Assessing the influence of <i>d</i> -orbital radius on formation of localized photogenerated states in corundum metal oxides

Photogenerated polarons are fundamental to the photophysics of transition metal oxide semiconductors. It is, therefore, imperative to understand the mechanisms by which polarons form upon photoexcitation of transition metal oxides to realize their potential in photoapplications. Hematite (α-Fe2O3) is known to form photoexcited small polarons, which limit its performance as a photoelectrocatalyst for water oxidation. Here, we report a systematic comparison of the electronic, optical, and vibrational properties of hematite to those of other metal oxides in the corundum crystal family that elucidates the impact of d-orbital radius on carrier-phonon coupling. Three corundum metal oxides - α-Al2O3 (no d-electrons), α-Fe2O3 (3d), and α-Rh2O3 (4d) - are analyzed with a combined approach of resonance Raman spectroscopy, thermal difference optical spectroscopy, and computational modeling of electronic and vibrational states. We find that the Raman spectrum of α-Al2O3 does not change as the Raman excitation is varied across the visible region, as there is no optical absorption. In contrast, both α-Fe2O3 and α-Rh2O3 exhibit strong coupling of phonons to optical transitions at the onset of absorption, which is evidence of excitation into a polaronic state. Closely comparing the optical polaronic properties of α-Fe2O3 and α-Rh2O3, we establish that increased lattice covalency in α-Rh2O3 arising from the increased radial extension of the 4d orbitals influences which phonon modes mediate photogenerated polaron formation.