The d Orbital Multi Pattern Occupancy in a Partially Filled d Shell: The KFeF₃ Perovskite as a Test Case

Area of Science:

• Solid-state chemistry
• Computational materials science
• Magnetism

Background:

• Potassium iron fluoride (KFeF3) exhibits a complex electronic structure with t2g4eg2 d-shell occupancy in iron (Fe) atoms.
• The Jahn-Teller effect theoretically predicts a tetragonal distortion in KFeF3, which contradicts experimental findings.

Purpose of the Study:

• To resolve the discrepancy between theoretical predictions and experimental observations of KFeF3's crystal structure.
• To investigate the role of minority spin electron configuration on the electronic and structural properties of KFeF3.

Main Methods:

• Utilized a 2 × 2 × 2 supercell model (40 atoms per cell) to explore 6561 possible local electronic configurations.
• Employed three hybrid density functional theory (DFT) functionals: B3LYP, PBE0, and HSE06.
• Calculated energies for ferromagnetic and anti-ferromagnetic spin arrangements.

Main Results:

• Identified numerous low-energy configurations where Fe atoms exhibit varied t2g orbital occupancies, often more stable than the fully ordered state.
• Demonstrated that the orientation of the beta (β) spin electron on Fe atoms influences electrostatic interactions by optimizing Fe quadrupole arrangements.
• Confirmed that these electronic configurations do not significantly alter magnetic interactions within the material.

Conclusions:

• The study resolves the contradiction regarding KFeF3's structure by highlighting the importance of local electronic configurations.
• The electrostatic effects arising from minority spin electron orientation are key to understanding KFeF3's observed properties.
• Hybrid functionals like B3LYP, PBE0, and HSE06 consistently model these complex electronic interactions.