Received Date: Dec 22, 2017; Accepted Date: Jan 24, 2018; Published Date: Jan 31, 2018
Copyright: © 2018 Saitoh G, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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We synthesized the phosphors of RE3+-doped Li1+x (Ta1-zNbz)1-xTixO3 (LTNT) with various emission colors by solid-state reaction in air using a conventional electric furnace, where RE=Eu, Er, Tm, or Dy. The optimal host composition with each dopant was determined for the highest photoluminescence (PL) intensity; the relevant chemical formulas were Li1.11Ta0.89Ti0.11O3 (x=0.11 and z=0) with Eu3+, Li1.03 (Ta0.2Nb0.8)0.97Ti0.03O3 (x=0.03 and z=0.8) with Er3+, Li1.08 (Ta0.4Nb0.6)0.92Ti0.08O3 (x=0.08 and z=0.6) with Tm3+, and Li1.14 (Ta0.6Nb0.4)0.86Ti0.14O3 (x=0.14 and z=0.4) with Dy3+. The crystal structures of the phosphors were refined by the XRD-Rietveld method to clarify the relationship between the PL properties and crystal structures. In the LTNT host material, the most effective activator was the Eu3+ ion, with an internal quantum efficiency of 97%. The efficient PL emission is closely related to the coordination environment of Eu3+ in the [Li (Eu) O12] polyhedron of the host LTNT.
Oxide phosphors, Photoluminescence, XRD, Rietveld method
In the Li2O–Nb2O5–TiO2 system, the crystals of Li1+x-yNb1-x-3yTix+4yO3 (LNT) with 0.05 ≤ x ≤ 0.3 and 0 ≤ y ≤ 0.182 form a superstructure known as the M-phase, which was discovered by Villafuerte-Castrejon et al. [1,2]. Several researchers have investigated the origin of this superstructure, and clarified that it is formed by the periodical insertion of an intergrowth layer into a matrix having a basic trigonal basic structure [3-7]. With the crystals of Li1+ x -yTa1- x -3yTi x +4yO3 (LTT) in the Li2O–Ta2O5–TiO2 system (0.1≤ x ≤ 0.15 and 0.05≤ y ≤ 0.175), there are a few reports mentioning the relationship between the dielectric properties and compositions . However, to the best of the authors’ knowledge, there have been no reports on the detailed crystal structures. The compositional ranges of the superstructure formation, together with the periods of the intergrowth layer, were different between the crystals of LNT and LTT. Interestingly, the amount of Ti that was required for the superstructure formation was larger for LTT than for LNT .
For the application of LNT as a phosphor host material, rare earth (RE3+=Eu, Er, Tm, or Dy) doped LNT solid solutions have been prepared by heating in air using a conventional electric furnace and/or millimeter-wave furnace [10-12]. The RE3+-doped LTT phosphors were also successfully synthesized, and their photoluminescence (PL) properties were compared to those of LNT:RE3+ . Furthermore, we previously reported on a new red-emitting phosphor in the Li2O–Nb2O5–Ta2O5–TiO2 system, in which we used the quaternary Li1+x (Ta1−zNbz)1−xTixO3 (LTNT, 0 ≤ x ≤0.25, 0 ≤ z ≤ 1.0) solid solution as the host material . The PL intensity of the LTT:Eu3+ phosphor, which was found to be dependent on the concentration of Eu3+, showed a high internal quantum efficiency of 84%.
In this work, in order to further improve the PL intensities of the newly developed LTNT:RE3+ phosphors, we have clarified the optimal compositions of the host materials for each of the activators RE3+, and the relevant crystal structures. We also discussed the close relationship between the PL properties and crystal structures.
The starting materials used for the preparation of LTNT were the reagent-grade chemicals of Li2CO3, Nb2O5, Ta2O5, and TiO2 (>99.9% grade). Each of the rare earth oxides (Eu2O3, Sm2O3, Er2O3, Tm2O3, and Dy2O3>99.9% grade) was doped in the LTNT solid solution. The powder specimens were well mixed, pressed into pellets, and heated in air at 1423 K for 15 h in a conventional electric furnace.
