Distributions of Eu2þ and Eu3þ do affect the optical properties. However, the substitution of the activator ions hardly perturbed the crystal
structure of the host lattice. Without the differences in crystal structure, it is not trivial to develop an effective descriptor to investigate
stoichiometry-dependent mechanism to explore the effects of Eu species on valence states. In this study, through x-ray nanodiffraction, x-ray
fluorescence, x-ray absorption near edge structure, and x-ray excited optical luminescence, we mapped the valence state distributions of Eu
species to calculate the local mixing enthalpy and entropy. The calculated thermodynamics parameters show good agreement with the optical
properties. We found that Eu2þ of Eu-doped BaAl2O4 predominantly existed in a divalent state, which results in segregation phenomena of
local Eu2þ and Eu3þ ions