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 (α-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.