Tb-doped YPO4phosphors: Polyacrylamide gel synthesis and optical properties

Article abstract of DOI:10.1016/j.jallcom.2016.08.065

Single phase of Tb-doped YPO₄phosphors with xenotime structure were successfully synthesized via a polyacrylamide gel at different annealing temperatures (650–1050?°C). The crystal structure, thermal behavior and optical properties of the as-synthesized samples were investigated by XRD, TG-DTA, FT-IR, PL, and XPS. The effects of heat treatment on the structure and luminescence properties of samples were discussed. The PL spectra results reveal that the annealing temperature plays a key role in the luminescence properties of products. The samples obtained under 650–850?°C exhibit a strong green emission at 546?nm, attributing to the⁵D₄-⁷F₅transition of Tb³⁺, while the samples produced above 850?°C emit dominating blue lines at 381 and 415?nm, corresponding to the⁵D₃-⁷F₆and⁵D₃-⁷F₅transitions, respectively. The XPS results imply that a part of doping Tb³⁺ions may be oxidized to Tb⁴⁺on the high-temperature thermal treatment process, which could be the reason for the bluish color of Tb-doped YPO₄phosphors.

Full text of DOI:10.1016/j.jallcom.2016.08.065

Accepted Manuscript
Tb-doped YPO phosphors: Polyacrylamide gel synthesis and optical properties
4
Jinyu Yang, Lin Luo, Zhuo Chen, Xueyu Chen, Xiaoyu Tang
PII:
S0925-8388(16)32443-4
10.1016/j.jallcom.2016.08.065
JALCOM 38566
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 5 May 2016
Revised Date: 6 August 2016
Accepted Date: 8 August 2016
Please cite this article as: J. Yang, L. Luo, Z. Chen, X. Chen, X. Tang, Tb-doped YPO phosphors:
4
Polyacrylamide gel synthesis and optical properties, Journal of Alloys and Compounds (2016), doi:
10.1016/j.jallcom.2016.08.065.
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ACCEPTED MANUSCRIPT
Tb-doped YPO4 phosphors: Polyacrylamide gel synthesis and optical properties
Jinyu Yang a, b, c*, Lin Luo a, b, Zhuo Chen a, b, Xueyu Chen a, b and Xiaoyu Tang d
a. School of Chemistry and Materials Science, Guizhou Normal University, Guiyang 550001, China
b. Key Lab for Functional Materials Chemistry of Guizhou Province, Guiyang 550001, China
c. Physics Department, University of Rhode Island, Kingston, RI 02881, USA
d. Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada
Abstract: Single phase of Tb-doped YPO4 phosphors with xenotime structure were successfully synthesized
o
via a polyacrylamide gel at different annealing temperatures (650 - 1050 C). The crystal structure, thermal
behavior and optical properties of the as-synthesized samples were investigated by XRD, TG-DTA, FT-IR,
PL, and XPS. The effects of heat treatment on the structure and luminescence properties of samples were
discussed. The PL spectra results reveal that the annealing temperature plays a key role in the luminescence
properties of products. The samples obtained under 650 - 850 °C exhibit a strong green emission at 546 nm,
5
o
attributing to the D4-7F5 transition of Tb3+, while the samples produced above 850 C emit dominating blue
5
5
lines at 381 and 415 nm, corresponding to the D3-7F6 and D3-7F5 transitions, respectively. The XPS results
imply that a part of doping Tb3+ ions may be oxidized to Tb4+ on the high-temperature thermal treatment
process, which could be the reason for the bluish color of Tb-doped YPO4 phosphors.
Keywords: Polyacrylamide gel, Tb-doped, YPO4, Optical properties
1. Introduction
Rare-earth (RE) based phosphors have attracted considerable attention during the past decades owing to
their numerous advantages, such as low toxicity, excellent photostability, outstanding thermal and chemical
stability, high photoluminescence quantum yield, and sharp emission lines. Yttrium orthophosphate (YPO4)
has been widely applied in phosphors, laser hosts, biolabeling, and sensor [1-8]. Many researchers have
devoted efforts to developing the synthesis of YPO4 phosphors with tunable particle size, sharp and doping
different rare-earth ions [6-14]. Several methods have been reported for the synthesis of YPO4 crystals, such
as hydrothermal/solvothermal [12-15], Sol-gel [11], high-temperature solid-state [2, 3], precipitation [8-10,
16, 17], combustion [18], microwave [7], evaporation [19], and so on. Polyacrylamide gel method, a new
* Corresponding author: Tel.: +86 851 86700050; fax: +86 851 86700050. E-mail address: jinyuyang@gmail.com

