English

• 1. INTRODUCTION

  In recent years, rare-earth (Ln³⁺) activated phosphors have been widely used in modern society to meet the increasing need for the creative applications in everyday devices, such as TV screens, computer monitors, electronic billboards, portable electronics, and white light emitting diodes (WLED)^[1-5]. Typically, the method of commercial white LEDs produced by Nichia Corporation is to combine a blue InGaN chip with a yellow phosphor YAG: Ce³⁺, wherein blue light excites the phosphor^{[6, 7]}. To improve the luminous efficiency, a new way which combined a near-ultraviolet (UV) LED chip with red, green and blue phosphors has been suggested. Such a combination can yield high color reproducibility and rendering index. Therefore, green and red phosphors with high thermal and chemical stability which can be excited by near UV around 400 nm attracted much attention in the past few years because of the amazing phenomenon of multi-color and tunable emission, such as K₈Nb₇P₇O₃₉: Eu^3+[8], Ba₃Y(PO₄)₃: Ce³⁺/Tb^3+[9] and so on^[10-13].

  It is well-known that the energy transfer process usually has many effects on altering the luminescence intensity, color and lifetime of a phosphor^[14]. As two famous Ln³⁺ ions, Tb³⁺ and Eu³⁺ show intense green and red emissions due to the 4f → 4f transitions, respectively^{[9, 15]}. Moreover, Tb³⁺ and Eu³⁺ co-doped phosphors usually show an effective energy transfer from Tb³⁺ to Eu³⁺, which have been produced many references^[16-20]. However, the common feature of most reports is that Eu³⁺ and Tb³⁺ ions are co-doped in a third-party host, whereas Eu³⁺-doped terbium based compounds have rarely been reported.

  Rare-earth double tungstate with the general formula ALn(WO₄)₂ (A = Li, Na, K, Rb, Cs; Ln = Y, La-Lu) has attracted much attention for their applications as luminescent host materials^[21-23]. Most of these crystals feature a scheelite-type structure with tetragonal I4₁/a symmetry at room temperature^[24]. For LiTb(WO₄)₂, the structure can be thought as the substitution of two Ca²⁺ ions in CaWO₄ by a couple of Li⁺ and Tb³⁺ ions in a statistical manner. Although the crystal structure of LiTb(WO₄)₂ is characterized by powder X-ray diffraction analysis in the literature^[25], no single crystal data have been given until now.

  In this work, we report the single crystal growth and structure determination of LiTb(WO₄)₂. Moreover, a series of LiTb_1-xEu_x(WO₄)₂ (x = 0, 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) solid solutions were prepared, which show multi-color light emitting performance due to the efficient Tb³⁺→Eu³⁺ charge transfer.

  2. EXPERIMENTAL

  2.1 Materials and methods

  All raw materials of Li₂CO₃ (≥99.5%), WO₃ (≥99.9%), Eu₂O₃ (≥99.95%) and Tb₄O₇ (≥99.95%) were purchased from Sinopharm Chemical Reagent Shanghai limited company and used without any further purification. The X-ray powder diffraction (XRD) analysis was performed by Rigaku DMax2500 diffractometer with graphite-monochromatic CuKα characteristic radiation (2θ = 5~70°). Photoluminescence spectra were carried out by FLS920 Edinburgh Analytical Instrument apparatus. The infrared absorption spectrum was recorded on a Perkin Elmer (FTIR 2000) spectrometer in the change of 1000~450 cm^-1. Sample in powder form was pressed into a disk using pellet of KBr.

  2.2 Synthetic procedure

  Single crystals of LiTb(WO₄)₂ were prepared by the high-temperature flux method by using Li₂O−WO₃ as a flux. It was grown from a mixture of Li₂CO₃ (0.3400 g, 4.6009 mmol), Tb₄O₇ (0.4300 g, 0.5751 mmol), and WO₃ (2.0000 g, 8.6267 mmol) with the molar ratio of 8:1:15 in a 20 mL Pt crucible. The mixture was placed in a vertical programmable temperature furnace, and then heated at 900 ℃ and kept this temperature until the solution became transparent and clear enough in the open air, then cooled at a rate of 2 ℃∙h⁻¹ to 600 ℃, to grow single crystals. Finally, the production was washed by hot water to dissolve the addition flux and then a few colorless crystals of high optical quality were obtained.

