1. Energy levels arising from a d³ configuration for a free transition metal ion (C=4.5B) (a), Tanabe-Sugano diagram for the d³ electron configuration in an octahedral crystal field (C=4.5B) (b), orientation of the five d-orbitals with respect to the ligands of an octahedral complex (black dots showing the ligands around the transition metal ion) (c), and crystal field splitting for the d-orbitals in an octahedral crystal field (d)^[16]

2. Regular octahedron coordination and distorted octahedra coordination

3. Unit cell of Na₂WO₂F₄ (a), highly-distorted [WO₂F₄] octahedra (b), and emission spectrum of Na₂WO₂F₄:Mn⁴⁺ (c) ^[21] with inset showing phosphor image under 460 nm light

4. (a) Unit cell of Cs₂WO₂F₄ which contains slightly- distorted [W(O,F)₆] octahedra, with the bottom-right showing the local coordination of Mn⁴⁺ in K₂MnF₆; (b) Excitation and emission spectra of Cs₂WO₂F₄:Mn⁴⁺ with inset showing the phosphor image under 365 nm light^[23]

5. PLE and DRS spectra of the Cs₂NbOF₅:Mn⁴⁺ phosphor (a) and temperature-dependent emission spectra of Cs₂NbOF₅:Mn⁴⁺ (b)^[24] with the inset showing the intensity evolution of the integrated emission (I_e), the stokes emission (I_s) and the anti-stokes emmission (I_a)

6. The PLE (a) and PL (b) spectra of the BaNbOF₅:Mn⁴⁺ phosphor at temperature of 78 and 298 K with insets showing the phosphor images under natural or UV light^[26]

7. Unit cell of Sr₂ScO₃F (a) and temperature-dependent emission spectra of Sr₂ScO₃F:Mn⁴⁺ (b)^[27]Sr: yellow; Sc: blue; O: red; F: gray

8. Excitation and emission spectra of BaTiOF₄:Mn⁴⁺ at room temperature (a), emission spectra of BaTiOF₄:Mn⁴⁺ at 77 K and 293 K (b), unit cell of BaTiOF₄ (c), and distorted octahedron coordination of [Ti₂OF₄] (d)^[28]Ba: yellow; Ti: blue; O: red; F: gray

9. Comparison of the calculated Mn⁴⁺ energy levels in Mg₂₈Ge_7.55O₃₂F_15.04 for all possible Mn⁴⁺ positions in Ge/Mg sites with the measured spectrum^[29]

10. Unit cell of LiAl₄O₆F and coordination of Al³⁺/Li⁺ (a) and emission spectra of LiAl₄O₆F:Mn⁴⁺ at temperature of 298-523 K (b) ^[30]

Table 1. The reported Mn⁴⁺ activated oxyfluoride phosphors