Review—Tanabe−Sugano Energy-Level Diagram and Racah Parameters in Mn4+-Activated Red and Deep Red-Emitting Phosphors

The electronic energy states in the Mn 4+ -activated phosphors are examined based on the Tanabe−Sugano energy-level diagram. Investigation is focused on the determination of the Racah and crystal-field parameters. The previous review [ ECS J. Solid State Sci. Technol. , 9, 016001 (2020)] suggested that some excited energy states 2 E g and 4 T 2 g of Mn 4+ ion can be relatively accurately determined from photoluminescence (PL) and PL excitation spectra with performing the Franck−Condon principle analysis based on the configurational-coordinate model. However, it is very difficult to accurately determine the higher-lying 4 T 1 g ,a energy from any optical spectrum. This higher-lying energy state needs in the calculation of the Racah parameters. Therefore, a set of expressions is proposed for an accurate determination of the Racah and also crystal-field-related parameters only by introducing 2 E g and 4 T 2 g values. Results are in good agreement with the Tanabe−Sugano energy-level diagram for the Mn 4+ -activated phosphors of various host materials like fluorides, oxides, and oxyfluorides.