  Powder sample of LiTb(WO₄)₂ was prepared by the high-temperature solid state reaction method according to the following solid-state reactions:

  $ \begin{aligned} & 4 \mathrm{Li}_2 \mathrm{CO}_3+16 \mathrm{WO}_3+2 \mathrm{~Tb}_4 \mathrm{O}_7 \rightarrow \\ & 8 \mathrm{LiTb}\left(\mathrm{WO}_4\right)_2+4 \mathrm{CO}_2+\mathrm{O}_2 \end{aligned} $

  The raw materials of Li₂CO₃ (0.0797 g, 1.0785 mmol), WO₃ (1.0000g, 4.3134 mmol) and Tb₄O₇ (0.4031g, 0.5391 mmol), with the stoichiometric molar ratio of 2:8:1, were thoroughly grounded and pressed into a pellet. Then the mixture was transferred into a proper platinum crucible and preheated at 500 ℃ for 5 h at a muffle furnace. After that, the mixture was ground again to guarantee the best homogeneity and then continue to heat to 800 ℃ for 48 h in the open air. In this stage, it is very important to grind and mix the mixture every ten hours to improve crystallinity. Moreover, powder samples of LiTb_1-xEu_x(WO₄)₂ (x = 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) were prepared by the same way according to the following solid-state reactions.

  $\begin{aligned} & 4 \mathrm{Li}_2 \mathrm{CO}_3+16 \mathrm{WO}_3+2(1-x) \mathrm{Tb}_4 \mathrm{O}_7+4 x \mathrm{Eu}_2 \mathrm{O}_3 \rightarrow \\ & 8 \mathrm{LiTb}_{1-x} \mathrm{Eu}_x\left(\mathrm{WO}_4\right)_2+4 \mathrm{CO}_2+\mathrm{O}_2 \end{aligned} $

  The purity of the powders was confirmed by powder XRD (2θ = 5~70°) analysis, as shown in Fig. 1. The experimental patterns were well indexed to the pure tetragonal phase of LiTb(WO₄)₂ and no other phase was observed, which indicated that the powder samples are pure and the structure model is right.

  Figure 1

  

  Figure 1.  XRD patterns of solid solution LiTb_1-xEu_x(WO₄)₂ (x = 0, 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) and simulated patterns of LiTb(WO₄)₂

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  2.3 Crystallography

  Several proper crystals (0.20mm × 0.05mm × 0.05mm) were selected and carefully mounted in fiberglass for single-crystal X-ray diffraction (SC-XRD) analysis. The set of intensity data was carried out using a Bruker Smart Apex2 CCD with graphite-monochromatic MoKα (λ = 0.71073 Å) radiation at 296 K. All of the atoms were refined with anisotropic thermal parameters. The important bond distances and bond angles are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  W(1)–O(1)	1.784(5)		Tb(1)–O(1)iv	2.405(5)		Tb(1)–O(1)	2.416(5)
  W(1)–O(1)i	1.784(5)		Tb(1)–O(1)v	2.405(5)		Tb(1)–O(1)vi	2.405(5)
  W(1)–O(1)ii	1.784(5)		Tb(1)–O(1)ix	2.416(5)		Tb(1)–O(1)vii	2.405(5)
  W(1)–O(1)iii	1.784(5)		Tb(1)–O(1)x	2.416(5)		Tb(1)–O(1)viii	2.416(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–W(1)–O(1)i	107.03(16)		O(1)i–W(1)–O(1)ii	114.5(3)		O(1)i–W(1)–O(1)iii	107.03(16)
  O(1)–W(1)–O(1)ii	107.04(16)		O(1)–W(1)–O(1)iii	114.5(3)		O(1)ii–W(1)–O(1)iii	107.03(16)
  Symmetry codes: (i) –y+1/4, x+1/4, –z+1/4; (ii) y–1/4, –x+1/4, –z+1/4; (iii) –x, –y+1/2, z; (iv) –x–1/2, –y+3/2, –z+1/2; (v) y–5/4, –x+3/4, z–1/4; (vi) –y+1/4, x+3/4, z–1/4; (vii) x–1/2, y, –z+1/2; (viii) y–5/4, –x+1/4, –z+1/4; (ix) –y+1/4, x+5/4, –z+1/4; (x) –x–1, –y+3/2, z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure

