Thermally Activated Optical Absorption into Polaronic States in Hematite

Polaron formation, whereby an electron or hole strongly couples to a lattice distortion, inhibits the carrier mobility of many first-row transition metal oxide semiconductors. Recently reported XUV transient absorption measurements of hematite (alpha-Fe2O3) demonstrate formation of electron small polarons upon photoexcitation into an undistorted charge-transfer state followed by subpicosecond lattice reorganization. Here, we show that polaronic states of hematite can be accessed directly via optical transitions from the ground state in a thermally activated lattice. Thermal difference spectra collected from 30 to 573 K combined with Stokes resonance Raman spectra indicate strong coupling between optical transitions near the band-edge (2.1-2.3 eV) and zone-center a(1g) and longitudinal (LO) optical phonons. Density functional theory calculations of the electronic and vibrational structures of pristine and polaron-distorted hematite lattices confirm that the geometric distortion corresponding to electron small polaron formation lies along the 28-meV a(1g) and 81-meV LO phonon coordinates and reproduce the features observed in the experimental thermal difference and resonance Raman spectra.

Automated summary

The formation of small polarons in hematite upon photoexcitation and subsequent lattice reorganization can inhibit carrier mobility in first-row transition metal oxide semiconductors. Accessing polaronic states in hematite via optical transitions from the ground state is confirmed through thermal difference and Stokes resonance Raman spectra, showing strong coupling between optical transitions near the band-edge and specific optical phonons. Density functional theory calculations of electronic and vibrational structures support these findings, indicating a geometric distortion corresponding to electron small polaron formation along specific phonon coordinates.