Influence of isolated and clustered defects on electronic and dielectric properties of wustite

TL;DR

This study uses density functional theory to explore how intrinsic Fe defects, including vacancies and clusters, affect the electronic and dielectric properties of FeO (wustite), revealing defect-induced changes in conductivity and optical behavior.

Contribution

It provides new insights into the role of isolated and clustered Fe defects on FeO's electronic structure and dielectric response, highlighting the impact of Coulomb interactions and defect concentration.

Findings

Defects reduce the Mott insulating gap in FeO.

High defect content can induce metallic behavior.

Defects cause phonon splitting and broaden dielectric loss spectra.

Abstract

The influence of intrinsic Fe defects in FeO (either single cation vacancies or prototypical 4:1 vacancy clusters) on the electronic and dielectric properties is studied within the density functional theory. The importance of local Coulomb interactions at Fe atoms is highlighted and shown to be responsible for the observed insulating Mott gap in FeO which is reduced by the presence of defects. We investigate nonstoichiometric configurations of Fe1-xO with x ranging from 3 to 9 % and find the aliovalent Fe cations in both the regular and interstitial lattice sites of the considered configurations. Furthermore, we show that the trivalent Fe ions are induced by both isolated and clustered Fe-vacancies and introduce the empty band states inside the insulating gap, which decreases monotonically with increasing cation vacancy concentration. The Fe1-xO systems with high defect content become metallic for small values of the Coulomb interaction U, yielding the increase in the dielectric functions and optical reflectivity at low energies in agreement with the experimental data. Due to the crystal defects, the infrared-active transverse optic phonons split and distribute over a wide range of frequencies clarifying the origin of the exceptionally large spectral linewidths of the dielectric loss functions observed for wustite in recent experiments.