  SC-XRD analysis indicates that compound LiTb(WO₄)₂ belongs to the tetragonal centrosymmetric space group I4₁/a and exhibits a three-dimensional (3D) network consisting of two-direction packing of isolated WO₄ tetrahedra and Li/Tb atoms (Fig. 2). The reason that Li and Tb atoms were expressed as Li/Tb is that they are disordered and randomly occupy the same 4a site. The Li|Tb atom is coordinated by eight oxygen atoms with Li/Tb–O bond distances ranging from 2.405(5) to 2.416(5) Å (Table 1). W atom locates at the center of a tetrahedron with the four equal W–O bond distances of 1.784(5) Å, and O–W–O bond angles fall in the range of 107.04(16)~114.5(3)^°. These values are common compared with other reports^[26-28].

  Figure 2

  

  Figure 2.  View of the crystal structure of LiTb(WO₄)₂ composed of [WO₄] tetrahedra and (Li/Tb)²⁺ ions

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  3.2 IR spectroscopic characterization

  The Fourier transform infrared (FT-IR) spectrum of LiTb(WO₄)₂ was recorded to characterize the structural detail, as shown in Fig. 3. A series of absorption peaks from 450 to 1000 cm⁻¹ originate from the vibrations of WO₄ tetrahedra. Among them, the bands at 874 and 515 cm^-1 can be assigned to stretching modes of W–O in WO₄ tetragon. The bands at 829 and 706 cm⁻¹ correspond to the ν₁ and ν₃ modes of the WO₄ tetrahedra, respectively. The band at 609.5 cm⁻¹ was due to the symmetrical vibrations of bridge oxygen atoms of Li/Tb–O–W groups^{[29, 30]}. The IR spectrum confirms the existence of tetrahedrally coordinated tungsten atoms, which is consistent with the results obtained from the single-crystal X-ray structural analysis.

  Figure 3

  

  Figure 3.  FT-IR spectra of the LiTb(WO₄)₂ sample

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  3.3 Excitation and emission spectra

  Fig. 4a shows the excitation spectra of undoped LiTb(WO₄)₂ phosphor by monitoring green emission 547 nm (⁵D₀→⁷F₂) in the range of 200~500 nm at room temperature. There exist a series of narrow bands in the range from 315 to 390 nm, which can be assigned to the spin-forbidden 4f→4f transitions of Tb³⁺: ⁷F₆→⁵D₀ (319 nm), ⁷F₆→⁵G₂ (341 nm), ⁷F₆→⁵D₂ (353 nm), ⁷F₆→⁵G₅ (360 nm), ⁷F₆→⁵G₆ (370 nm), ⁷F₆→⁵D₃ (379 nm) and ⁷F₆→⁵D₄ (487 nm), as listed in Table 2. The peak at 379 nm is the strongest, which is a common character compared with other reported phosphors^{[31, 32]}. Under excitation of 379 nm, four typical emission peaks from blue to red region are generated, which can be assigned to the ⁵D₄→⁷F_J (J = 3, 4, 5, 6) transition of Tb³⁺, that is, 491 nm (⁵D₄→⁷F₆) in the blue region, 547 nm (⁵D₄→⁷F₅) in the green region, and 589 nm (⁵D₄→⁷F₄) and 616 nm (⁵D₄→⁷F₃) in the red region. The strongest one is located at 547 nm, which is the reason why compound LiTb(WO₄)₂ exhibits strong green emission. This result is essentially in agreement with other Tb³⁺ activated phosphors, such as K₂Ln(PO₄)(WO₄): Tb^3+[31] and Li₃Sc(BO₃)₂: 0.01Tb^3+[32].

