Verwey-Type Charge Ordering and Site-Selective Mott Transition in Fe₄O₅ under Pressure

Abstract

The metal–insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which “unifies” it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the “trimeron”/“dimeron” charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure–temperature phase diagram of Fe4O5.

Electronic properties of single-crystalline Fe4O5

“…To date, band structure calculations have been inconsistent in the prediction of the electronic properties of Fe 4 O 5 , reporting it to be a metal, 49,101 a wide-gap semiconductor, 50 or to be a narrow-gap semiconductor with E g = 0.2 eV. 102 Our study resolves these discrepancies.…”

“…99 However, the above small difference in the energies of the lowermost d-orbitals of Fe1, Fe2, and Fe3 atoms in Fe 4 O 5 is challenging to trace both experimentally and by band structure calculations. 100 To date, band structure calculations have been inconsistent in the prediction of the electronic properties of Fe 4 O 5 , reporting it to be a metal, 49,101 a wide-gap semiconductor, 50 or to be a narrow-gap semiconductor with E g = 0.2 eV. 102 Our study resolves these discrepancies.…”

“…4c), its electrical conductivity indeed may look as semiconducting. 50,102 Furthermore, in polycrystalline samples, a thermal activation energy of electrical conductivity can be significantly increased due to grain-boundary effects (Fig. 3).…”

“…While band structure calculations predicted a metallic nature of both Fe 4 O 5 48 and Fe 5 O 6 , 49 recent experiments showed seemingly semiconducting properties of Fe 4 O 5 and its metallization was observed only under high pressure above 80 GPa at room temperature. 50,51…”

“…While band structure calculations predicted a metallic nature of both Fe 4 O 5 48 and Fe 5 O 6 , 49 recent experiments showed seemingly semiconducting properties of Fe 4 O 5 and its metallization was observed only under high pressure above 80 GPa at room temperature. 50,51 A typical splitting of the d-orbitals of Fe 2+ (3d 6 ) and Fe 3+ (3d 5 ) ions in highly-symmetric octahedral and trigonal-prismatic oxygen coordinations 52,53 is depicted in Fig. 2.…”