Pressure-induced high-spin/low-spin disproportionated state in the Mott insulator FeBO₃

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Mott insulators exhibit pressure-induced transitions to metallic states, often linked to electron delocalization and spin-state changes.
• Iron borate (FeBO3) is a canonical Mott insulator, but its high-pressure behavior remains debated.
• Standard models often assume a simple high-spin to low-spin (HS-LS) transition, which may not fully capture complex phenomena.

Purpose of the Study:

• To resolve the long-standing controversy surrounding the high-pressure behavior of the Mott insulator FeBO3.
• To investigate the electronic properties of FeBO3 under extreme pressures beyond the scope of conventional HS-LS transition models.
• To elucidate the nature of spin-state transitions in Mott insulators at high pressures.

Main Methods:

• High-resolution X-ray diffraction (XRD) measurements up to ~150 GPa.
• Mössbauer spectroscopy to probe electronic and magnetic states.
• Analysis of crystal structure and electronic properties under varying pressures.

Main Results:

• An unusual "mixed" high-spin/low-spin (HS/LS) electronic state was observed in FeBO3 within a wide pressure range (50-106 GPa).
• This mixed state exhibited a stable abundance ratio of HS and LS Fe sites within the R-3c crystal structure.
• The findings indicate the insufficiency of standard HS-LS transition models for describing Mott insulators at high pressures.

Conclusions:

• The high-pressure behavior of FeBO3 is characterized by an unconventional, cooperative, and potentially dynamical ordering of mixed HS/LS Fe sites.
• This complex spin-state ordering leads to the frustration of magnetic moments.
• The study necessitates revised theoretical approaches for understanding Mott insulators under extreme conditions.