Phase identification was made based on the X-ray powder diffraction (XRPD) data (CuKα), which were obtained on a RINT 2500 device (Rigaku Co., Ltd., Japan) operated at 40 kV and 200 mA. The phase compositions were determined from the XRPD data (CuKα1) collected on another diffractometer in the 2θ range of 15.0-100.0° (X’Pert PRO Alpha-1, PAN alytical B.V., Almelo, the Netherlands) operated at 45 kV and 40 mA. The profile intensity data were investigated by the Rietveld method  using a computer program RIETAN-FP . The crystal-structure models were visualized with a computer program VESTA . Distortion parameters for the coordination polyhedra were determined using a computer program IVTON . Excitation and emission spectra were obtained using a fluorescence spectrophotometer (F-7000HITACHI, Japan). Quantum efficiency was measured by a spectral radiometer (MCPD-7000, Otsuka Electronics Co., Ltd., Osaka, Japan).
Optimal compositions and PL properties of LTNT: RE3+ phosphors
In our previous paper, we compared the emission spectra between the two-types of phosphors, RE3+-doped Li1.11Nb0.89Ti0.11O0 (LNT with x=0.11 and y=0) and RE3+-doped Li1.11Ta0.89Ti0.11O3 (LTT with x=0.11 and y=0) as shown in Figure 1 . The resulting materials showed various emission colors, ranging from red for Li1.11Ta0.89Ti0.11O3:Eu3+, yellow for Li1.11Ta0.89Ti0.11O3:Dy3+, green for Li1.11Ta0.89Ti0.11O3:Er3+, and blue for Li1.11Ta0.89Ti0.11O3:Tm3+. With the Li1.11Ta0.89 Ti0.11O3 host material, the most appropriate activator for effective emission was the Eu3+ ion. On the other hand, when activated with Er3+ and/or Tm3+, the emission intensities were lower for Li1.11Ta0.89Ti0.11O3 than for Li1.11Ta0.89Ti0.11O3. Hence, we concluded that the slight differences in the coordination environment of RE3+ between the host materials of Li1.11Ta0.89Ti0.11O3 and Li1.11Ta0.89Ti0.11O3 would effectively affect the emission energies of the RE3+ ions.
Figure 2 shows the relationship between PL intensity and the z value of L1.11(Ta1-zNbz)0.89Ti0.11:RE3+ (RE2O3: 2.5 wt%). The emission or excitation wavelengths were measured by monitoring them at maximum wavelengths. The PL intensities of the phosphors were measured at 399 nm for Eu3+, at 526 nm for Er3+, at 359 nm for Tm3+, and at 355 nm for Dy3+. We determined the optimal z-values of LTNT (x=0.11) with different RE3+ ions; they were z=0 for Eu, z=0.6 for Tm, z=0.8 for Er, and z=0.4 for Dy. We subsequently determined the most suitable quantities of RE3+ ions as activators for the host material of Li1.11Ta0.89Ti0.11O3. Among the phosphors doped with Eu2O3 up to 4.5 wt%, the highest PL intensity was observed when doped with 2.5 wt% Eu2O3 (Figure 3). For the other phosphors doped with Er2O3 up to 3.0 wt%, those doped with Tm2O3 up to 3.5 wt%, and those doped with Dy2O3 up to 4.0 wt%, the PL intensities were the highest for 1.0 wt% Er2O3, 1.0 wt% Tm2O3, and 1.0 wt% Dy2O3 (Figure 3). Finally, we determined the optimal Ti content in the host materials under the best condition of the z-values and RE2O3 contents (Figure 4). As a result, the optimal host composition showing the highest PL intensity was eventually determined for each of the dopants; the relevant chemical formulas were Li1.11Ta0.89Ti0.11O3 (x=0.11 and z=0) with Eu3+, Li1.03(Ta0.2Nb0.8)0.97Ti0.03O3 (x=0.03 and z=0.8) with Er3+, Li1.08(Ta0.4Nb0.6)0.92Ti0.08O3 (x=0.08 and z=0.6) with Tm3+, and Li1.14(Ta0.6Nb0.4)0.86Ti0.14O3 (x=0.14 and z=0.4) with Dy3+. In general, the electronic dipole intensities in the lanthanide 4f–4f transitions exhibit extraordinary sensitivity to the ligand environment, hence the optimal host compositions as determined above must be close to those most appropriate in view of the lattice vibrations and environment of RE3+ ions.
