1. Introduction

Phosphors for excitation in the vacuum ultraviolet region, especially the Xe resonance emission line (147 nm) and/or the Xe₂ molecular emission band (172 nm), are required for application in plasma display panels (PDPs) and Hg-free fluorescent tubes. Nowadays, (Y,Gd)BO₃:Eu³⁺, Zn₂SiO₄:Mn²⁺ (ZSM), and BaMgAl₁₀O₁₇:Eu²⁺ (BAM) with red/green/blue-emitting are usually used as tricolor phosphors in these devices [1-3]. But there are some shortcomings for each phosphor. In detail for green and blue phosphors, ZSM has a long decay time and BAM exhibits serious degradation under heating and VUV irradiation. In recent years, much work was devoted to decrease the degradation of BAM, such as atomic substitution with Nd³⁺ and Er³⁺ for reducing the damage caused by VUV-irradiation [4,5], coating the phosphor surface with MgO [6,7], SiO₂ [8] for improving thermal degradation. Another way to solve the degradation problem is to develop a new blue phosphor for PDP application, such as Si₃Al₁₀SiO₂₀:Eu²⁺ [9], CaMgSi₂O₆:Eu²⁺ [10]. It is the feature of these approaches that blue emission is obtained by Eu²⁺.

In this paper, we prepared a novel blue-emitting VUV phosphor at 700 °C, which is activated by Tb³⁺ instead of Eu²⁺. Moreover, when we change the doping concentration of Tb³⁺ in the samples, the chromaticity coordinates of these Tb³⁺ doped Na₂GdF₂PO₄ phosphors are tunable from blue to whitish and further to yellowish-green range. By a single host compound and with a sole doping ion, the intensive emission with a large color gamut is fulfilled.

2. Experimental

The powder samples were prepared using a high-temperature solid-state reaction technique. The reactants are (NH₄)₂HPO₄ (A.R.), NaF (A.R.), Tb₄O₇ (99.9%), and Gd₂O₃ (99.9%). After a good mixing as stoichiometric raw materials, mixtures were pre-heated at 350 °C, and fired at 700 °C in CO atmosphere for several hours.

The structure of the samples was examined by powder X-ray diffraction using Cu Kα radiation (λ= 1.5405 Å) on a Rigaku D/max 2200 vpc X-ray diffractometer.

The luminescence decay curves were measured at an EDINBURGH FLS 920 combined fluorescence lifetime and steady-state spectrometer, which is equipped with a time-correlated single photon counting (TCSPC) card.

The VUV spectra at room temperature were measured at the VUV spectroscopy experimental station on beam line U24 of National Synchrotron Radiation Laboratory (NSRL, Hefei, China). The electron energy of the storage ring is 800 MeV, and the beam current is about 150–250 mA. A Seya-Namioka monochromator (1200 g·mm^-1, 100–400 nm) is used for the synchrotron radiation excitation photons, while an Acton-275 monochromator (1200 g·mm^-1, 330–700 nm) for the emission photons and the signal is detected by a Hamamatsu H5920-01 photomultiplier (PMT). The system has been corrected for the monochromators but not for the PMT recently. Therefore the systematic error may occur when we evaluate spectral efficiency by intensity integration. However, we think the result can be considered as a reference to evaluate relative luminance of phosphor in the same spectral range. The resolution of the instruments is about 0.2 nm. The pressure in the sample chamber is about 1×10^-3 Pa. The relative VUV excitation intensities of the samples are corrected by dividing the measured excitation intensities of the samples with that of sodium salicylate (o-C₆H₄OHCOONa) in the same excitation conditions.

