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

Abstract

The electronic energy states in the Mn4+-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 2Eg and 4T2g of Mn4+ 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 4T1g,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 2Eg and 4T2g values. Results are in good agreement with the Tanabe−Sugano energy-level diagram for the Mn4+-activated phosphors of various host materials like fluorides, oxides, and oxyfluorides.

Deep‐red‐emitting SrLaLiTeO6: Mn4+ double perovskites: Correlation between Mn4+‐O2− bonding and photoluminescence

“…This, in turn, result in variation in Racah parameter, B and the effect can be quantitatively estimated using Nephelauxetic ratio, β. 13,14 The energy level diagram of Mn 4+ ions in SrLaLiTeO 6 is shown in Figure 9B. As shown in the energy level diagram, electrons get excited from the ground state 4 A 2g to excited energy levels 4 T 1g , 2 T 2g , and 4 T 2g and nonradiatively relaxes to first excited state 2 E g ( 4 T 1g → 2 T 2g → 4 T 2g → 2 E g ).…”

“…The excitation maximum at 305 nm matches with the emission spectrum of commercial UV LED chips and hence these phosphors can be excited using UV LEDs. The PLE spectrum is deconvoluted using Gaussian function and four fitted peaks are obtained at 293, 317, 396, and 466 nm which can be attributed to Mn 4+ -O 2− CT band, 4 13,14 Here, the emission maximum centered at 696 nm becomes electric dipole allowed transition since the center of symmetry of Mn 4+ ions in octahedral coordination environment deviates considerably from inversion symmetry.…”

“…The three electrons in d 3 configuration fill the lowest t 2g states. The wide energy gap between t 2g and e g states results in the stabilization of 4+ oxidation state in octahedral fields 11–15 . The excitation spectrum of Mn 4+ ‐activated oxide phosphors is dominated by Mn 4+ ‐ O 2− charge‐transfer transitions and other spin‐allowed 3 d ‐3 d optical transitions within Mn 4+ ions.…”

“…The wide energy gap between t 2g and e g states results in the stabilization of 4+ oxidation state in octahedral fields. [11][12][13][14][15] The excitation spectrum of Mn 4+ -activated oxide phosphors is dominated by Mn 4+ -O 2− charge-transfer transitions and other spin-allowed 3d-3d optical transitions within Mn 4+ ions. However, according to Tanabe-Sugano diagram, the spin-forbidden 2 E g → 4 A 2g emission transition is independent of D q /B (crystal field strength/Racah Parameter) and is dependent on the covalency of Mn 4+ -O 2− bonding which in turn depends on the host structure.…”

Rare Earth‐Free Luminescent Materials for WLEDs: Recent Progress and Perspectives

“…Very recently, this topic has been extensively investigated and reviewed, therefore, these materials have not been included in this report. [ 19 ]…”