Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer in Ca₆La₂Na₂(PO₄)₆F₂:Eu, Tb phosphors

Abstract

Novel phosphors Ca₆La₂Na₂(PO₄)₆F₂:0.10Eu²⁺/Eu³⁺, xTb³⁺ (x = 0, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50) were prepared by a solid state reaction in a CO-reducing atmosphere. The fluorescence spectra of samples Ca₆La₂Na₂(PO₄)₆F₂:xEu²⁺ reveal that still a small number of Eu³⁺ ions are detected in the host. The Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer process in Ca₆La₂Na₂(PO₄)₆F₂:0.10Eu²⁺/Eu³⁺, xTb³⁺ phosphors was discussed in detail through the excitation, emission spectra and the fluorescence decay. As a result of Eu²⁺ sensitization, the relative emission intensity of Ca₆La₂Na₂(PO₄)₆F₂:0.10Eu²⁺/Eu³⁺, xTb³⁺ is enhanced under near-ultraviolet light excitation. Furthermore, tunable emission with a large color gamut can be obtained by changing the Tb³⁺ doping concentration. These results indicate that the Ca₆La₂Na₂(PO₄)₆F₂:Eu²⁺/Eu³⁺, Tb³⁺ phosphors will have potential use in n-UV chip pumped white LED devices.

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1. Introduction

Nowadays, white light emitting diodes (w-LEDs) are been considered as the fourth-generation lighting source, and will take the place of conventional fluorescent lamps in the near future.¹ For phosphor-converted w-LEDs, white light can be generated mainly in two ways: (1) fabricate a blue LED chip with the yellow-emitting phosphor Y₃Al₅O₁₂:Ce³⁺; (2) fabricate an ultraviolet (UV) or near-UV (350–420 nm) LED chip with tri-color (red, green and blue) phosphors. The first way fabricated by blue-emitting chips coated with the yellow-emitting Y₃Al₅O₁₂:Ce³⁺ garnet, have several drawbacks such as a high correlated color temperature (CCT) and a low color-rendering index (CRI). Accordingly, much attention has been paid to the second way in which tri-color phosphors are combined with ultraviolet (UV) or n-UV LEDs.^2–4 As we know, the blue components of the n-UV based LEDs often come from the Ce³⁺ or Eu²⁺ doped phosphors,^5–8 and Tb³⁺ and Eu³⁺ ions are usually used as green-emitting and red-emitting activators, because Tb³⁺ and Eu³⁺ ions can produce green and red emission due to ⁵D₄ → ⁷F_J and ⁵D₀ → ⁷F_J transitions, respectively.^9,10 Furthermore, in Ce³⁺–Tb³⁺–Eu³⁺ and Eu²⁺–Tb³⁺–Eu³⁺ co-doped phosphors, Tb³⁺ ions can act as an energy transfer bridge like Ce³⁺/Eu²⁺ → Tb³⁺ → Eu³⁺ between Ce³⁺/Eu²⁺ and Eu³⁺ ions. It is well known that Ce³⁺ and Eu²⁺ ions are excellent sensitizers, however, the energy transfer efficiency for Ce³⁺/Eu²⁺ → Eu³⁺ is usually low. This is largely due to the metal–metal charge transfer (MMCT), in which excited charge-transfer state Ce⁴⁺–Eu²⁺ is easily occur.^11,12 So it is a good way to enhance Eu³⁺ emission by energy transfer like Ce³⁺/Eu²⁺ → Tb³⁺ → Eu³⁺ ions. In present, some Ce³⁺–Tb³⁺–Eu³⁺ and Eu²⁺–Tb³⁺–Eu³⁺ co-doped phosphors were reported, and it is a novel way to obtain bright phosphors and control the emission color.^13–18

It is well known that the compounds with the apatite structure are considered as effective host lattices for luminescent materials. Among these compounds, the fluoro-apatite compounds with general formula M_10−2xLn_xNa_x(PO₄)₆F₂ (M = Ca, Sr, Ba; Ln = rare earths) are studied widely for luminescent materials.^19–21 In our previous work, RE³⁺ (Ce³⁺, Tb³⁺, Eu³⁺, Dy³⁺, Tm³⁺ and Sm³⁺)-doped M_10−2xLn_xNa_x(PO₄)₆F₂ (M = Ca, Sr, Ba; Ln = rare earths) phosphors were studied.^22–27 In this work, novel phosphors Ca₆La₂Na₂(PO₄)₆F₂:Eu²⁺/Eu³⁺ (CLPF:Eu) and Ca₆La₂Na₂(PO₄)₆F₂:Eu²⁺/Eu³⁺,Tb³⁺ (CLPF:Eu, Tb) were synthesized, and the detailed photoluminescence properties were investigated for their potential application in n-UV based LEDs.