  Figure 4

  

  Figure 4.  Excitation and emission spectra of LiTb(WO₄)₂ (a) and LiTb_0.99Eu_0.01(WO₄)₂ (b) phosphors

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  Table 2

  

  Table 2.  Excitation Peaks of LiTb_1-xEu_x(WO₄)₂ Phosphors

  DownLoad: CSV

  No.	Wavelength (nm)	Activation ion	Excitation transition
  1	319	Tb3+	7F6→5D0
  2	341	Tb3+	7F6→5G2
  3	354	Tb3+	7F6 →5D2
  4	360	Tb3+	7F6 →5G5
  5	370	Tb3+	7F6 →5G6
  6	379	Tb3+	7F6 →5D3
  7	395	Eu3+	7F0 →5L6
  8	417	Eu3+	7F0 →5D3
  9	465	Eu3+	7F0 →5D2
  10	487	Tb3+	7F0 →5D4

  3.4 Energy transfer between Tb³⁺ and Eu³⁺

  The excitation and emission spectra of LiTb_0.99Eu_0.01(WO₄)₂ at different excitation and emission wavelengths are given in Fig. 4b. It is obvious that the green emission of LiTb(WO₄)₂ changes greatly when doped with Eu³⁺. The excitation spectrum by monitoring at 547 nm (⁵D₄→⁷F₅) is similar to undoped sample LiTb(WO₄)₂ (Fig. 4a). By monitoring at 616 nm (⁵D₀→⁷F₂), the characterize excitation peaks of both Tb³⁺ (319, 341, 353, 360, 370, 379, 487 nm) and Eu³⁺ (395, 417, 465 nm) appear together (Table 2), indicating that an energy transfer occurs from Tb³⁺ to Eu³⁺. On the other hand, the characterize emission peaks of both Tb³⁺ (491, 547 nm) and Eu³⁺ (594, 616, 656, 703 nm) appear together in the emission spectrum excited by monitoring at wavelength 379 nm (⁷F₆→⁵D₃), whereas by 395 nm (⁷D₀→⁵L₆) is similar to pure Eu³⁺ samples or Eu³⁺ doped materials^[33-35]. This clearly proves that energy transfer takes place from Tb³⁺ to Eu³⁺ but the back transfer Eu³⁺→Tb³⁺ does not exist in the LiTb_0.99Eu_0.01(WO₄)₂ sample. It is a good property for solid solution phosphor LiTb_0.98Eu_0.02(WO₄)₂ that a broad near-UV (350~400 nm) light can be strongly absorbed, which is nicely in agreement with the excitation wavelength around 400 nm for near-UV LEDs and blue LEDs in commercial LED, respectively^[36-40].

  To further investigate the energy, transfer process of Tb³⁺→Eu³⁺, the concentration-dependent luminescent properties of a serials of LiTb_1-xEu_x(WO₄)₂ (x = 0, 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) phosphors were studied. As shown in Fig. 5, all spectra reveal the characteristics of emission bands of Tb³⁺ and Eu³⁺. Simultaneously, the emission spectrum clearly shows the decrease of Tb³⁺ characterizes peaks (487, 547 nm) with increasing the Eu³⁺ concentration from 0 to 0.1. On the contrary, the Eu³⁺ characteristic emissions (595, 616, 656 and 703 nm) firstly increase with the increase of Eu³⁺ concentration from 0 to 0.01, and then decrease when the concentration exceeds 0.01. There are two possible reasons for this phenomenon. One is the concentration quenching of excess Eu³⁺, and the other is that 378 nm is not more suitable than 395 nm^{[41, 42]}.

