Fig. 5: The impact of charge-transfer quenching on the onset of thermal equilibrium.

a Decay rates of the ⁵D₁ emission at various temperatures. The dashed black line is a fit of the experimental rates (blue dots) up to 423 K to Eq. 1, while the solid blue line is a fit to Eq. 7. b Intensity ratios between the ⁵D₁ and ⁵D₀ emissions for various temperatures extracted from luminescence spectra. The dashed black line is a fit of the experimental ratios (blue dots) to Eq. 4, while the solid blue line is a fit of Eq. 8. c Configurational coordinate diagram of the ⁵D_0–1 and ⁷F_J levels of Eu³⁺ including the charge-transfer state (CTS) of Y₂O₂S. Coupling between the ⁵D₁ and ⁵D₀ levels takes place directly via phonon emission and absorption (red arrows) and indirectly via crossover to the CTS (green arrows)