Comparative Study of Absorption Spectra of V2+, Cr3+, and Mn4+in α-Al2O3Based on First-Principles Configuration–Interaction Calculations

First-principles configuration–interaction (CI) calculations of the multiplet structures and the ground-state (GS) absorption spectra of isoelectronic 3d 3 ions such as V 2+ , Cr 3+ , and Mn 4+ in α-Al 2 O 3 have been performed. The results of the molecular orbital (MO) calculations without lattice-relaxation effect indicated that the GSMOs are not appropriate for the calculation of the absorption spectra in the case of V 2+ in α-Al 2 O 3 (α-Al 2 O 3 :V 2+ ) due to the strong mixing between the V 3d orbitals and the conduction band. Therefore we investigated the effect of orbital-relaxation by tentatively performing CI calculations using MOs obtained for several excited states and the CI calculation using the MOs in the intermediate t 2 g 1.5 e g 1.5 configuration was found to give reasonable theoretical spectra. The theoretical peak energies and the relative peak intensities were improved further for α-Al 2 O 3 :V 2+ and α-Al 2 O 3 :Cr 3+ by consideration of energy corrections such as configuration-dependent correction (CDC) and correlation correction (CC). The comparison between the theoretical spectra and the experimental ones indicated that the theoretical spectra were significantly improved for α-Al 2 O 3 :V 2+ and α-Al 2 O 3 :Cr 3+ by consideration of the lattice-relaxation effect. As a result, the tendency of the variation of the peak energies among the isoelectronic 3d 3 ions was clearly reproduced by the first-principles calculations. It was also found that none of the orbital-relaxation, the lattice-relaxation, CDC, and CC has significant effects on the absorption spectra of Mn 4+ in α-Al 2 O 3 .