3. Results and discussion

3. 1. The VUV-UV excitation spectrum and emission spectra under VUV-UV excitation

VUV-UV excitation spectrum and emission spectra for phosphor Na₂Gd_0.999Tb_0.001F₂PO₄ at RT are exhibited in Fig. 1. Fig. 1(a) shows two broad absorption bands (A and B) peaking at 224 and 174 nm, respectively. The band A should correspond to the lowest spin-allowed (LSA) f-d transition of Tb³⁺ in the host Na₂GdF₂PO₄, which is near that in NaGdFPO₄ [11]. A series of sharp lines around 194, 202, 252, 273, 311 nm in Fig. 1(a) corresponds to the ⁸S_7/2→⁶G_J, ⁶D_J, ⁶I_J, ⁶P_J transitions of Gd³⁺ ion, respectively. The occurrence of the intense peaks indicates efficient energy transfer from Gd³⁺ to Tb³⁺. That is, ⁶G_J, ⁶D_J, ⁶I_J, ⁶P_J states of Gd³⁺ play an intermediate role and can sensitize Tb³⁺ luminescence.

 

Fig. 1. The VUV excitation spectrum (a: λ _em =413 nm) and emission spectra at VUV-UV excitation (b: λ _ex = 226 nm, c: λ _ex = 147 nm, and d: λ _ex = 172 nm) for Na₂Gd_0.999Tb_0.001F₂PO₄.

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The absorptions below wavelength 190 nm which include the band B, are mainly the host-related absorption. At the same time, we do not exclude the probable occurrence of f-d transitions of Tb³⁺ and Gd³⁺←O^2- charge transfer band (CTB) in this spectral range. It is known that the host-related absorption of most phosphates appears in the range of 140~180 nm [12, 13]. The host absorption edge ~190 nm in the present case moves to longer wavelength region than that in GdPO₄ (~165 nm) and NaGdFPO₄ (~180 nm) [14, 15]. We think that the presence of F^- in the crystal may show important influence on this change, more F^- ions in crystal lattice of Na₂GdF₂PO₄ than in that of NaGdFPO₄ and GdPO₄ might incline to increase covalency of P-O bond for PO₄ ^3- groups [14]. Therefore, the host absorption edge of Na₂GdF₂PO₄ moves to long wavelength. The f-d transitions of Tb³⁺ are complicated in excitation spectrum, which is in relation with two factors. First, when one electron is promoted from the ground states 4f⁸ (⁷F₆) to 4f⁷5d¹ excitation levels, it results in two types of f-d transitions: the spin-allowed ⁷F₆-⁷D_J transitions (with higher energies and intensities) and the spin-forbidden ⁷F₆-⁹D_J transitions (with lower energies and intensities); Second, the crystal field will split both ⁷D_J and ⁹D_J levels into many sublevels, and each transition from ground state to CFS (crystal field splitting) ⁷D_J and ⁹D_J sublevels will probably occur in the excitation spectrum. To elucidate the probable overlapping of Tb³⁺ 5d states to the host absorption, we can evaluate the highest 5d level of Tb³⁺ in Na₂GdF₂PO₄ by the magnitude of 5d CFS of Ce³⁺ (around 17 430 cm^-1) in the same host site. If Ce³⁺ and Tb³⁺ keep nearly same CFS values in same host lattice, the highest 5d level of Tb³⁺ in Na₂GdF₂PO₄ can be evaluated to be about 161 nm. For the CTB of Gd³⁺←O^2-, its energy can only be roughly estimated by the Jørgensen empirical formula [16]: E _CT = [χ _opt(X) - χ _opt(M)] × 30 × 10³ (cm^-1) [χ _opt(X) and χ _opt(M) are the optical electronegativities of the anion X and central metal cation M, respectively]. Using χ _opt(O^2-)=3.2 and χ _opt(Gd³⁺)= 0.91 [17], the CTB energy of Gd³⁺ in most oxides can be approximately estimated to be 68 700 cm^-1 (146 nm).

