1. Introduction

Nowadays, tricolor luminescence lamps are widely used in lighting field. Usually, the white-emitting light is obtained by three phosphors in these lamps, that is, the blend of red-emitting phosphor Y₂O₃:Eu³⁺ (YOE), the green-emitting Zn₂SiO₄:Mn²⁺ (ZSM), CeMgAl₁₁O₁₉:Tb³⁺ (CAT), (La,Ce)PO₄:Tb³⁺(LAP), or (Ce,Gd)MgB₅O₁₀:Tb³⁺ (CBT), and the blue-emitting BaMgAl₁₀O17:Eu²⁺ (BAM). [1,2] The main drawback of these lamps is that the electric discharge of mercury(Hg) atoms is used as excitation source, and the mercury is harmful to the environment when the lamps become broken or expired. In order to avoid the use of harmful mercury, Hg-free luminescence lamps are proposed, in which the phosphors convert the vacuum ultraviolet (VUV, wavelength λ<200 nm and energy E>50,000 cm^-1) photons, that is generated by the discharge of Xe (with wavelength 147 nm) and Xe₂ (172 nm), to blue, green, and red light. The tricolor phosphors, (Y,Gd)BO₃:Eu³⁺ (YGB: Eu³⁺) with red emission, Zn₂SiO₄:Mn²⁺ (ZSM) with green emission, and BaMgAl₁₀O₁₇:Eu²⁺ (BAM) with blue emission, are usually recommended to be used in Hg-free luminescence lamps. [3,4] It costs more rare earth consumption through blending tricolor phosphors to obtain white light, and the rare-earth europium is expensive. Moreover, the preparation temperature of the above three commercial tricolor phosphors is higher than 900 °C by the solid-state reaction technique.

The inorganic condensed polyphosphates with general formula M^IRE^III(PO₃)₄ (where M^I are alkali metal ions and RE^III rare earth metal ions) are relatively stable under normal conditions of temperature and humidity [5,6]. These compounds can be kept for many years in a perfect state of crystallinity and they are not water soluble [7]. They have been extensively investigated in the past years due to their interesting optical properties [8–10]. In this work, only one type of rare-earth Dy³⁺ ion was doped in NaGd(PO₃)₄, and intensive white emission was obtained when the sample NaGd(PO₃)₄:Dy³⁺ (NGP: Dy³⁺) was under 172 nm excitation. And dysprosium (Dy) is abundant in the ion adsorption type deposit of China and its price is cheap.

2. Experimental

A series of polycrystalline samples of NaGd_1-xDy_x(PO₃)₄ (x=0~1.00) were prepared by a high temperature solid-state reaction of stoichiometric amounts (Na/RE/P=1:1:4) of analytical reagent grade Na₂CO₃, NH₄H₂PO₄, and 99.99% pure rare-earth oxides (Gd₂O₃ and Dy₂O₃) using the following reactions:

${8\mathrm{NH}}_4{H}_2{\mathrm{PO}}_4+\left(1-x\right){\mathrm{Gd}}_2{O}_3+{\mathrm{xDy}}_2{O}_3+{\mathrm{Na}}_2{\mathrm{CO}}_3\stackrel{\frac{973K}{40h}}{\to }{2\mathrm{NaGd}}_{1-x}{\mathrm{Dy}}_x{\left({\mathrm{PO}}_3\right)}_4+{8\mathrm{NH}}_3+{12H}_{2}O+{\mathrm{CO}}_2$

The pulverous mixtures were ground in an agate mortar and then calcinated at 973 K (700 °C) for 40 h in a corundum crucible under air atmosphere.

The X-ray powder diffraction analyses were carried out with a Rigaku D/max 2200 vpc X-ray powder diffractometer (Cu Kα radiation, 40kV, 30mA) at room temperature (RT), and the data were collected with 2θ=10~60°, step size=0.02 °.

The UV luminescence spectra at RT were recorded on an Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer. A 450 W xenon lamp was used as the excitation source for the UV excitation spectra and a blue-sensitive photomultiplier tube (R1527 PMT) was used for the emission spectra recording.

