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

Eu3+ 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 Eu3+ activated Sr2MNbO6 (M = Ga, In, Gd). When Sr2GdNbO6:Eu3+ is excited by the photons in the range of charge transfer band (CTB), the dark red 5D0→7F4 transition predominates the emission, exhibiting an anomalous intensity even higher than that of 5D0→7F1 or 5D0→7F2. Whilst, if the sample is pumped by f-f characteristic transitions, the hypersensitive 5D0→7F2 transition emerges as the strongest. However, Eu3+ doped Sr2InNbO6 and Sr2GdNbO6 never demonstrate such results, 5D0→7F1 acts as the most intense one regardless of the excitation wavelength. It is found that Eu3+ substitutes Sr2+ site, and cuboctaherdron [SrO12] with Oh symmetry is likely to deform to triangular orthobicupola D3h symmetry, leading to two non-equivalent crystal sites in Sr2GaNbO6 host. But Eu3+ replaces In3+ and Gd3+ position with Oh symmetry and has only one site in Sr2InNbO6 and Sr2GdNbO6. Based on the comparative study on the intensity of 5D0→7F0 transition, as well as the influence factors of temperature, the doping concentration on the relative intensity among 5D0→7FJ transitions, and the band gaps of these three hosts, the origin of the abnormal intense 5D0→7F4 is ascribed to the mixing of CTB and low-lying 4f [6] configuration as well as two non-equivalent Eu3+ sites.