2. Experimental

Eu²⁺/Eu³⁺ doped phosphors Ca₆La₂Na₂(PO₄)₆F₂:xEu²⁺/Eu³⁺ (x = 0.01, 0.04, 0.07, 0.10, 0.13, 0.16, 0.20) and Eu²⁺/Eu³⁺–Tb³⁺ co-doped Ca₆La₂Na₂(PO₄)₆F₂ phosphors Ca₆La₂Na₂(PO₄)₆F₂:0.10Eu, xTb (x = 0, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50) were synthesized by a high-temperature solid-state reaction. The starting materials are analytical grade CaCO₃, NH₄H₂PO₄, NH₄F, Na₂CO₃ and rare earth oxides (La₂O₃, Eu₂O₃, 99.95%; Tb₄O₇, 99.99% purity). The raw materials were carefully weighed stoichiometrically and ground in an agate mortar. After mixing and thorough grinding, the mixtures were heated at 1050 °C for 4 h in a CO-reduced atmosphere, by covering the highly pure carbon grains on the inner crucible. The final products were cooled to room temperature by switching off the muffle furnace and ground again into white powder.

The phase purity and structure of the final products were characterized by a powder X-ray diffraction (XRD) analysis using Cu Kα radiation (λ = 1.5405 Å, 40 kV, 30 mA) on a PANalytilal X'pert Powder X-Ray Diffractometer at room temperature (RT). The photoluminescence properties were measured on a HITACHI F7000 fluorescence spectrometer equipped with a 450 W xenon lamp as the excitation source. The luminescence decay spectra were measured by a FLS 920 steady-state spectrometer equipped with a fluorescence lifetime spectrometer, and a 150 W nF900 ns flash lamp and a 60 W μF flash lamp were used as the flash-light source, respectively. All the measurements were performed at room temperature (RT).

3. Results and discussion

3.1 Phase characterization

As apaties, the compounds M_10−2xLn_xNa_x(PO₄)₆F₂ (M = Ca, Sr, Ba; Ln = rare earths) have general structure of the fluoapatites with P6₃/m space group.²⁷ Two cationic M²⁺ (Ln³⁺) sites can be found in these compounds. Site (1) are at the Wyckoff 4f positions with C₃ point symmetry which is surrounded by nine oxygen anions, while the other site (2) are at the Wyckoff 6h positions with C_s point symmetry which is surrounded by six oxygen anions plus one F⁻ anion. Though different amounts of cations M²⁺ and Ln³⁺ will bring different distortion of the tetrahedra structure, significant changes in the structure do not occur. So the JCPDS card of Ca₅(PO₄)₃F can be used to identify the phase purity of Ca₆La₂Na₂(PO₄)₆F₂ in previous work.^23,27

The phase purities of the as-prepared samples were measured by X-ray diffraction (XRD) at room temperature. Fig. 1 shows the XRD patterns of typical samples CLPF:xEu (a–c), CLPF:0.10Eu, xTb (d–f) and the Joint Committee on Powder Diffraction Standards (JCPDS) card file (no 71-0881) of Ca₁₀(PO₄)₆F₂ as a reference. It can be seen that the diffraction patterns of these samples are in good agreement with the standard JCPDS card, and no other peaks or impurities are detected. Hence, it can be concluded that the dopant Eu²⁺, Eu³⁺ and Tb³⁺ ions are completely incorporated into the host lattice without making significant changes to the crystal structure.

Fig. 1 XRD patterns of samples CLPF:xEu (a–c) and CLPF:0.10Eu, xTb (d–f).