  Figure 5

  

  Figure 5.  Emission spectra of representative LiTb_1-xEu_x(WO₄)₂ (x = 0~0.1) phosphors excited by 379 nm (⁷F₆ →⁵D₃ of Tb³⁺) as a function of Eu³⁺ doping content x. Inset is the variatation of 547 nm (⁵D₄→⁷F₅ of Tb³⁺) and 616 nm (⁷F₀→⁵D₂ of Eu³⁺) emission intensity

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  It should be mentioned that the emission spectrum with 487 nm excitation (⁷F₆ → ⁵D₄ of Tb³⁺) is similar with 379 nm (⁷F₆ → ⁵D₃ of Tb³⁺) excitation spectra, enhancing the probability of Tb³⁺ → Eu³⁺ energy transfer along with the increase of Eu³⁺ dopant. The energy transfer efficiencies of LiTb_1-xEu_x(WO₄)₂ (x = 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) phosphors can be calculated by using the equation as follows^{[15, 36]}:

  $ \eta_{\mathrm{T}}=1-I / I_0 $

  (1)

  where I and I₀ represent the 547 nm emission intensity of Tb³⁺ host in the presence and absence of activator Eu^3+[43]. The efficiency values were calculated to be 42.4%, 49.7%, 59.9%, 78.8%, 91.5%, 97.9% and 98.1% for Eu³⁺ concentrations of 0.4%, 0.8%, 1%, 2%, 5%, 8% and 10%, respectively. We can see that the value is 59.9% with Eu³⁺ concentration of 1%, and it increases significantly with increasing the Eu³⁺ concentration until 5%, 59.9%→91.5%^[31]. Then the η_CT(I) values increase slowly, 91.5%→98.1%, for the Eu³⁺ concentration of 5%→10%, showing the saturation of Tb³⁺→Eu³⁺ charge transfer. This result suggests an efficient Tb³⁺→Eu³⁺ charge transfer process and Eu³⁺ is an excellent activator in LiTb(WO₄)₂ host.

  As shown in Fig. 6, the PL decay curve of LiTb_1-x(WO₄)₂: xEu³⁺ phosphors (excitation: 379 nm, emission: 547 nm) are well fitted with the single-exponential function^{[43, 44]}:

  $ I_{(\mathrm{t})}=I_0+A_1 \exp (-t / \tau) $

  (2)

  Figure 6

  

  Figure 6.  Fluorescence decay profile for LiTb_1-xEu_x(WO₄)₂ under the excitation of 379 nm

  DownLoad: Full-Size Img PowerPoint

  where I₀ is the baseline correction, A₁ is a pre-exponential factor obtained from the curve fitting, and τ is the lifetime of exciting state. The fitting τ value is 2.46, 0.97, 0.91, 0.86, 0.61, 0.38, 0.22 and 0.14 ms for LiTb_1-x(WO₄)₂: xEu³⁺ samples with x = 0%, 0.4%, 0.8%, 1%, 2%, 5%, 8% and 10%, respectively. The luminescent decay lifetime of Tb³⁺ emission at 547 nm decreased rapidly with increasing the concentration of Eu³⁺ ions, indicating the existence of ET from Tb³⁺ to Eu³⁺.

  The energy transfer process between Tb³⁺ and Eu³⁺ ions is shown in Fig. 7. The electrons of Tb³⁺ ions are excited by the 379 nm photons to their ⁵D₃ level. Some of them relax to the ⁷F₆, ⁷F₅, ⁷F₄ and ⁷F₃ levels for generating the blue, green and red emissions, while the energy transfer is possible due to the nearby electrons of the Eu³⁺ to the excited ⁵D_{1, 0} level^[45]. Subsequently, the electrons of Eu³⁺ relax to the ⁷F_J (J = 1~6) levels quickly, which generates the orange and red emissions. Notably, the characteristic emission of Eu³⁺ is enhanced because of the effective overlap between ⁵D₄→⁷F_J (J = 3~6) emissions of Tb³⁺ and the ⁷F₁ → ⁵D_{1, 2} absorption of Eu³⁺.

