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

Area of Science:

• Condensed matter physics
• Materials science
• Solid-state chemistry

Background:

• Metal-insulator transitions driven by electronic correlations are fundamental in condensed matter.
• Mixed-valence compounds often exhibit charge ordering (CO), leading to complex phases.
• Magnetite (Fe3O4) is a classic example of a mixed-valence material with debated charge ordering.

Purpose of the Study:

• Investigate the electronic, magnetic, and structural properties of mixed-valence Fe4O5 under high pressure.
• Understand the nature of charge ordering and its transitions in Fe4O5.
• Provide a microscopic explanation unifying Fe4O5's behavior with other charge-ordered materials.

Main Methods:

• High-resolution X-ray diffraction.
• Electrical resistance measurements.
• Density Functional Theory plus Dynamical Mean-Field Theory (DFT+DMFT) calculations.

Main Results:

• At ambient conditions, Fe4O5 is a narrow-gap insulator with Verwey-type charge ordering.
• Under pressure (up to ~100 GPa), Fe4O5 exhibits electronic and magnetic transitions.
• A site-selective insulator-to-metal transition occurs at ~84 GPa, accompanied by valence changes and CO collapse.

Conclusions:

• Fe4O5 displays pressure-induced tuning of charge ordering, competing with 'trimeron'/'dimeron' states.
• The study unifies Fe4O5's CO mechanism with those in magnetite and rare-earth nickelates.
• A comprehensive pressure-temperature phase diagram for Fe4O5 is presented.