Polaronic Optical Transitions in Hematite (${\alpha}-Fe_{2}O_{3}$) Revealed by First-Principles Electron-Phonon Coupling

TL;DR

This study uses first-principles calculations to reveal how polaronic states form and influence optical properties in hematite, highlighting their temperature dependence and coupling with specific phonons, which impacts its optoelectronic behavior.

Contribution

The paper demonstrates the direct optical excitation of polaronic states in hematite using first-principles electron-phonon calculations, providing detailed insights into their formation and temperature evolution.

Findings

Polaronic states are optically accessible in hematite.

Optical absorption involves coupling to LO phonons >50 meV.

Polaron consists of an electron localized between two Fe atoms.

Abstract

Polaron formation following optical absorption is a key process that defines the photophysical properties of many semiconducting transition metal oxides, which comprise an important class of materials with potential optoelectronic and photocatalytic applications. In this work, we use hematite (${\alpha}-Fe_{2}O_{3}$) as a model transition metal oxide semiconductor to demonstrate the feasibility of direct optical population of band-edge polaronic states. We employ first-principles electron-phonon computations within the framework of the DFT+U+J method to reveal the presence of these states within a thermal distribution of phonon displacements and model their evolution with temperature. Our computations reproduce the temperature dependence of the optical dielectric function of hematite with remarkable accuracy and indicate that the band-edge optical absorption and second-order resonance Raman spectra arise from polaronic optical transitions involving coupling to longitudinal optical phonons with energies greater than 50 meV. Additionally, we find that the resulting polaron comprises an electron localized to two adjacent Fe atoms with distortions that lie primarily along the coordinates of phonons with energies of 31 and 81 meV.