Fig. 2: Mn⁴⁺ luminescence of K₂TiF₆:Mn⁴⁺.

a Photographic image of K₂TiF₆:Mn⁴⁺ (x%) phosphors with x = 0.01, 0.1, 0.8, 1.3, 3.8, 5.4, 9.4, and 15.7. The phosphors have a white to yellow body color under ambient light (top) and show red Mn⁴⁺ luminescence under 365 nm UV illumination (bottom). b Tanabe−Sugano energy level diagram of the d³ electron configuration in an octahedral crystal field. The ⁴A₂ → ⁴T₁, ⁴A₂ → ⁴T₂, and ²E → ⁴A₂ transitions of Mn⁴⁺ are indicated by the purple, blue and red arrows, respectively. Note that the excitation transitions are displaced for clarity. For a specific coordination all transitions take place around the same crystal field Δ_O. c Emission spectrum of K₂TiF₆:Mn⁴⁺ (0.8%) upon excitation with blue light (λ_exc = 450 nm). d Excitation spectrum of the red Mn⁴⁺ luminescence (λ_em = 630 nm) from K₂TiF₆:Mn⁴⁺ (0.8%). Spectra are recorded at ambient temperature