Difference of Eu³⁺ luminescent properties in YOCl and YOBr oxyhalide hosts

The photoluminescence spectra and luminescence lifetimes of Eu³⁺-doped oxyhalides, YOX:Eu³⁺ (X = Cl or Br), and their temperature dependence on different halide species of mixed-anion coordinations were investigated and analyzed. In terms of the ionic and covalent nature of bonding, Eu³⁺ ions form different coordination polyhedra in the isostructural YOCl and YOBr hosts: a ninefold [Eu³⁺O₄Cl₅] and an eightfold [Eu³⁺O₄Br₄] polyhedra. The Judd–Ofelt Ω₂ parameter for YOCl:Eu³⁺ takes a very large value (=8.81 × 10⁻²⁰cm²) due to the ninefold polyhedron with C_4v symmetry. On the other hand, despite the same C_4v symmetry, YOBr:Eu³⁺ shows a very small Ω₂ parameter (=2.72 × 10⁻²⁰cm²) because of its structural similarity to the square antiprism polyhedron with D₄ symmetry. The Ω₄ parameters for YOX:Eu³⁺ are much larger than those for other Eu³⁺-doped oxides, possibly related to the covalency of halide anions, Cl⁻ and Br⁻, showing an intense luminescence band (⁵D₀→ ⁷F₄) at around 700 nm. The Eu³⁺ ions in these YOX hosts were excitable by charge transfer bands in 270–280 nm regions. The relaxation pathways from the charge transfer states to the initial states for luminescence are discussed, using the configuration coordinate diagrams with the spectroscopic characterization.