Pressure-Induced Site-Selective Mott Insulator-Metal Transition in ${\mathrm{Fe}}_2{\mathrm{O}}_3$

We provide experimental and theoretical evidence for a pressure-induced Mott insulator-metal transition in ${\mathrm{Fe}}_2{\mathrm{O}}_3$ characterized by site-selective delocalization of the electrons. Density functional plus dynamical mean-field theory ($\mathrm{DFT}+\mathrm{DMFT}$) calculations, along with Mössbauer spectroscopy, x-ray diffraction, and electrical transport measurements on ${\mathrm{Fe}}_2{\mathrm{O}}_3$ up to 100 GPa, reveal this site-selective Mott transition between 50 and 68 GPa, such that the metallization can be described by ${({}^{\mathrm{VI}}{\mathrm{Fe}}^{3+\mathrm{HS}})}_2{\mathrm{O}}_3$ $[R\overline{3}c\text{structure}]\underset{50\mathrm{GPa}}{\to }({}^{\mathrm{VIII}}{\mathrm{Fe}}^{3+\mathrm{HS}}{{}^{\mathrm{VI}}\mathrm{Fe}}^{\mathrm{M}}){\mathrm{O}}_3$ $[P{2}_1/n\text{structure}]\underset{68\mathrm{GPa}}{\to }{({}^{\mathrm{VI}}{\mathrm{Fe}}^{\mathrm{M}})}_2{\mathrm{O}}_3[Aba2/PPv\text{structure}]$. Within the $P{2}_1/n$ crystal structure, characterized by two distinct coordination sites (VI and VIII), we observe equal abundances of ferric ions (${\mathrm{Fe}}^{3+}$) and ions having delocalized electrons (${\mathrm{Fe}}^{\mathrm{M}}$), and only at higher pressures is a fully metallic high-pressure structure obtained, all at room temperature. Thereby, 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 approximately 50 GPa, ${\mathrm{Fe}}_2{\mathrm{O}}_3$ is a strongly correlated metal with reduced electron mobility (large band renormalizations) of $m^{*}/m\sim 4$ and 6 near the Fermi level. Importantly, upon decompression, we observe a site-selective (metallic) to conventional Mott insulator phase transition $({}^{\mathrm{VIII}}{\mathrm{Fe}}^{3+\mathrm{HS}}{{}^{\mathrm{VI}}\mathrm{Fe}}^{\mathrm{M}}){\mathrm{O}}_3\underset{50\mathrm{GPa}}{\to }({}^{\mathrm{VIII}}{\mathrm{Fe}}^{3+\mathrm{HS}}{{}^{\mathrm{VI}}\mathrm{Fe}}^{3+\mathrm{HS}}){\mathrm{O}}_3$ within the same $P{2}_1/n$ structure, indicating a decoupling of the electronic and lattice degrees of freedom. Our results offer a model for understanding insulator-metal transitions in correlated electron materials, showing that the interplay of electronic correlations and crystal structure may result in rather complex behavior of the electronic and magnetic states of such compounds.

Popular Summary

The distinction between insulator and metal is one of the most fundamental concepts of condensed-matter physics, having relevance to fields ranging from materials science to planetary physics. In particular, the Mott transition from insulator to metal, which is controlled by mutual interactions between electrons, plays a key role in transition-metal oxides. Here, we provide theoretical and experimental evidence for a complex pressure-induced insulator-to-metal transition in an archetypal “$3d$ Mott insulator” ${\mathrm{Fe}}_2{\mathrm{O}}_3$, whose insulating behavior is a result of strong on-site Coulomb repulsion of electrons.

This new transition is site selective—electrons in the $3d$ orbital of the iron atoms are delocalized first at one location and then more generally throughout the crystal. Above a pressure of 50 GPa, the material forms a strongly correlated metal with reduced electron mobility compared to a normal metal. When the pressure is reduced, ${\mathrm{Fe}}_2{\mathrm{O}}_3$ reverts to a conventional Mott insulator.

Our work highlights the interplay between electronic states and crystal structures and not only addresses a long-standing controversy regarding the high-pressure behavior of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ but also suggests that site-selective Mott transitions are important for transition-metal compounds in general.

Figure 1

Pressure dependence of the unit-cell volume of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ at room temperature [$V_0$ is the unit-cell volume of the hematite phase at 1 bar and 300 K (for details, see Table S-III [46]); lattice volume is normalized to 2 formula units of ${\mathrm{Fe}}_2{\mathrm{O}}_3$, and volume error bars are within the symbol sizes]. Solid and open symbols indicate compression and decompression (powder), respectively, and half-filled symbols correspond to the single-crystal compression experiments (DPv and Aba2 structures) [28]. Results of first-principles ($\mathrm{DFT}+\mathrm{DMFT}$) calculations for the hematite, double-perovskite (DPv) and postperovskite (PPv, ${\mathrm{CaIrO}}_3$ structure) phases are shown by solid lines. Additional abbreviations are defined as follows: double-perovskite phase formed on decompression (${\mathrm{DPv}}_{\text{dec}}$), high-pressure phase (HP), high-spin (HS), Mott insulator (MI), nonmagnetic (NM).

Figure 2

Total energy and local magnetic moment $\sqrt{⟨{\widehat{m}}_z^2⟩}$ of paramagnetic ${\mathrm{Fe}}_2{\mathrm{O}}_3$ calculated by $\mathrm{DFT}+\mathrm{DMFT}$ for different lattice volumes at $T=1160\mathrm{K}$, where MI stands for Mott insulator, SSMI for site-selective MI, and PM for paramagnetic metal. Above 45 GPa, the hematite-structured ($R\overline{3}c$) MI phase transforms to the double-perovskite (DPv) SSMI phase, which, above 75 GPa, further collapses to the postperovskite (PPv) PM phase. The corresponding phase transitions are shown by vertical dashed lines. Our results for the lattice volume collapse are marked by red shading.

