Theoretically probing the physics underlying the photoluminescence of phosphors and predicting their thermal quenching properties are significant issues in the field of phosphor research. The electronic ground states of a series of Eu(II)-doped alkali-earth sulfide phosphors, i.e., MS:Eu2+ (M = Mg, Ca, Sr, Ba), have been analyzed using density functional theory calculations to characterize and analyze their photoluminescence properties in terms of quantum efficiency and its thermal decay tendency. Anderson’s impurity model to MS:Eu2+ enables devising a physical picture of how the electronic ground states |ψEu5d representing the Eu(II)-5d orbitals are mixed with those of the conduction bands (CBs) of host materials. The focus is on quantitatively deducing the electron delocalization nature of |ψEu5d over |CB〉, especially k|ψkMdk, which represents the bands formed by the d orbitals of M atoms. The ratio of the probability amplitudes of |ψEu5d and k|ψkMdk, i.e., CEu5d/CMd, proves to be correlated with the electron localization nature of |ψEu5d, thereby suggesting that this ratio can be an effective parameter for evaluating the thermal quenching tendency of photoluminescence without more precise information on the electronic excited states. Energetically small gaps and large spatial overlaps between |ψEu5d and |CB〉 delocalize electrons in a hybridized state, which gives these electrons the tendency to dissipate without luminescence. The results explain the rankings of the quantum yield and its temperature dependence in the MS:Eu2+ (M = Ca, Sr, Ba) systems, which follow the Dorenbos thermal quenching model, while MgS:Eu2+ does not have the same mechanistic origin.

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