Upon exciting Tb³⁺ 5d states at 226 nm, strong blue emission from Tb^{3+ 5}D₃ to ⁷F_6,5,4,3,2 transition appears in the emission spectrum [Fig. 1(b)]. In addition, weak emission around 311 nm from Gd^{3+ 6}P_J→⁸S_7/2 transitions appears whose intensity is 4% of total Tb³⁺ emission, exhibiting weak energy transfer from Tb³⁺ to Gd³⁺. With excitation at 172 nm, much stronger emission from Gd^{3+ 6}P_J→⁸S_7/2 transitions appears, whose intensity is 47% of total Tb³⁺ emission, as shown in Fig. 1(d). This situation is the same for 147 nm excitation [Fig. 1(c)], which shows efficiently energy transfer from host to both Gd³⁺ and Tb³⁺ ions.

3.2. The concentration quenching and tunable CIE chromaticity coordinates

 

Fig. 2. Emission intensities of Tb^{3+ 5}D₃ and ⁵D₄ levels with 147, 172, and 226 nm excitation as a function of x values in Na₂Gd_1-xTb_xF₂PO₄ phosphors.

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The dependence of emission intensities of ⁵D₃ and ⁵D₄ levels on the Tb³⁺ doping content in Na₂Gd_1-xTb_xF₂PO₄ upon 147, 172, and 226 nm excitation are shown in Fig. 2, respectively. At relative low Tb³⁺ content (below about 1.5 mol %), emission intensity of ⁵D₃ is stronger than that of ⁵D₄. ⁵D₃ emission shows maximum intensity around 0.3 mol %, and then rapidly decreases due to cross-relaxation between ⁵D₃, ⁷F₀→⁵D_4, ⁷F₆ states. Above 20 mol % Tb³⁺ content, the ⁵D₃ emission is so weak as to nearly disappear. At the same time, ⁵D₄ emission intensity fast increases, and reach to luminescence saturation above 25 mol %, this phenomenon is especially obvious for 172 nm excitation. The concentration quenching has not been evidently observed up to 100 mol % of Tb³⁺ substituted for Gd³⁺, showing the interaction between Tb³⁺ ion and host lattice is very weak [18].

When Tb³⁺ content increases, blue emission from ⁵D₃ levels decreases and green emission from ⁵D₄ levels increases. Therefore, samples Na₂Gd_1-xTb_xF₂PO₄ show tunable emission from blue to whitish and further to yellowish-green depending on Tb³⁺ content, as shown in the CIE chromaticity diagram of Fig. 3 in which the dots indicate the CIE chromaticity coordinate positions.

 

Fig. 3. The CIE chromaticity diagram for samples Na₂Gd_1-xTb_xF₂PO₄ (x= 0.001, 0.003, 0.01, 0.04, 0.1, 0.2).

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It is worth pointing out that this series of phosphors shows high efficient luminescence. In order to evaluate the possibility of using the phosphors as potential components in PDPs and Hg-free fluorescent tubes, here we give comparisons of three samples Na₂Gd_1-xTb_xF₂PO₄ (x = 0.003, 0.01, 0.1) with two commercially available PDP phosphors Zn₂SiO₄:Mn²⁺ (ZSM, with green emission) and BaMgAl₁₀O₁₇:Eu²⁺ (BAM, with blue emission). To compare, we normalized all parameters including slit width, integrated time, beam intensity and relative intensity of energy at excitation wavelength. That is, all spectra are thought to be measured with nearly the same condition. The Spectroscopic curves of five phosphors, Na₂Gd_1-xTb_xF₂PO₄ (x = 0.003, 0.01, 0.1) and commercial phosphors BAM and ZSM, are shown in Fig. 4(a). Table 1 lists peaking emission wavelength λ _peak and relative luminance of emission spectra of those five samples under λ _ex= 147/172 nm excitation.

