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 Sr2GaNbO6:Eu3+ is excited by the photons in the range of charge transfer band (CTB), the dark red D-5(0)-> F-7(4) transition predominates the emission, exhibiting an anomalous intensity even higher than that of D-5(0)-> F-7(1) or D-5(0)-> F-7(2). Whilst, if the sample is pumped by f-f characteristic transitions, the hypersensitive D-5(0)-> F-7(2) transition emerges as the strongest. However, Eu3+ doped Sr2InNbO6 and Sr2GdNbO6 never demonstrate such results, D-5(0)-> F-7(1) acts as the most intense one regardless of the excitation wavelength. It is found that Eu3+ substitutes Sr2+ site, and cuboctaherdron [SrO12] with O-h symmetry is likely to deform to triangular orthobicupola D-3h symmetry, leading to two non-equivalent crystal sites in Sr2GaNbO6 host. But Eu3+ replaces In3+ and Gd3+ position with O-h symmetry and has only one site in Sr2InNbO6 and Sr2GdNbO6. Based on the comparative study on the intensity of D-5(0)-> F-7(0) transition, as well as the influence factors of temperature, the doping concentration on the relative intensity among D-5(0)-> F-7(J) transitions, and the band gaps of these three hosts, the origin of the abnormal intense D-5(0)-> F-7(4) is ascribed to the mixing of CTB and low-lying 4f(6) configuration as well as two non-equivalent Eu3+ sites.