Figure 3

Spectral function for paramagnetic DPv ${\mathrm{Fe}}_2{\mathrm{O}}_3$ calculated by $\mathrm{DFT}+\mathrm{DMFT}$ at temperature $T=390\mathrm{K}$ and various lattice volumes, based on crystal structure parameters taken from the x-ray diffraction results at approximately 54 GPa. Fe $3d$ (black, red, and blue curves) and O $2p$ (blue-shaded area) orbitals are shown relative to the Fermi energy ($E_F=0$). $\mathrm{DFT}+\mathrm{DMFT}$ results reveal the existence of a site-selective Mott phase at $V=0.71V_0$ (b), in which the $3d$ electrons of only half of the Fe sites (right: $B$ sites) are metallic, while the other Fe (left: $A$ sites) remain insulating. For $V=0.97V_0$ (6 GPa), we obtain a conventional Mott insulating phase in which both the Fe $A$ and $B$ sublattices are insulating (a). Under a high pressure of approximately 219 GPa ($0.55V_0$), $\mathrm{DFT}+\mathrm{DMFT}$ give a metallic solution for both the Fe $A$ and $B$ sites (c).

Figure 4

Partial Fe $3d$ occupations of paramagnetic DPv ${\mathrm{Fe}}_2{\mathrm{O}}_3$ calculated by $\mathrm{DFT}+\mathrm{DMFT}$ as a function of lattice volume.

Figure 5

Local spin-spin correlation function $\chi (\tau )=⟨{\mathrm{m}}_{\mathrm{z}}(\tau ){\mathrm{m}}_{\mathrm{z}}(0)⟩$ calculated by $\mathrm{DFT}+\mathrm{DMFT}$ for paramagnetic DPv ${\mathrm{Fe}}_2{\mathrm{O}}_3$.

Figure 6

${}^{57}\mathrm{Fe}$ Mössbauer spectra of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ at various pressures and temperatures: at room temperature (a),(c) and reduced temperatures (b),(d), upon compression (a),(b),(d) and decompression (c). Red, blue, green, and magenta lines and shaded areas represent fits to the hematite, magnetic DPv (${\mathrm{DPv}}_m$), nonmagnetic DPv (${\mathrm{DPv}}_{nm}$), and ${\mathrm{DPv}}_{\text{dec}}$ components; the black solid line is the sum of all components. The high-pressure phase (HP) component is shown in solid grey.

Figure 7

Pressure evolution of the relative abundances of the Mössbauer spectral components, and the abundance of the metallic phase. The abundances of the Mössbauer spectral components are shown for compression (a) and decompression (b) as computed from the relative areas of the absorption bands. Note the onset of the new ${\mathrm{DPv}}_{\text{dec}}$ component upon decompression below 47 GPa, with significant hysteresis in the reappearance of the DPv and hematite components. Upon recompression from 25 GPa (not shown), the double-perovskite nonmagnetic (${\mathrm{DPv}}_{nm}$) and doube-perovskite magnetic (${\mathrm{DPv}}_m$) components appear again at 50 GPa, with a relative abundance in agreement with the compression trend. The volumetric abundance of the metallic phase is shown for decompression and recompression (from 37 GPa) (c), as derived from the room-temperature measurements of electrical resistance (see Supplemental Material [46]). Note the two distinct steps in the $V_M/V_0(P)$ dependence. Solid curves are guides for the eye.

Figure 8

X-ray powder diffraction patterns of ${\mathrm{Fe}}_2{\mathrm{O}}_3$ at $T=298\mathrm{K}$ at various pressures. The double-perovskite (DPv) intermediate-pressure phase, which first appears at approximately 53 GPa, is clearly seen at 55.2 GPa. Up- and down-facing arrows represent an increase and decrease in the DPv and hematite phases, respectively. Upon decompression, the distinctive doubling of the diffraction peaks of the DPv phase is observed at about 51 GPa, especially (002), (111), and ($11\overline{1}$). A part of the spectrum in the $2\theta$ range of 4.5°–6.0° is expanded in the inset to emphasize this doubling. With further decompression below 35 GPa, the peaks of the hematite $R\overline{3}c$ structure appear, and, at 25 GPa, the transition to the original hematite phase is complete. The main diffraction peak of the Ne pressure medium is marked with an asterisk. Italics correspond to the diffraction peaks of the DPv phase.

Figure 9

Monoclinic distortion of the double-perovskite phase of ${\mathrm{Fe}}_2{\mathrm{O}}_3$. Pressure evolution of the $\beta$ angle, characteristic of the monoclinic distortion of the unit cell with $P{2}_1/n$ symmetry (DPv structure), upon compression and decompression (solid and open symbols, respectively). Note the significant decrease of the monoclinic distortion coinciding with the reverse electronic transition to the strongly correlated state upon decompression; the $\beta$ angle changes from 91.4° to 90.1°.

Figure 10

Pressure dependence of electrical resistance at 298 K. The solid triangles, open hexagons, and half-filled pentagons show data recorded upon compression, decompression, and successive recompression, respectively. Inset (a): our results for pressures of 60–90 GPa; inset (b): temperature dependence of electrical conductance $S$ of ${\mathrm{Fe}}_2{\mathrm{O}}_3$, with measurements on the DPv phase performed during decompression to 37 GPa, followed by recompression. Measurements on the hematite phase were collected during a separate decompression cycle to ambient pressure (symbols “square”) [55]. The change in sign of the slope $\mathrm{dln}(S)/dT$ documented at approximately 53 GPa is the signature of metallic conductivity.