Optical Absorption and Band Gap Reduction in (Fe_1–xCr_x)₂O₃ Solid Solutions: A First-Principles Study

We provide a detailed theoretical analysis of the character of optical transitions and band gap reduction in (Fe 1– x Cr x ) 2 O 3 solid solutions using extensive periodic model and embedded cluster calculations. Time-dependent density functional theory is used to calculate and assign optical absorption bands for x = 0.0, 0.5, and 1.0 and photon energies up to 5 eV. Consistent with recent experimental data, a band gap reduction of as much as 0.7 eV with respect to that of pure α-Fe 2 O 3 is found. This result is attributed predominantly to two effects: (i) the higher valence band edge for x ≈ 0.5, as compared to those in pure α-Fe 2 O 3 and α-Cr 2 O 3, and (ii) the onset of Cr → Fe d – d excitations in the solid solutions. Broadening of the valence band due to hybridization of O 2p with Fe and Cr 3d states also contributes to band gap reduction.