3.2 Luminescent properties of CLPF:xEu

The PL spectra (λ_ex = 325 nm) of CLPF:xEu (x = 0.01, 0.04, 0.07, 0.10, 0.13, 0.16, 0.20) phosphors are presented in Fig. 2. It can be seen that all these phosphors exhibit a broad blue emission band with a peak at around 465 nm, which corresponds to the 4f⁶5d¹ → 4f⁷ allowed transition of Eu²⁺. It is clear that the PL intensity increases greatly with Eu concentration increasing first, reaches a maximum at a low Eu concentration (x = 0.04), and then decreases with increasing Eu doping concentrations, which should be due to the concentration quenching effect. Furthermore, it is evident that Eu³⁺ is still detected in all these phosphors, especially for samples with high doping concentration. There are two small narrow emission lines with peaks centred at 593 nm and 618 nm in the spectra, which correspond to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transition of Eu³⁺, respectively.²³

Fig. 2 PL spectra of samples CLPF:xEu (x = 0.01, 0.04, 0.07, 0.10, 0.13, 0.16, 0.20) under 325 nm excitation.

As is known to all, the detailed electronic structure of Eu²⁺ is [Xe]4f⁷5d⁰ while it is [Xe]4f⁶ for Eu³⁺. Thus, the allowed 4f⁷5d⁰ → 4f⁶5d¹ transition of Eu²⁺ would occur when it is excited, and show a broad emission band, while Eu³⁺ ions emit narrow lines owing to the f–f transitions. According to the observation of Eu valence state in phosphors in previous reports,^16,28–32 we propose that both Eu²⁺ and Eu³⁺ can exist stably in the CLPF host. To prove it, a Eu³⁺ single-doped CLPF phosphor CLPF:0.10Eu³⁺ was prepared in air and a Eu²⁺ single-doped phosphor CLPF:0.10Eu²⁺ was prepared in H₂-reducing atmosphere for comparing. The PLE and PL spectra of samples CLPF:0.10Eu³⁺, CLPF:0.10Eu²⁺/Eu³⁺ and CLPF:0.10Eu²⁺ are shown in Fig. 3. As shown in Fig. 3(a), a broad excitation band centered at 305 nm originated from 4f⁷5d⁰ → 4f⁶5d¹ of Eu²⁺ can be observed. The PLE spectrum (Fig. 3(b)) of Eu³⁺ single-doped phosphor CLPF:Eu³⁺ consists of a broad excitation band in the 200–300 nm wavelength region and a series of excitation peaks from 300 to 500 nm, which are attributed to charge transfer band and f–f transitions of Eu³⁺ ions. The PLE spectrum (Fig. 3(c) and (d)) profile of CLPF:0.10Eu²⁺/Eu³⁺ are nearly identical to that of Eu²⁺ singly-doped phosphor CLPF:0.10Eu²⁺ and Eu³⁺ singly-doped CLPF:0.10Eu³⁺ except for intensity when they are monitored at 470 and 594 nm, respectively. It confirms that Eu²⁺ and Eu³⁺ ions should coexist in the CLPF host.

Fig. 3 PLE and PL spectra of samples CLPF:0.10Eu²⁺ ((a): λ_em = 470 nm; (e): λ_ex = 305 nm), CLPF:0.10Eu³⁺ ((b): λ_em = 617 nm; (f): λ_ex = 393 nm) and CLPF:0.10Eu²⁺/Eu³⁺ ((c): λ_em = 470 nm; (d): λ_em = 617 nm; (g): λ_ex = 393 nm).

The emission spectrum (Fig. 3(e)) of sample CLPF:0.10Eu²⁺ under 305 nm excitation consists of a broad emission band in the blue region with a peak centred at 465 nm. It can be seen clearly that the emission peak position is almost coincident with that of CLPF:Eu phosphors as in Fig. 2. And the emission spectrum (Fig. 3(f)) of CLPF:0.10Eu³⁺ under 393 nm excitation shows series of narrow emission lines with peaks centred at 591 nm, 617 nm and 700 nm, which correspond to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transition of Eu³⁺, respectively. The emission peak position of Eu³⁺ ions is the same as that in our previous work.²³ As seen Fig. 3(g), it is not difficult to conclude that the emission spectrum of sample CLPF:0.10Eu²⁺/Eu³⁺ under 393 nm excitation consists of both Eu²⁺ emission band and Eu³⁺ emission lines.