The VUV spectra were recorded at Beamline 4B8 in Beijing Synchrotron Radiation Facilities (BSRF) under dedicated synchrotron mode (2.5 GeV, 150-60mA). A 1 m Seya monochromator (1200 g/mm, 120–350nm, 1 nm bandwidth) was used for the synchrotron radiation excitation spectra measurement, and an Acton SP-308 monochromator (600 g/mm, 330–900nm) was used for the emission spectra measurement. The signal was detected with a Hamamatsu H8259-01 photon counting unit. The vacuum in the sample chamber was about 1 × 10^-5 mbar. The effect of the experimental set-up response on the relative VUV excitation intensities of the samples were corrected by dividing the measured excitation intensities of the samples with the excitation intensities of sodium salicylate (o-C₆H₄OHCOONa) measured simultaneously in the same excitation conditions.

3. Results and discussion

3.1 X-ray Powder Diffraction

In order to characterize the phase purity of the samples, X-ray powder diffraction (XRD) measurements were performed for all samples. As examples, the XRD patterns of samples NaGd(PO₃)₄, NaGd_0.95Dy_0.05(PO₃)₄ and NaDy(PO₃)₄ were plotted in Fig. 1, indicating that all samples are of single phase and in good agreement with the reported powder patterns in JCPDS standard card numbered 47-0657 [NaGd(PO₃)₄]. These XRD patterns comparisons also show that the polyphosphate samples were synthesized successfully at 700°C. This temperature is much lower than the preparation temperature of current commercial tricolor phosphors by the solid-state reaction technique.

 

Fig. 1. XRD patterns of samples NaGd(PO₃)₄, NaGd_0.95Dy_0.05(PO₃)₄ and NaDy(PO₃)₄.

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The compound NaGd(PO₃)₄, crystallizing in a monoclinic system with P2₁/n space group, can be described as a long chain polyphosphate containing alternating zigzag (PO₃)_n chains linked by distorted GdO₈ dodecahedra. It was testified there is only one site for Gd³⁺ ions in NaGd(PO₃)₄ with Eu³⁺ ions as a probe[11]. Because of the much small ionic radii difference between rare-earth ions Dy³⁺ (102.7 pm) and Gd³⁺ (105.3 pm) in the eight-fold coordination environment [12], the compound NaDy(PO₃)₄ is iso-structure with NaGd(PO₃)₄, and the XRD patterns of NaGd_0.95Dy_0.05(PO₃)₄ and NaDy(PO₃)₄ are the same with that of NaGd(PO₃)₄, although all Gd³⁺ ions were substituted by Dy³⁺ ions in NaDy(PO₃)₄.

3.2 The UV-νisible luminescence properties

Trivalent dysprosium (Dy³⁺) ion has two dominant bands in the emission spectrum. The yellow band (574 nm) corresponds with the hypersensitive transition ⁴F_9/2→⁶H_13/2 (Δ L=2, Δ J=2), and the blue band (480 nm) corresponds with the ⁴F_9/2→⁶H_15/2 transition. Figure 2 shows the UV-visible excitation and emission spectra of the sample NaGd_0.95Dy_0.05(PO₃)₄ at RT. A number of absorption peaks in the 240–500 nm region can be seen in Fig. 2(a). These peaks at around 251 nm, 273 nm and 311 nm are attributed to the ⁸S_7/2→⁶D_J, ⁶I_J, ⁶P_J transitions within Gd³⁺ ions respectively [13], indicating the existence of the energy transfer process from Gd³⁺ to Dy³⁺ in this sample. The other absorption peaks in the range of 280~500 nm, marked by the Arabic numerals 1–9 in Fig. 2(a), correspond to the f-f transitions of Dy³⁺ ions in the host lattice. The ground state of Dy³⁺ is ⁶H_15/2, and peaks 1–9 are attributed to the transitions from this ground state to different excitation levels: ⁴K_13/2+⁴H_13/2 (1), ⁴K_15/2 (2), ⁴I_9/2+⁴G_9/2 (3), ⁴M_15/2+ ⁶P_7/2 (4), ⁴I_11/2 (5), ⁴M_21/2+ ⁴I_13/2+ ⁴K_17/2+ ⁴F_7/2 (6), ⁴G_11/2 (7), ⁴I_15/2 (8), and ⁴F_9/2 (9), respectively. The emission spectra under the excitation of 349 nm UV radiation are exhibited in Fig. 2(b), in which blue emission at about 479 nm (peak 10) and the yellow emission at about 573 nm (peak 11) are strong. They correspond to the transitions from the ⁴F_9/2 excited state to the ⁶H_15/2 and ⁶H_13/2 ground states, respectively.

