A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu³⁺) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu³⁺-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu³⁺:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu³⁺ ion, can directly affect the optical properties of Eu³⁺. We review the luminescence properties of Eu³⁺ ions in mixed-anion compounds of the oxynitride YSiO₂N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the ⁵D₀ → ⁷F_0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu³⁺ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu³⁺-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu³⁺-doped phosphors are presented.