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

• Published in: Journal of the American Chemical Society Vol. 144; no. 23; pp. 10259 - 10269
• Main Authors: Layek, Samar, Greenberg, Eran, Chariton, Stella, Bykov, Maxim, Bykova, Elena, Trots, Dmytro M., Kurnosov, Alexander V., Chuvashova, Irina, Ovsyannikov, Sergey V., Leonov, Ivan, Rozenberg, Gregory Kh
• Format: Journal Article
• Language: English
• Published: American Chemical Society 15.06.2022
• Subjects: magnetism; magnetite; phase transition; physical phases; rare earth elements; temperature; X-ray diffraction
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.2c00895

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.