 

Fig. 2. Excitation and emission spectra of NaGd_0.95Dy_0.05(PO₃)₄

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Figure 3 exhibits the emission spectra of the samples NaGd_1-xDy_x(PO₃)₄ (x=0.01~1.00) under 349 nm UV excitation in the same conditions. When the value of x exceeds 0.05, the blue and yellowish emission peaks become weaker and weaker due to the concentration quenching. The concentration quenching might be elucidated by the following two factors. (i) The excitation migration due to resonance between the activators is enhanced when the doping concentration is increased, and thus the excitation energy reaches quenching centers. (ii) The activators are paired or coagulated and are changed to a quenching center.

 

Fig. 3. Emission spectra of NaGd_1-xDy_x(PO₃)₄ (λ_ex=349 nm)

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3.2 The VUV excitation spectrum and emission spectra under VUV excitation

 

Fig. 4. VUV spectra of NaGd_0.95Dy_0.05(PO₃)₄ (labeled as NGP:Dy³⁺) and commercial phosphor (Y,Gd)BO₃:Eu³⁺ (YGB: Eu³⁺) and Zn₂SiO₄:Mn²⁺ (ZSM)

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The emission intensity of phosphor NaGd_1-xDy_x(PO₃)₄ for x=0.05 under 349 nm excitation is higher than that of other concentration samples, so the sample NaGd_0.95Dy_0.05(PO₃)₄ (labeled as NGP:Dy³⁺) is chosen to measure the VUV excitation spectrum and emission spectra. Figure 4(a) shows the VUV excitation spectrum upon blue emission (479 nm). In order to describe the VUV absorption peaks clearly, the VUV spectra of undoped NaGd(PO₃)₄ are also showed in Fig. 5.

After comparing the VUV excitation spectra of NGP:Dy³⁺ and that of undoped NaGd(PO₃)₄, it can be concluded that the peaks in the 190–320 nm region (Fig. 4(a)) are mainly ascribed to the absorption of Gd³⁺ ions, and the absorption peak at about 325 nm is due to the transition ⁶H_15/2→⁴K_15/2 of doped Dy³⁺ ions. Broad bands below the wavelength 190 nm in Fig. 4(a) are considered to include the host-related absorption, the f-d transitions of Dy³⁺ in the host lattice, and Dy³⁺⃖O^2- charge transfer band (CTB) from the following standpoints.

1. In our previous work, the host-related absorption bands of some phosphates and fluorophosphates were investigated. [14–16] Though the compositions and the structure of these phosphates, fluorophosphates and polyphosphates are different, they all show absorption band around wavelength 150–170 nm. We consider that the intrinsic absorption of PO₃ ^- is located around this range 140–170 nm (band D in Fig. 4(a)), which is confirmed by the inset spectrum in Fig. 5.

 

Fig. 5. The VUV spectra of undoped NaGd(PO₃)₄ at RT

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2. For Dy³⁺ ions, when one electron is promoted from ground states 4f⁹ to 4f⁸5d¹ excited levels, it can give rise to two groups of f-d transitions: spin-allowed (SA) transitions are stronger and with higher energies, while spin-forbidden (SF) are weaker and with lower energies. The energies of the lowest SA and the lowest SF f-d transitions can be evaluated according to the method proposed by Dorenbos and the spectroscopic data of Ce³⁺ ions in the host lattice [17]. In our previous work[13], the decreasing of the lowest 5d state for Ce³⁺ in the host lattice NaGd(PO₃)₄ (D value) is about 15.67 × 10³ cm^-1 in comparison with free gaseous Ce³⁺. Because the influence of the crystal field and covalency of the host lattice on the red shift of 5d levels are approximately equal for all rare-earth ions, we consider this D value is adopted by Dy³⁺ in NaGd(PO₃)₄. The energies of the lowest SA and the lowest SF f-d transitions for free Dy³⁺ ions are reported to be 74.44 × 10³ cm^-1 and 68.74 × 10³ cm^-1, respectively[20]. Then we predicate that the lowest SA transitions for Dy³⁺ ions in NaGd(PO₃)₄ is 58.77 × 10³ cm^-1 (170 nm, strong), which are coincidence with the positions of band B in Fig. 4(a), and the lowest SF f-d is 53.07 × 10³ cm^-1 (188 nm, weak), just because this band has overlap with the peak of ⁸S_7/2→⁶F_J transition (183 nm) within Gd³⁺ ions, band A has a little shift toward high energy region.

