Energy diagrams of fluorescence excitation and emission processes for displacement ΔR>0 (a) and ΔR = 0 (b). (a) The configurational coordinate diagram of fluorescence center of the phosphor crystal. The energy is plotted against coordinate Q comprising activator ion Eu2+ and its ligand bonds, e.g., Si-N bond. Abs refers to absorption of light and the related vertical transition from ground state to excited state. The horizontal lines indicate the vibrational levels. Em refers to the light emitting process. The crossover is a radiationless process of transitioning from excited state to ground state due to the energy closeness at the crossing point of the two parabolas. R0 and R0’ are the equilibrium distance at ground state and excited state, respectively. (b) The ideal situation where the equilibrium distance R0 at ground state is not displaced at excited state, i.e., ΔR = R0 - R0’ = 0. The diagram is here oversimplified to point to the direction of search for materials with minimal displacement, ΔR, and should not include those with f-f transition (e.g., Eu3+).