3.3 Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer in CLPF:Eu, Tb

As mentioned above, Eu³⁺ ions are hard to be directly sensitized by Ce³⁺ or Eu²⁺ ions ascribed to the existence of metal–metal charge transfer (MMCT), which will quenche the luminescence of the sensitizer.^11,12 However, Eu²⁺ → Tb³⁺ and Tb³⁺ → Eu³⁺ can be found in many hosts. So can Tb³⁺ ion be used as a terbium bridge to realize a Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer in CLPF host? Based on this idea, we prepared series of samples CLPF:0.10Eu²⁺/Eu³⁺, xTb³⁺ (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50), which is shortened as CLPF:0.10Eu, xTb. The PL and PLE spectra of phosphors CLPF:0.10Eu²⁺, CLPF:0.10Eu³⁺, CLPF:0.50Tb³⁺ and CLPF:0.10Eu, 0.10Tb are shown in Fig. 4. By comparing the PL spectra of these single-doped and co-doped, it is easy to find characteristic Eu²⁺ emission band and Eu³⁺, Tb³⁺ emission lines in the emission spectrum of sample CLPF:0.10Eu, 0.10Tb under 368 nm excitation (curve f). By comparing the PLE spectra of samples CLPF:0.10Tb³⁺ and CLPF:0.10Eu, 0.10Tb (curve c and d), it can be found that the excitation intensity of CLPF:0.10Eu, 0.10Tb phosphor is enhanced when Eu is introduced. It is clear that Eu²⁺ → Tb³⁺ energy transfer occurs in this case. The PLE spectra of samples CLPF:0.10Eu³⁺ and CLPF:0.10Eu, 0.10Tb (curve g and h) show that Tb³⁺ excitation lines (340, 350 and 368 nm) are observed by monitoring Eu³⁺ emission at 617 nm, which indicates that Tb³⁺ → Eu³⁺ energy transfer is realized. The detail energy transfer process is shown in Fig. 5. When Eu²⁺ ions are excited by n-UV light, part of energy can be transfer to the ⁵D₄ level of Tb³⁺ ions from Eu²⁺, and then the energy is further transferred to the ⁵D₀ level of Eu³⁺ ions, and finally Eu³⁺ emission intensity is enhanced. Therefore, the Tb³⁺ ions act as both activators and sensitizers at the same time in CLPF:Eu, Tb phosphors, which we call it as energy transfer bridge. Further discuss about this energy transfer model can be found in latter part by PL spectra and decay curves.

Fig. 4 PL and PLE spectra of CLPF:0.10Eu²⁺, CLPF:0.10Eu³⁺, CLPF:0.50Tb³⁺ and CLPF:0.10Eu, 0.10Tb phosphors.

Fig. 5 Schematic diagram of energy transfer Eu²⁺ → Tb³⁺ → Eu³⁺ in CLPF:Eu, Tb.

Fig. 6 shows emission spectra of CLPF:0.10Eu, 0.10Tb phosphor under different light excitation. Except for curves a and f excited by 260 and 400 nm, in which only Eu³⁺ emission peaks are observable, the Eu²⁺ emission band (∼470 nm), Tb³⁺ emission lines (∼487, 543 nm) and the Eu³⁺ emission lines (∼591, 617 and 700 nm) can be found from other emission spectrum curves. Still the relative intensities of Eu²⁺, Eu³⁺ and Tb³⁺ in these curves are different. This is not surprising, because different wavelength may directly excite different luminescent centre ions, and the energy transfer behaviors are also different. Interestingly, 305 nm light can excite neither Tb³⁺ nor Eu³⁺ ions, and 340, 368 nm light can hardly excite Eu³⁺ ions, which we can conclude from the excitation spectra of CLPF:0.50Tb³⁺ and CLPF:0.10Eu (curve c and g in Fig. 4). However, Tb³⁺ and Eu³⁺ emission peaks can be clearly seen in curve b, c and d excited by 305, 340, 368 nm light. This result further indicates that energy transfer process Eu²⁺ → Tb³⁺ → Eu³⁺ occurs. In order to further discuss Eu²⁺ → Tb³⁺ and Tb³⁺ → Eu³⁺ energy transfer, the emission spectra under 340 and 305 nm excitation for samples CLPF:0.10Eu²⁺/Eu³⁺, xTb³⁺ with different Tb³⁺ doping concentration are investigated latter.