3. The energy of the Dy³⁺⃖O^2- CTB can be roughly estimated by the Jørgensen empirical formula[18]:

E _CT=[χ_opt(X)-χ_opt(M)] × 30 × 10³ cm^-1

Here, E _CT gives the energy of CTB in unit cm^-1, while 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(Dy³⁺)=1.21[19], the CTB energy of Dy³⁺ in oxides can be approximately estimated to be 59 700 cm^-1 (168 nm), which is overlapped with the lowest SA f-d transitions of Dy³⁺ ions (band B in Fig. 4(a)).

In addition, a sharp peak in band C (peaking at about 162 nm) can be observed in Fig. 4(a), which is probably caused by the high energy f-f transition of Gd³⁺ or Dy³⁺ ions.

According to above considerations, we thought that the PO₃ ^- ligand’s absorption, the Dy³⁺⃖O^2- CTB, the f-d transitions of Dy³⁺, and part of f-f transitions within Gd³⁺ or Dy³⁺ occur and overlap in VUV excitation spectrum, resulting in strong absorption of the sample NGP:Dy³⁺ at 172 nm wavelength.

In Fig. 4(b), the emission spectrum of the sample NGP:Dy³⁺ under 172 nm excitation is exhibited (curve 1). The positions of the intensive blue and yellowish emission under VUV excitation are in agreement with that under UV excitation, but the relative intensities show some differences as the emission under UV excitation was not corrected by the instrumental response of blue-sensitive PMT.

For the purpose of obtaining the relative emission intensity of the sample NGP:Dy³⁺ under 172 nm excitation, the intensity in whole visible (380–730 nm) range was integrated, and compared with that of commercial phosphors YGB:Eu³⁺ and ZSM measured in the same conditions (curves 2 and 3 in Fig. 4(b)). The results were determined through the ratios (K) as following:

$K\left(\frac{\mathrm{NGP}:{\mathrm{Dy}}^{3+}}{\mathrm{Comm}.}\right)=\frac{\int I_{\mathrm{NGP}:{\mathrm{Dy}}^{3+}}\left(\lambda \right)d{\lambda }_{\mathrm{NGP}:{\mathrm{Dy}}^{3+}}}{\int I_{\mathrm{Comm}.}\left(\lambda \right)d{\lambda }_{\mathrm{Comm}.}}$ .

It was obtained that:

$K\left(\frac{\mathrm{NGP}:{\mathrm{Dy}}^{3+}}{\mathrm{YGB}:{\mathrm{Eu}}^{3+}}\right)=1.99$ , $K\left(\frac{\mathrm{NGP}:{\mathrm{Dy}}^{3+}}{\mathrm{ZSM}}\right)=0.95$ .

The integrated intensity of NGP:Dy³⁺ is almost twice than that of commercial red phosphor YGB:Eu³⁺ and also almost equal to that of commercial green phosphor ZSM in the same conditions. These results show that the title sample has intensive emission when it is excited with the wavelength 172 nm VUV light, which is generated by the discharge of excimer Xe₂.

The chromaticity coordinate (x, y) of the sample NGP:Dy³⁺ was calculated in term of the emission under 172 nm excitation and is showed in Fig. 6. The CIE color coordinate of NGP: Dy³⁺ is (0.33, 0.38), which is located in warm-white light region.

 

Fig. 6. The CIE color coordinates of NGP:Dy³⁺ together with that of commercial phosphor YGB:Eu³⁺ and ZSM.

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4. Conclusion

The phosphor NaGd_1-xDy_x(PO₃)₄ for x=0.05 shows broad and strong absorption in the VUV range and high intensive emission under 172 nm excitation, and its CIE color coordinate enter the warm-white light region. Additionally, this efficient white emitting phosphor NaGd_1-xDy_x(PO₃)₄ is activated by a single Dy³⁺ ion and with a lower preparation temperature than that of current commercial tricolor phosphors, which tend to decrease the consumption of rare earth resource and energy. Therefore, the phosphor NGP:Dy³⁺ may be considered as a suitable candidate for Hg-free lamps application.