experiments at about 50 GPa.[7,12]
Summary
Being model objects for studying the Mott transition phenomenon, the iron-bearing oxides play an important role in the mineralogy of Earth’s lower mantle and outer core.[1–8] Because of their complex electronic, magnetic, and crystal structure behavior under high pressure–temperature conditions, these compounds have been of considerable recent interest.[6–12] It is known that upon compression these materials exhibit a magnetic collapse—a crossover from a high-spin (HS) to low-spin (LS) state of iron ions, resulting in drastic changes of their physical properties.[13–16] the anomalous behavior of their bulk modulus, density, and elastic properties is essential to understanding the seismic observations and dynamic processes in the Earth’s lower mantle and outer core,[1–5] e.g., for interpretation of the anomalous seismic behavior at the bottom 400 km of Earth’s mantle, in the so-called D′′ region. Being model objects for studying the Mott transition phenomenon, the iron-bearing oxides play an important role in the mineralogy of Earth’s lower mantle and outer core.[1–8] Because of their complex electronic, magnetic, and crystal structure behavior under high pressure–temperature conditions, these compounds have been of considerable recent interest.[6–12]. The calculated critical pressure of MIT ~72 GPa is significantly higher, by ~40%, than that found in the experiments.[10,12] This manifests crucial importance of the interplay of the electronic state and lattice near the Mott transition in.
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