In the phosphors emitting various colors, the Li1.11Ta0.89Ti0.11O3:Eu3+ phosphor showed a relatively high internal quantum efficiency of 84% as previously reported . Furthermore, we reported that the Sm3+ ion can act as an effective sensitizer for Eu3+-activated phosphors because of the energy transfer process . In the present experimental stage, we synthesized the Eu3+ and Sm3+ co-doped phosphor, and characterized the PL property. Figure 5 shows the excitation and emission spectra of Li1.11Ta0.89Ti0.11O3:Eu3+, Sm3+, in which the contents of Eu2O3 and Sm2O3 were, respectively, 0.1 wt% and 2.5 wt%. The sharp emission peaks with relatively strong intensities were observed at a wavelength of around 625 nm (5D0-7F2, electric-dipole), when excited by the purple light of 399 nm. The relatively weak emission appeared at wavelengths around 600 nm (5D0-7F1, magnetic-dipole transition in the Eu3+ ion) and 700 nm (5D0-7F4, electric-dipole transition in the Eu3+ ion). The inner quantum efficiency of the phosphor achieved 97%, which was very close to the theoretical value.
Figure 6 shows the emission and excitation spectra of LTNT phosphors with the most suitable compositions as determined above. Table 1 summarizes the chromaticity coordinates (x, y) of the emitting light. A green emission peak at 526 nm (4S3/2-4I15/2) was observed for the Li1.03 (Ta0.2Nb0.8)0.97Ti0.03O3:Er3+ upon excitation at 551 nm (4I15/2-2H11/2). Blue emission was observed at 363 nm (1D2-3F4 transition) for the Li1.08 (Ta0.4Nb0.6)0.92Ti0.08O3:Tm3+ upon excitation at 461 nm (3H6-1D2). Yellow emission peaks at around 581 nm (4F9/2-6H13/2) were observed for the Li1.14 (Ta0.6Nb0.4)0.86Ti0.14O3:Dy3+ upon excitation at 356 nm (6H15/2 -4M15/2). The PL behavior of the RE3+-doped phosphors was not affected by the host material’s structure due to the 4f-4f transitions [13,19].
Table 1. Optimal compositions and chromaticity of LTNT phosphors.
Morphologies and Crystal Structures of LNTN: RE3+ Phosphors
In this chapter, we will discuss the micro-textures and crystal structures of the LTNT:RE3+ phosphors. The SEM images in Figure 7 show that the average grain sizes were about 1.5 μm for Li1.11Ta0.89Ti0.11O3:Eu3+,Sm3+, 5.3 μm for Li1.08 (Ta0.4Nb0.6)0.92Ti0.08O3:Tm3+, 8.0 μm for Li1.03 (Ta0.2Nb0.8)0.97Ti0.03O3:Er3+, and 4.2 μm for Li1.14 (Ta0.6Nb0.4)0.86Ti0.14O3:Dy3+. The LNT grain’s shape changes from spherical to plate-like when it forms a superstructure . The SEM images indicated that the LTNT:RE3+ phosphors had no superstructure because their shapes were not platelike. The difference in sizes was due to the speed of the grain growth owing to the various doping rare-earth ions and their concentrations. The reflection indices in Figure 8 were based on the hexagonal unit cells with a ~ 0.5 nm, c ~ 1.4 nm. Parts of the crystal structures of the host materials were determined, and shown in Figures 9 and 10. We successfully constructed the structural models, in which all of the RE3+ ions occupy the Li site. It should be noted that all of these structures were isomorphous with LiNbO3 (space group R3c), and free from the superstructure peculiar to the M-phase.