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development of the Sol–gel routes, has received more attention in the preparation of phosphor materials due
to the significant benefits, including the high purity of products, homogeneity, and well-controlled properties
[20, 21], whereas few studies have been published on the synthesis of rare-earth ions doped YPO4 phosphors
via polyacrylamide gel method.
Terbium is one of the preferred doping rare-earth ions and has drawn great attention because of their wide
applications in phosphors. Tb3+ emits strong sharp green light around 546 nm, which is close to the theoretical
optimum wavelength for the green component of three primary color centers. Many studies have been
reported that the Tb-doped YPO4 phosphors exhibit green emission since the doped Tb ions exist in trivalent
state (Tb3+) [1, 2, 7, 15, 16]. It is well known that Tb4+ ions also exist in nature, while Tb4+ can act as a
luminescence quenching center for Tb3+ luminescence in hosts [22-24]. However, the simultaneous presence
of Tb3+ ions and Tb4+ ions in YPO4 host have not been reported.
In this study, a polyacrylamide gel route was used to prepare phase pure Tb3+ ions and Tb4+ ions co-
doped YPO4 crystals. The effect of annealing on the structure and optical properties of the as-prepared
products were discussed.
2. Materials and methods
Y2O3 (99.99%) and Tb4O7 (99.99%) were obtained from the Aladdin Reagent Corporation. Ammonium
dihydrogen phosphate (NH4H2PO4), Acrylamide (AM), N,N-methylenebisacrylamide (MBAM), Ammonium
persulfate (APS) and Nitric acid (HNO3) were purchased from the TianJin Kemiou Chemical Reagent Co.,
Ltd. All the reagents in this study were analytical grade and used as received without further purification.
Deionized water was used throughout the experiments. A certain amount of Y2O3 and Tb4O7 were dissolved
in HNO3 and adjusted to 1 mol.L-1 Y(NO3)3 and Tb(NO3)3 solution, respectively.
Samples were synthesized according to the previously reported method with minor modifications [20]. In
a typical procedure, 0.345 g of NH4H2PO4 was dissolved in 30 ml deionized water. Then 2.985 ml of Y(NO3)3
solution (1 mol.L-1) and 0.015 ml of Tb(NO3)3 solution (1 mol.L-1) were introduced into the NH4H2PO4
solution, respectively, and stirred for 10 min to form a clear solution. After that, 0.30 g of AM and 0.05 g of
MBAM were added to the mixed solution while stirring vigorously with a magnetic stirrer. After 20 min of
stirring, the solution was kept in a water bath at 60 °C. Then 0.30 ml of freshly prepared 10% (w/v) APS
solution was added. After vigorous stirring for 1 h, an ivory-white polymeric gel was formed, and the gel was
dried at 80 °C for 24 h in air. Finally, the dried gel was divided into five parts and each part was separately

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calcined at 650, 750, 850, 950 and 1050 °C for 5 h in air. The obtained samples were respectively denoted as
YPT05-650, YPT05-750, YPT05-850, YPT05-950, and YPT05-1050. For comparative purposes, two high-
doping-concentration samples were prepared by varying the volume of 1 mol.L-1 Y(NO3)3 solution and 1
mol.L-1 Tb(NO3)3 solution as 2.85 ml and 0.15 ml, and calcined at 650 °C and 1050 °C, respectively, via a
similar method. The prepared samples were denoted as YPT5-650 and YPT5-1050.
The crystalline structure of the prepared samples was obtained on a Bruker D8 Advance x-ray
diffractometer using Cu Kα radiation, and the scan range was set from 10° to 80° (2θ) with a step of 0.05° s-1.
Fourier transform infrared (FT-IR) spectra were recorded on a Scientific Nicolet iS5 spectrometer by
dispersing the powders in KBr. The thermal decomposition behavior of the xerogel was studied by
simultaneous thermogravimetry and differential thermal analysis (TG-DTA) on a TA instruments TGA 2050
equipment in air with a heating rate of 10 °C·min-1 from room temperature to 1,500 °C. The emission spectra
of the samples were identified by a Hitachi F-4600 fluorescence spectrophotometer at room temperature. X-
ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 photoelectron spectrometer with
a Mono Al Kαsource (1486.6 eV, 10 mA*12kV) using a pressed tablet.
3. Result and discussion
Fig. 1 shows the XRD patterns of the as-prepared samples annealed at different temperatures. As shown
in Fig. 1, all the diffraction lines of the as-synthesized samples can be readily indexed to the reference pattern
(JPCDS 11-0254), indicating that the samples have a xenotime structure of YPO4 with the space group
I4l/amd (141). It was also found that a small quantity of doped Tb ions has a negligible effect on the crystal
structure of samples. No additional diffraction peaks, except for the patterns of YPO4, can be observed in Fig.
1, confirming that the as-prepared samples are pure phase YPO4 crystals. With the increase of calcination
temperature, the intensities of the diffraction peaks increase and the half-width of the diffraction peak
decrease for all YPO4 samples, revealing that the samples gradually grow into large regular crystals with
xenotime structure.
The crystallite size of the samples was calculated from the half-width of the 200 diffraction peak located
at 25.8o (2-theta) by the Debye-Scherrer’s formula [25]. The calculated crystallite size is depicted in the inset
of Fig. 1. It can be observed that the crystallite size of the samples increases from 8.6 nm to 51.2 nm with
increasing calcination temperature from 650 °C to 1050 °C. The lattice parameters and unit-cell volumes of
samples were also obtained (Table 1) from XRD data using the whole pattern fitting (WPF) by Jade 6.0

