[Paper Review] Theoretical and experimental evidence of a site-selective Mott transition in Fe2O3 under pressure

Eran Greenberg, I. Leonov+7 more|arXiv (Cornell University)|Jun 8, 2017

High-pressure geophysics and materials60 references17 citations

TL;DR

This study presents experimental and theoretical evidence for a site-selective Mott transition in hematite (Fe2O3) under high pressure, where only half the Fe³⁺ ions become delocalized and metallic while the other half remain localized. The transition, occurring between 50–68 GPa, is marked by a site-dependent collapse of local moments and strong electron correlations, with effective mass renormalizations of m*/m ~4–6 near the Fermi level, indicating a novel form of pressure-induced metallization distinct from conventional Mott transitions.

ABSTRACT

We provide experimental and theoretical evidence for a novel type of pressure-induced insulator-metal transition characterized by site-selective delocalization of the electrons. Mössbauer spectroscopy, X-ray diffraction and electrical transport measurements on Fe$_2$O$_3$ to 100 GPa, along with dynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by ($^ m{VI}$Fe$^{3+ m{HS}}$)$_2$O$_3$ [$R\bar{3}c$ structure] $\overrightarrow{ iny m{50~GPa}}$ ($^ m{VIII}$Fe$^{3+ m{HS~VI}}$Fe$^ m{M}$)O$_3$ [$P2_1/n$ structure] $\overrightarrow{ iny m{68~GPa}}$ ($^ m{VI}$Fe$^ m{M}$)$_2$O$_3$ [$Aba2$ structure]. Within the $P2_1/n$ crystal structure, characterized by two distinct coordination sites (VI and VIII), we observe equal abundances of ferric ions (Fe$^{3+}$) and ions having delocalized electrons (Fe$^ m{M}$), and only at higher pressures is a fully metallic $Aba2$ structure obtained, all at room temperature. The transition is characterized by delocalization/metallization of the $3d$ electrons on half the Fe sites, with a site-dependent collapse of local moments. Above $\sim$50 GPa, Fe$_2$O$_3$ is a strongly correlated metal with reduced electron mobility (large band renormalizations) of m*/m$\sim$4 and 6 near the Fermi level. Upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition ($^ m{VIII}$Fe$^{3+ m{HS~VI}}$Fe$^ m{M}$)O$_3$ $\overrightarrow{ iny m{50~GPa}}$ ($^ m{VIII}$Fe$^{3+ m{HS~VI}}$Fe$^{3+ m{HS}}$)O$_3$ within the same $P2_1/n$ structure, indicating a decoupling of the electronic and lattice degrees of freedom, characteristic of a true Mott transition. Our results show that the interplay of electronic correlations and lattice may result in rather complex behavior of the electronic structure and magnetic state.

Motivation & Objective

• To investigate the electronic and magnetic response of Fe2O3 under extreme pressure, particularly the nature of its insulator-to-metal transition.
• To determine whether the transition involves site-selective delocalization of 3d electrons, rather than a uniform metallization.
• To explore the role of electronic correlations and lattice structure in driving a non-uniform Mott transition.
• To examine the reversibility and electronic structure evolution upon decompression, testing the decoupling of electronic and lattice degrees of freedom.

Proposed method

• High-pressure Mössbauer spectroscopy to probe local electronic and magnetic environments of Fe sites.
• X-ray diffraction to track structural evolution and coordination changes under pressure.
• Electrical transport measurements to detect metallization and quantify carrier mobility.
• Dynamical mean-field theory (DFT+DMFT) calculations to model electronic correlations and predict site-selective behavior.
• Analysis of crystal structure changes from R¯3c to P2₁/n and finally to Aba2, with distinct Fe coordination sites (VI and VIII).
• Comparison of experimental data with theoretical predictions to confirm site-selective Mott transition characteristics.

Experimental results

Research questions

• RQ1 Does Fe2O3 undergo a site-selective Mott transition under high pressure, with only a subset of Fe sites becoming metallic?
• RQ2 What is the role of electronic correlations in driving the selective delocalization of 3d electrons in Fe2O3 at high pressure?
• RQ3 How do the local magnetic moments and electronic structure evolve across the transition, and is this behavior reversible upon decompression?
• RQ4 To what extent do the lattice and electronic degrees of freedom decouple during the transition, indicating a true Mott transition?
• RQ5 What is the effective mass enhancement of charge carriers near the Fermi level in the site-selective metallic phase?

Key findings

• A site-selective Mott transition occurs between 50 and 68 GPa in Fe2O3, where only half the Fe sites become metallic while the other half remain localized.
• The transition is characterized by a site-dependent collapse of local magnetic moments, with Fe³⁺(HS) ions on the VI site remaining localized and Fe ions on the VIII site becoming delocalized (Feᴹ).
• Effective mass renormalizations of m*/m ~4–6 are observed near the Fermi level in the P2₁/n phase, indicating strong electron correlations and reduced electron mobility.
• Upon decompression, the system reverts to a conventional Mott insulator within the same P2₁/n structure, confirming the decoupling of electronic and lattice degrees of freedom.
• The final metallic phase at higher pressures adopts an Aba2 structure, where all Fe sites are delocalized and fully metallic.
• The transition is not uniform: the system exhibits coexistence of localized and delocalized electrons on distinct Fe sites, defining a novel class of site-selective Mott transitions.