Fig. 6 PL spectra of sample CLPF:0.10Eu, 0.10Tb excited by different wavelength.

Fig. 7 shows the concentration dependence of PL spectra for the samples CLPF:0.10Eu, xTb (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) under 340 nm excitation. The characteristic Eu²⁺, Tb³⁺ and Eu³⁺ emission bands and peaks can be found from all these PL spectra. As discussed above, Tb³⁺ and Eu³⁺ strong emission peaks in this case is due to Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer. The emission intensity of Eu²⁺, Tb³⁺ and Eu³⁺ is plotted in the inset of Fig. 8. It can be seen that the intensity of Eu²⁺ does not change much, and has the optimum emission intensity with doping Tb³⁺ concentration at x = 0.10. The Eu³⁺ and Tb³⁺ relative intensities increase evidently with Tb³⁺ concentration increasing. It is interesting that the Eu³⁺ emission intensity of CLPF:0.10Eu, 0.50Tb is enhanced over fifty times that of CLPF:0.10Eu due to the Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer. As compared, the PL spectra of the samples CLPF:0.10Eu, xTb under 305 nm excitation are also measured and presented in Fig. 9. It is evident that Eu²⁺ relative intensities are more obvious in this case than that in Fig. 7. It is ascribed to the fact that 305 nm light can excite Eu²⁺ more effective than 340 nm.

Fig. 7 PL spectra for 340 nm excitation of samples CLPF:0.10Eu, xTb (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50).

Fig. 8 Eu²⁺, Eu³⁺ and Tb³⁺ emission intensities in dependent of Tb³⁺ concentrations in CLPF:0.10Eu, xTb (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50).

Fig. 9 PL spectra of samples CLPF:0.10Eu, xTb (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) under 305 nm excitation.

Fig. 10 presents the room temperature decay curves of Eu²⁺ in CLPF:0.10Eu, xTb (x = 0, 0.05, 0.10, 0.30 and 0.50). The lifetime of Eu²⁺ can be fitted by a single-order exponential equation in the absence of Tb³⁺, and the lifetime value is calculated to be 2.04 μs. When Tb³⁺ is introduced, the lifetime decreases evidently and becomes a second-order exponential decay. With Tb³⁺ concentration increasing, the lifetime of Eu²⁺ gradually decrease. This result clearly indicates that an energy transfer process between Eu²⁺ and Tb³⁺ exists in CLPF:0.10Eu, xTb phosphors. Similar results have been discussed extensively in previous work.¹⁶ The energy transfer efficiency from Eu²⁺ to Tb³⁺ can be calculated according to the following equation:^16,33

where τ_Eu0 and τ_Eu are the decay time of Eu²⁺ in the absence and presence of acceptor Tb³⁺, respectively. The calculated results of η_T are listed in Table 1, which shows that the energy transfer efficiency from Eu²⁺ to Tb³⁺ change increases from 77.9% to 86.3% with Tb³⁺ doping concentration increasing.

Fig. 10 Decay curves of Eu²⁺ in CLPF:0.10Eu, xTb (x = 0, 0.05, 0.10, 0.30 and 0.50).

Table 1 The lifetime values of Eu²⁺ in CLPF:0.10Eu, xTb and the energy transfer efficiency (η_T) from Eu²⁺ to Tb³⁺

No.	x value	TEu (μs)	ηT (%)
1	0	2.04	NA
2	0.05	0.45	77.9
3	0.10	0.32	84.3
4	0.30	0.30	85.3
5	0.50	0.28	86.3

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Fig. 11 gives the decay curves for the Eu³⁺ emission at 617 nm in CLPF:0.10Eu, xTb (x = 0.05, 0.10 and 0.50) and CLPF:0.10Eu³⁺ phosphors. When the Tb³⁺ concentration is low (x = 0.05), the decay curves show a clear rise immediately after the excitation pulse which indicates feeding of the Eu³⁺ excited states. This further confirms the energy transfer process form Tb³⁺ to Eu³⁺. As the Tb³⁺ concentration increases, the rise process gets faster and faster, and the decay of the Eu³⁺ become single exponential, which are in good agreement with the Eu³⁺ decay of in Eu³⁺ singly-doped phosphor CLPF:0.10Eu³⁺.