  Figure 7

  

  Figure 7.  Schematic diagram of the energy transfer between Tb³⁺ and Eu³⁺ in a host

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  3.5 CIE chromaticity coordinates

  Because the red emissions of Eu³⁺ and green emissions of Tb³⁺ simultaneously exist in solid solution materials LiTb_1-xEu_x(WO₄)₂ (x = 0~0.1) under UV or near-UV light excitation, the multicolor tunable luminescence can be obtained by adjusting the x value. The color of any light can be represented as an (x, y) coordinate on the basis of Commission International de L'Eclairage (CIE) functions. As shown in Fig. 8, the luminescent color (excited by 379 nm) tuned from green, yellow, orange to red as the concentration of Eu³⁺ increases from 0 to 10% and the corresponding CIE coordinates are listed in Table 3. The other noteworthy thing is that the CIE chromaticity coordinate of LiTb_0.9Eu_0.1(WO₄)₂ is so close to the standard red chromaticity coordinates (0.667, 0.330) of the National Television System Committee (NTSC). These presented results indicate the potential application of LiTb_1-xEu_x(WO₄)₂ phosphors for white-LEDs.

  Figure 8

  

  Figure 8.  CIE chromaticity diagram for the emission spectra of LiTb_1-xEu_x(WO₄)₂ samples excited by 379 nm and inset digital photograph of the emission from samples

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  Table 3

  

  Table 3.  CIE Chromaticity Coordinates of LiTb_1-xEu_x(WO₄)₂

  DownLoad: CSV

  	LiTb1-xEux(WO4)2	λex (nm)	λem (nm)	CIE
  1	x = 0	379	Embase = 450~750	(0.3099, 0.612)
  2	x = 0.004	379	Embase = 450~750	(0.4459, 0.5062)
  3	x = 0.008	379	Embase = 450~750	(0.4795, 0.4801)
  4	x = 0.01	379	Embase = 450~750	(0.5131, 0.4525)
  5	x = 0.02	379	Embase = 450~~750	(0.5538, 0.4194)
  6	x = 0.05	379	Embase = 450~750	(0.596, 0.3851)
  7	x = 0.08	379	Embase = 450~750	(0.6223, 0.3614)
  8	x = 0.1	379	Embase = 450~750	(0.6287, 0.3566)

  4. CONCLUSION

  In summary, a single crystal of LiTb(WO₄)₂ was prepared using high-temperature molten salt method and structurally determined through single-crystal X-ray diffraction analysis. The results show that it features the scheelite-type structure and crystalizes in the tetragonal space group I4₁/a. Li⁺ and Tb³⁺ ions in the structure are disordered and randomly occupy the same site with the molar ratio of 1:1. A series of LiTb_1-xEu_x(WO₄)₂ phosphors were prepared via a high-temperature solid-state reaction method. Due to the concentration-dependent energy transfer, the emission color of the phosphor can be continuously changed from green through yellow and eventually to red by changing the doping concentration of Eu³⁺ because of the energy transfer from Tb³⁺ to Eu³⁺ ions. Hence, we think that the as-prepared multicolor tunable phosphors could be a promising candidate for near UV convertible phosphor for LED and display applications.

  
  
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  Figure 1  XRD patterns of solid solution LiTb_1-xEu_x(WO₄)₂ (x = 0, 0.004, 0.008, 0.01, 0.02, 0.05, 0.08, 0.1) and simulated patterns of LiTb(WO₄)₂

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  Figure 2  View of the crystal structure of LiTb(WO₄)₂ composed of [WO₄] tetrahedra and (Li/Tb)²⁺ ions

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  Figure 3  FT-IR spectra of the LiTb(WO₄)₂ sample

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  Figure 4  Excitation and emission spectra of LiTb(WO₄)₂ (a) and LiTb_0.99Eu_0.01(WO₄)₂ (b) phosphors