Here, we used the standard visual spectral efficiency (integrated intensities in the whole 380 ~ 730 nm visible range) curves to calculate relative luminance. For sample Na₂Gd_0.997Tb_0.003F₂PO₄, strong blue emission originates from Tb^{3+ 5}D₃ levels and its relative luminance is calculated to be about 42% (for 147 nm excitation) and 80% (for 172 nm excitation) of that commercially available BAM phosphor according to the values in Table 1. On the other hand, a good thermal stability for luminescence is an important requirement for a phosphor in PDPs. For Na₂Gd_0.997Tb_0.003F₂PO₄ phosphor, its thermal quenching effects under 273 nm excitation are not clearly observed in temperature range from 273 K to 420 K (see Fig. 4(b)), showing that the luminescence exhibits a good thermal stability. In addition, the sample Na₂Gd_0.997Tb_0.003F₂PO₄ exhibits a shorter 10% decay time which is about 3.2 ms (see Fig. 4(c)) and therefore has no any effect on the quality of rapid-moving pictures of plasma display panels. Moreover, the reprocessing of the expensive Tb³⁺ from the phosphate is easier than that of Eu²⁺ from aluminate phosphor (BAM), which can effectively reduce the consumption of rare earths, and low preparation temperature can further save energy.

 

Fig. 4. (a) The emission spectra of samples Na₂Gd_1-xTb_xF₂PO₄ (x=0.003, 0.01, 0.1) under 147/172 nm excitation, in comparison with commercial phosphors ZSM and BAM. (b) Luminescence intensity of Na₂Gd_0.997Tb_0.003F₂PO₄ phosphor excitation at 273 nm as a function of temperature. (c) The decay curves of Na₂Gd_0.997Tb_0.003F₂PO₄, Na₂Gd_0.9Tb_0.1F₂PO₄, and commercial phosphor ZSM.

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The Sample Na₂Gd_0.9Tb_0.1F₂PO₄ shows strong green emission under 147/172 nm. According to the values in Table 1, its relative luminance is estimated to be about 104% (for 147 nm excitation) and 126% (for 172 nm excitation) in comparison with that of commercially available ZSM phosphor, respectively. Furthermore, the 10% decay time of Na₂Gd_0.9Tb_0.1F₂PO₄ phosphor (~ 7.4 ms, see Fig. 4(c)) is shorter than that of commercial ZSM phosphors.

As for medial content sample Na₂Gd_0.99Tb_0.01F₂PO₄, its relative luminance is evaluated about 53% of that commercial blue phosphor BAM and 104% of that commercial green phosphor ZSM in the same conditions under 147 nm excitation, and these values are respectively about 85% and 148% for 172 nm excitation. The CIE color coordinate of Na₂Gd_0.99Tb_0.01F₂PO₄ (0.23, 0.28) is near cold-white light region.

Table. 1. Peaking wavelength λ_peak, relative luminance of emission spectra of samples Na2Gd1- xTbxF2PO4 (x = 0.003, 0.01, 0.1) compared with that of the commercial phosphors BAM, ZSM under λ_ex= 147/172 nm wavelength excitation:

View Table

4. Conclusion

In summary, VUV-vis spectroscopic properties of phosphors Na₂GdF₂PO₄:Tb³⁺ under 147/172 nm excitation are investigated, the emitted color of Na₂GdF₂PO₄:Tb³⁺ can be easily tailored from blue to whitish and further to yellowish-green by varying the Tb³⁺ content, especially for three high efficient phosphors Na₂Gd_1-xTb_xF₂PO₄ (x = 0.003, 0.01, 0.1) under 147/172 nm excitation: (1) Phosphor Na₂Gd_0.997Tb_0.003F₂PO₄ exhibits strong blue emission and good thermal stability for luminescence performance. A very low Tb³⁺ content, easier recovery and a lower preparation temperature tend to decrease the consumption of rare earth resource and save energy. (2) Green-emitting phosphor Na₂Gd_0.9Tb_0.1F₂PO₄ shows a stronger luminescence and a shorter decay time than the commercially available ZSM phosphor. (3) The emission of phosphor Na₂Gd_0.99Tb_0.01F₂PO₄ exhibits chromaticity coordinates (0.23, 0.28) in cold-white light region. These results demonstrate that this series of phosphors is expected to be promising candidates for application in PDPs and Hg-free fluorescent tubes.