Multiple-Valence and Visible to Near-Infrared Photoluminescence of Manganese in ZnGa2O4: A First-Principles Study

The transition metal manganese is attractive for magnetism and luminescence engineering due to the nontrivial interplay of charge, spin, lattice, and orbital freedom degrees. Here, we report the manganese site occupancy and valence states by taking experimental synthesis conditions into consideration and the optical features of target intra-atomic Mn2+, Mn3+, and Mn4+ in the framework of density functional theory in a prototype spinel ZnGa2O4 host. The formation energy results show that Mn2+ dominates in tetrahedral ligands at the Zn2+ site and Mn3+ and Mn4+ dominate in octahedral ligands at the Ga3+ site under the corresponding different chemical potential conditions, and the distortion of the local coordination environment of Mn3+ in octahedral ligands due to the Jahn–Teller effect is studied in detail. The excited-state equilibrium geometries and energies are obtained via spin or orbital occupancy constraints, which are further complemented by the Tanabe–Sugano diagram to predict the energy spectra and to interpret the experimental results on Mn2+, Mn4+, and Mn3+. In particular, the competition of 5T2 and 1T2 excited states and the relaxation processes leading to near-infrared luminescence of rarely reported Mn3+ are analyzed. First-principles calculations complemented by the Tanabe–Sugano diagram analysis may serve as an effective and predictive tool for exploring valence states, energy structures, and luminescence of complexes containing 3dn transition-metal ions.