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  Figure 5  Emission spectra of representative LiTb_1-xEu_x(WO₄)₂ (x = 0~0.1) phosphors excited by 379 nm (⁷F₆ →⁵D₃ of Tb³⁺) as a function of Eu³⁺ doping content x. Inset is the variatation of 547 nm (⁵D₄→⁷F₅ of Tb³⁺) and 616 nm (⁷F₀→⁵D₂ of Eu³⁺) emission intensity

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  Figure 6  Fluorescence decay profile for LiTb_1-xEu_x(WO₄)₂ under the excitation of 379 nm

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  Figure 7  Schematic diagram of the energy transfer between Tb³⁺ and Eu³⁺ in a host

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  Figure 8  CIE chromaticity diagram for the emission spectra of LiTb_1-xEu_x(WO₄)₂ samples excited by 379 nm and inset digital photograph of the emission from samples

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  W(1)–O(1)	1.784(5)		Tb(1)–O(1)iv	2.405(5)		Tb(1)–O(1)	2.416(5)
  W(1)–O(1)i	1.784(5)		Tb(1)–O(1)v	2.405(5)		Tb(1)–O(1)vi	2.405(5)
  W(1)–O(1)ii	1.784(5)		Tb(1)–O(1)ix	2.416(5)		Tb(1)–O(1)vii	2.405(5)
  W(1)–O(1)iii	1.784(5)		Tb(1)–O(1)x	2.416(5)		Tb(1)–O(1)viii	2.416(5)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–W(1)–O(1)i	107.03(16)		O(1)i–W(1)–O(1)ii	114.5(3)		O(1)i–W(1)–O(1)iii	107.03(16)
  O(1)–W(1)–O(1)ii	107.04(16)		O(1)–W(1)–O(1)iii	114.5(3)		O(1)ii–W(1)–O(1)iii	107.03(16)
  Symmetry codes: (i) –y+1/4, x+1/4, –z+1/4; (ii) y–1/4, –x+1/4, –z+1/4; (iii) –x, –y+1/2, z; (iv) –x–1/2, –y+3/2, –z+1/2; (v) y–5/4, –x+3/4, z–1/4; (vi) –y+1/4, x+3/4, z–1/4; (vii) x–1/2, y, –z+1/2; (viii) y–5/4, –x+1/4, –z+1/4; (ix) –y+1/4, x+5/4, –z+1/4; (x) –x–1, –y+3/2, z

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  Table 2.  Excitation Peaks of LiTb_1-xEu_x(WO₄)₂ Phosphors

  No.	Wavelength (nm)	Activation ion	Excitation transition
  1	319	Tb3+	7F6→5D0
  2	341	Tb3+	7F6→5G2
  3	354	Tb3+	7F6 →5D2
  4	360	Tb3+	7F6 →5G5
  5	370	Tb3+	7F6 →5G6
  6	379	Tb3+	7F6 →5D3
  7	395	Eu3+	7F0 →5L6
  8	417	Eu3+	7F0 →5D3
  9	465	Eu3+	7F0 →5D2
  10	487	Tb3+	7F0 →5D4

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  Table 3.  CIE Chromaticity Coordinates of LiTb_1-xEu_x(WO₄)₂

  	LiTb1-xEux(WO4)2	λex (nm)	λem (nm)	CIE
  1	x = 0	379	Embase = 450~750	(0.3099, 0.612)
  2	x = 0.004	379	Embase = 450~750	(0.4459, 0.5062)
  3	x = 0.008	379	Embase = 450~750	(0.4795, 0.4801)
  4	x = 0.01	379	Embase = 450~750	(0.5131, 0.4525)
  5	x = 0.02	379	Embase = 450~~750	(0.5538, 0.4194)
  6	x = 0.05	379	Embase = 450~750	(0.596, 0.3851)
  7	x = 0.08	379	Embase = 450~750	(0.6223, 0.3614)
  8	x = 0.1	379	Embase = 450~750	(0.6287, 0.3566)

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