Comparative study on the anomalous intense 5D0→7F4 emission in Eu3+ doped Sr2MNbO6 (M=Ga, In, Gd) phosphors

Eu 3+ ions are extensively employed as spectroscopic probes because of the particular emissions due to the unique 4f 6 electronic configuration. Herein, we have developed a series of interesting fluorescent powders of Eu 3+ activated Sr 2 MNbO 6 (M = Ga, In, Gd). When Sr 2 GaNbO 6 :Eu 3+ is excited by the photons in the range of charge transfer band (CTB), the dark red 5 D 0 → 7 F 4 transition predominates the emission, exhibiting an anomalous intensity even higher than that of 5 D 0 → 7 F 1 or 5 D 0 → 7 F 2 . Whilst, if the sample is pumped by f-f characteristic transitions, the hypersensitive 5 D 0 → 7 F 2 transition emerges as the strongest. However, Eu 3+ doped Sr 2 InNbO 6 and Sr 2 GdNbO 6 never demonstrate such results, 5 D 0 → 7 F 1 acts as the most intense one regardless of the excitation wavelength. It is found that Eu 3+ substitutes Sr 2+ site, and cuboctaherdron [SrO 12 ] with O h symmetry is likely to deform to triangular orthobicupola D 3h symmetry, leading to two non-equivalent crystal sites in Sr 2 GaNbO 6 host. But Eu 3+ replaces In 3+ and Gd 3+ position with O h symmetry and has only one site in Sr 2 InNbO 6 and Sr 2 GdNbO 6 . Based on the comparative study on the intensity of 5 D 0 → 7 F 0 transition, as well as the influence factors of temperature, the doping concentration on the relative intensity among 5 D 0 → 7 F J transitions, and the band gaps of these three hosts, the origin of the abnormal intense 5 D 0 → 7 F 4 is ascribed to the mixing of CTB and low-lying 4f 6 configuration as well as two non-equivalent Eu 3+ sites.