Fig. 11 Decay curves of Eu³⁺ in CLPF:0.10Eu, xTb (x = 0.05, 0.10 and 0.50) and CLPF:0.10Eu³⁺.

3.4 CIE chromaticity coordinates and diagram for CLPF:Eu, Tb

In order to investigate the application in n-UV phosphor converted LEDs, n-UV lights (368 and 377 nm) are also chosen to excite the CLPF:Eu, xTb phosphors, and the emission spectra are shown in Fig. 12. It can be seen that the relative intensities of Eu²⁺, Eu³⁺ and Tb³⁺ vary with the increasing of Tb³⁺ concentration. By comparing the panels a and b in Fig. 12, it is not difficult to find out the difference: Eu²⁺ emission in panel b is weaker than that in panel a. It is because 368 nm light can excite Eu²⁺ ions more efficiency than 377 nm. Based on these emission spectra, we can easily predict that the emitting color of CLPF:Eu, xTb phosphors is tunable under n-UV light excitation. The CIE chromaticity diagram and CIE chromaticity coordinates for the CLPF:Eu, xTb under 368 nm excitation are shown in Table 2 and Fig. 13. As seen in Fig. 13, It can be seen that the emitting color of the CLPF:Eu, xTb phosphors under 368 nm excitation can be modulated from blue to white, to yellow-green, and even to yellow. Similarly, under 377 nm excitation, the CIE coordinates of phosphors with low Tb³⁺ doping concentrations (x = 0.05 and 0.10) locate at white light region, and change to yellow region gradually with Tb³⁺ doping concentrations increasing. Thus, the emitting colors can be tunable in a large color gamut by changing the Tb³⁺ doping concentration. Furthermore, the white-light emission can be obtained in a single host, and the CIE coordinates can be further modulated to be closer to the white point (x = 0.33, y = 0.33) by varying the Tb³⁺ doping concentration.

Fig. 12 PL spectra of CLPF:Eu, xTb phosphors under 368 and 377 nm excitation.

Table 2 CIE chromaticity coordinates for CLPF:Eu, xTb under 368 and 377 nm excitation

No.	x value	CIE coordinates (x, y) (λex = 368 nm)	CIE coordinates (x, y) (λex = 377 nm)
1	0	(0.193, 0.193)	NA
2	0.05	(0.258, 0.229)	(0.380, 0.273)
3	0.10	(0.284, 0.301)	(0.381, 0.324)
4	0.20	(0.332, 0.428)	(0.382, 0.433)
5	0.30	(0.345, 0.457)	(0.383, 0.460)
6	0.40	(0.399, 0.508)	(0.395, 0.480)
7	0.50	NA	(0.425, 0.498)

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Fig. 13 CIE chromaticity diagram for CLPF:Eu, xTb phosphors under 368 (red dot) and 377 nm (blue dot) excitation.

4. Conclusions

Eu³⁺ ions were detected in the so-called CLPF:xEu²⁺, CLPF:0.10Eu²⁺, xTb³⁺ phosphors which were prepared by a solid state reaction in a CO reducing atmosphere. In CLPF:0.10Eu, xTb phosphors, Tb³⁺ ions act as a Tb energy transfer bridge, and the Eu²⁺ → Tb³⁺ → Eu³⁺ energy transfer is realized. The energy transfer efficiency of Eu²⁺ → Tb³⁺ can reach 86.3% in CLPF:0.10Eu, 0.50Tb. The emission intensity n-UV light excitation can be enhanced obviously because of Eu²⁺ sensitizing. The emission color of CLPF:0.10Eu, xTb have a large color gamut by changing the Tb³⁺ doping concentration. Under 368 and 377 nm excitation, the emission color changes continuously from blue to white and eventually to yellow region as the concentration of the Tb³⁺ increases. Hence, CLPF:0.10Eu, Tb phosphors can act as a single-component white-light phosphor for wLEDs.

Acknowledgements

The work is financially supported by National Natural Science Foundation of China (Grant no. 21401165, 11404283), Natural Science Foundation of Guangdong Province (Grant no. 2014A030307040, 2014A030307028), Training Program for Excellent Youth Teachers in Universities of Guangdong Province (No. YQ2015110), Overseas Scholarship Program for Elite Young and Middle-aged Teachers of Lingnan Normal University, and the Fundamental Research Funds for the Central University (No. 21614334).

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