Abstract: BaFX:Eu²⁺(X = Cl, Br) and their solid solution BaFCl_1-xBr_x:Eu²⁺(x = 0.25, 0.50, 0.75) are excellent X^-ray phosphors and photostimulated luminescence materials.We have found that the fluorescence decay curves of the solid solution are composed of a slow and a fast component.Their origins have been discussed on the basis of different luminescence centers.The samples were prepared by solid state reaction.The luminescence of Eu²⁺ mostly consists of a 5d-4f narrow-band emission.The band emission is centered at 385 nm for BaFCl:Eu²⁺ and at 389 nm for BaFBr:Eu²⁺.Their excitation band originating from 4f→5d transition is centered at near 277 nm.BaFX:Eu²⁺(X = Cl, Br) exhibits a single exponential deccy.For the solid solution BaFCl_1-xBr_x:Eu²⁺, only double exponential decays can fit the experimental values well.By a nonlinear least squares method, the lifetime of these samples is given in Table 1.It was found that the fast component τ_e1 of BaFCl_1-xBr_x:Eu²⁺ is a constant and approximately e-qual to that of BaFBr:Eu2+_τe.The slow compeonent τsub>τe decreases with the increase of Br content x.The time τe0 taken as the intensity decays to 1/e of the initial intensity is consistent with the values reported in literatures where they were treated as a single exponential decay.In BaFX:Eu²⁺, the Eu²⁺ is on the site of Ba²⁺.Four F^- ions and five X^- ions are coordinated with it.In the solid solution BaFCl_1-xBr_x:Eu²⁺, Cl^- and Br^- occupy the sites of X^- randomly, thus forming different luminescence centers.The ligands Cl and Br around Eu²⁺ have different configuration with different probabilities as shown in Table 2.The most probable ligand configuration is Cl₄Br for x=0.25, Cl₃Br₂ and Cl₂Br₃ for x = 0.50, and ClBr₄ for x = 0.75, respectively.The Eu²⁺ with such coordination might be responsible for the slow component in Table 1.The fast component exists in all the solid solution and can be attributed to what resulting from the Eu²⁺ with Br₅ coordination even though the probability of this ligand configuration is small for x-0.25 and 0.50.On the basis of Table 3 and 4, a possible explanation for this result is that the luminescence from the Eu²⁺ with Br₅ configurartion i.e.BaFBr:Eu²⁺ component in solid solution may be strong in some extent in the band emission and there might exist energy transfer from other centers to this center.

Key words: Europium doped barium fluoride halide solid solution, Luminescence center, Fluorescence lifetime