Synthesis and luminescence enhancement of Eu3+, Sm3+ co-doped Li1.11Ta0.89Ti0.11O3 phosphor
Graphical abstract
Enhancement of red emission intensity and internal- and external-quantum efficiencies by co-doping of Eu3+ and Sm3+ ion in Li1.11Ta0.89Ti0.11O3 phosphor.
Introduction
In the ternary Li2O–Nb2O5–TiO2 system, the compounds with the general composition Li1+x−yNb1−x−3yTix+4yO3 (LNT), known as the “M-phase” solid solutions, form superstructures, where M = Nb or Ta with 0.06 ≤ x ≤ 0.33 and 0 ≤ y ≤ 0.17. Since the discovery of the M-phase for LNT by Castrejon et al. [1], [2], the physical properties as well as the crystal structures of the solid solutions have been investigated in detail [3], [4], [5], [6]. The M-phase superstructures have been found to be formed by the periodical insertion of intergrowth layers in the matrix having a trigonal structure. The relationship between dielectric properties and periods of the intergrowth layers of the M-phase have been studied successively [7], [8].
The LNT compounds have been applied to the host materials of new phosphors [9], [10]. The photoluminescence (PL) intensities at 625 nm were much higher for LNT:Eu3+ than for LiNbO3:Eu3+ [11], [12]. The RE-doped LiTaO3 phosphors have also been reported by other groups, where RE = Pr3+ [13], Er3+ [14], Tb3+ [15] Eu3+ [16] and Tm3+ [17]. Recently, we have succeeded in synthesizing new red phosphors based on the quaternary Li1+x(Ta1−zNbz)1−xTixO3 (LTNT, 0 ≤ x ≤ 0.25, 0 ≤ z ≤ 1.0) solid solutions as the host materials [18]. We have confirmed that the superstructures are actually formed in the higher Ti content regions of 0.06 ≤ x ≤ 0.2 in the Ta-containing system (0.05 ≤ z ≤ 0.175). The PL intensities of the LTNT:Eu3+ phosphors were not affected by the structural distortions induced by the superstructure formation, but dependent on the x- and z-values as well as the concentration of Eu3+.
The RE (= Sm3+, Er3+, Dy3+ and/or Tm3+) doped Li1+xTa1−xTixO3 (LTT) phosphors, showing various emission colors, have been synthesized by the conventional electric furnace in air to compared their PL properties to those of the LNT phosphors [19]. We have concluded that the small differences in environment of the dopant sites between the host materials of LNT and LTT eventually affect the emission energy of the RE3+ ions. In the LTT host materials, the most effective activator was Eu3+. The internal quantum efficiency attained the value of over 84% for the host composition of Li1.11Ta0.89Ti0.11O3 (x = 0.11) [19]. The Li1.11Ta0.89Ti0.11O3:Eu3+ phosphor, when excited by the near ultraviolet (UV) light, emits strong red light, hence it could be suitable for the backlight of projectors; the backlight contains an array of red, green, and blue LEDs whose combined light forms white light [20]. To apply the LTT-host phosphors to the backlight, much higher PL intensities are required. Because the Sm3+ ions have been reported to act as sensitizers for various types of Eu-doped phosphors [21], [22], [23], [24], we have tried to enhance the emission intensities of the LTT phosphors by co-doping of Eu3+ and Sm3+. We have successfully prepared the red-light emitting Li1.11Ta0.89Ti0.11O3:Eu3+, Sm3+ phosphors with the improved PL properties.
Section snippets
Experimental procedure
The starting materials used were Li2CO3, Ta2O5, and TiO2 (>99.9% grade) powders to prepare the solid solutions of Li1+x−yTa1−x−3yTix+4yO3. As activators, Eu2O3 and/or Sm2O3 were doped in the host materials. The mixed and pressed powders were pre-annealed at 1273 K for 3 h to drive off CO2, and continuously sintered at 1423 K for 15 h using a conventional electric furnace.
Structural analysis was carried out by X-ray powder diffraction (XRD) using a RINT 2500 (Rigaku Co., Ltd. Tokyo, Japan) operating
Photoluminescence property
Phosphors based on the Li1+x−yM1−x−3yTix+4yO3 (M: Ta, Nb, 0 ≤ x ≤ 0.25, y = 0) solid solutions as host materials were investigated. The optimal composition of the host material for a red phosphor was found to be Li1.11Ta0.89Ti0.11O3 (M = Ta, x = 0.11 and y = 0) by Eu3+-ion doping, as previously reported [18]. The PL spectrum of the Li1.11Ta0.89Ti0.11O3:Eu3+ phosphor is characterized by the charge transfer band in the range of 220–340 nm and sharp excitation peaks in the near-UV to green light (Fig. 1).
Conclusion
The Eu3+- and Sm3+-doped Li1.11Ta0.89Ti0.11O3 phosphors have been successfully synthesized by the solid-state reaction. The highest PL intensity was attained for the phosphor doped with Eu2O3 at 2.5 wt% and Sm2O3 at 0.1 wt%. The red light was efficiently emitted at 624 nm when excited by the near-UV light, with the internal quantum efficiency of 98%. The PL intensity was ca. 1.4 times higher for Li1.11Ta0.89Ti0.11O3:Eu3+, Sm3+ than for Li1.11Ta0.89Ti0.11O3:Eu3+, indicating that the small amount of
Acknowledgement
This work (H.N.) was partially supported by a Grant-in-Aid for Scientific Research (c) No. 25420709 by the Japan Society for the Promotion of Science.
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