Mixing entropy and enthalpy effects on europium ions in Eu-doped BaAl₂O₄

Distributions of Eu²⁺ and Eu³⁺ 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 Eu²⁺ of Eu-doped BaAl₂O₄ predominantly existed in a divalent state, which results in segregation phenomena of local Eu²⁺ and Eu³⁺ ions.