The addition of relatively small amounts of Ti4+ effectively increased the PL intensities for Li1+xTa1-xTixO3:Eu3+ and Li1+x(Ta0.4Nb0.6)1-xTixO3:Tm3+ as shown in Figure 4. Hence, we refined the crystal structures of Li1+xTa1-xTixO3:Eu3+ with 0 ≤ x ≤ 0.25. Figure 9 shows parts of the refined structural models of Li0.901Eu0.033TaO3 (x=0), (Li0.977Eu0.023) (Ta0.89Ti0.11)O2.968 (x=0.11), and (Li0.952Eu0.026)(Ta0.75Ti0.25)O2.901 (x=0.25). These chemical compositions were determined as the result of the Rietveld refinement process, and for this reason there are small deviations from those used hitherto. The table included in Figure 9 describes the polyhedral details of Δ (centroid-to-cation distance; eccentricity), VS (sphere volume), and VP (volume of coordination polyhedron). The distortion-parameter values (VS/VP) of [(Li, Eu)O12] polyhedra were close to each other among the three-types of phosphors with x=0, 0.11, and 0.25, as were the VS/VP-values of [(Li, Eu)O12] polyhedra. However, the (Li0.977Eu0.023)(Ta0.89Ti0.11)O2.968 phosphor (x=0.11) showed a remarkably large Δ-value (= 0.047 nm) in [(Li, Eu)O12] polyhedron as compared with the two other phosphors with x=0 and 0.25. Because the (Li0.977Eu0.023)(Ta0.89Ti0.11)O2.968 phosphor showed the highest PL intensity, the displacement of Eu3+ position from the centroid of [(Li, Eu)O12] polyhedra must contribute to the highly enhanced intraconfigurational 7F0–5D1 transition.
Figure 10 shows parts of the crystal structures of (Li0.991Tm0.007) (Ta0.400Nb0.598)O3 and (Li0.990Tm0.011) (Ta0.361Nb0.558Ti0.081)O2.970. These chemical formulas were determined by the Rietveld method, and hence they show slight deviations from the chemical compositions of Li1+x(Ta0.4Nb0.6)1-xTixO3:Tm3+ with x=0 and 0.08. The Δ-values as well as the VS/VP-values in [(Li, Tm)O12] polyhedra were almost the same between the two phosphors, although the PL intensity was effectively enhanced for x=0.08 than for x=0 as shown in Figure 4c. Thus, the displacement of the Tm3+ position from the centroid position of [(Li, Tm)O12] polyhedra would be totally unrelated to the enhancement of PL intensity. Additionally, the displacements of Eu3+ and Sm3+ positions along the c-axis from the centroid of [(Eu, Sm)O12] polyhedra could be closely related to the enhancement mechanism of the red-light emission through the energy transfer from Sm3+ to Eu3+ [20,21].
We synthesized LTNT:RE3+ (RE=Eu, Er, Tm, and Dy) phosphors with various emission colors by solid-state reaction in air at 1423 K for 15 h using a conventional electric furnace. The phosphors emitted red light for Eu3+, green light for Er3+, blue light for Tm3+, and yellow light for Dy3+. The optimal host compositions of the phosphors were Li1.11Ta0.89Ti0.11O3 with Eu3+, Li1.03 (Ta0.2Nb0.8)0.97Ti0.03O3 with Er3+, Li1.08 (Ta0.4Nb0.6)0.92Ti0.08O3 with Tm3+, and Li1.14 (Ta0.6Nb0.4)0.86Ti0.14O3 with Dy3+. These compositions would be close to those most appropriate in view of the lattice vibrations and environment of RE3+ ions, since the electronic dipole intensities in the lanthanide 4f–4f transitions generally exhibit extraordinary sensitivity to the ligand environment. For the LTNT host phosphor, the most effective activator was the Eu3+ ion. The red-emitting phosphor of Li1.11Ta0.89Ti0.11O3: Eu3+,Sm3+ had a high internal quantum efficiency of 97%. A structural study of the phosphors using the XRD-Rietveld method indicated that the high PL intensity of the Eu3+-activated phosphor was closely related to the environmental symmetry of the Eu3+ ion. The displacement of the Eu3+ position from the centroid of [(Li, Eu)O12] polyhedra would effectively contribute to the highly enhanced intraconfigurational 7F0–5D1 transition.
This work was partially supported by a Grant-in-Aid for Scientific Research (c) No. 16K06721 (H. N.) by the Japan Society for the Promotion of Science.