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 omega(2) parameter for YOCl:Eu3+ takes a very large value (=8.81 x 10(-20 )cm(2)) 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 omega(2) parameter (=2.72 x 10(-20 )cm(2)) because of its structural similarity to the square antiprism polyhedron with D-4 symmetry. The omega(4) 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 (D-5(0 )-> F-7(4)) 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.