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software (MDI, USA). The results shown in Table 1 indicate that the unit-cell dimension decreases with
increasing annealing temperature due to the reduction in defect density. Increase of sintering temperature
improves crystallinity and increases average grain size, which results in decreasing defect density. Therefore,
unit-cell dimension decreases [26, 27].
Table 1. Lattice parameters and unit-cell volumes of the samples annealed at different temperatures
α (o) β (o) γ (o) Cell Volume (Å3
)
Sample No.
a (Å
)
b (Å
)
c (
Å
)
YPT05-650 6.8680±0.0049 6.8680±0.0049 6.0410±0.0042
YPT05-750 6.8671±0.0044 6.8671±0.0044 6.0321±0.0039
YPT05-850 6.8660±0.0022 6.8660±0.0022 6.0254±0.0037
YPT05-950 6.8654±0.0016 6.8654±0.0016 6.0244±0.0023
YPT05-1050 6.8654±0.0017 6.8654±0.0017 6.0194±0.0021
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
284.95
284.45
284.05
283.95
283.72
The thermal decomposition of the gel precursor was investigated by TG-DTA to understand the chemical
reaction occurring during the calcination as demonstrated in Fig. 2. Three weight loss regions can be observed
in the TG curve from 50 °C to 740 °C, indicating that the gel precursor degradation and the formation of
YPO4 is a multi-step process. In addition, the great weight loss is accompanied by obvious exothermic DTA
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peaks from 210 to 740 C. The first step of weight loss (15.24 %) arising at 50 - 302 °C is due to the loss of
residual water and hydroxide in the polymeric network, accompanied with an exothermic peak appearing at
271.6 °C on the DTA curve [20]. From 302 to 430 oC, the second weight loss of 25.53 % was found in the TG
curve. At the same time, two exothermic DTA peaks were observed at 374 °C and 405 °C in Fig.2, attributed
to the decomposition of ammonium dihydrogen phosphate and of the branched chain of polyacrylamide gel
[20]. A remarkable exothermic peak located at 556.2 °C and an obvious weight loss step with 44.65 % were
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exhibited within 430 – 740 C on the DTA and TG curves, respectively. This may be corresponding to the
burning of polyacrylamide gel and the degradation of the nitrate [28]. Above 740 oC, the TG and DTA curves
almost unchanged, indicating that no obvious chemical reaction and phase transformation occur in the sample.
Fig. 3 describes the FT-IR spectra of YPT05-1050 and YPT05-650. In Fig. 3, all the FT-IR spectra have
similar absorption peaks. The two spectra showed a weak absorption at 1635 cm-1 due to the bending
vibrations of absorbed water, and a broad band at around 3415 cm-1 attributed to the stretching O-H vibration
of water molecules. The peaks at 1300 - 900 cm-1 indicate the characteristics of phosphate groups. The bands

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at 1061 cm-1 and 1023 cm-1 originate from the P-O asymmetrical stretching vibration. Additionally, the two
sharp absorption peaks at 646 cm-1 and 528 cm-1 are attributed to the bending vibrations of P-O [29]. It should
be noted that the absorbance intensities of the peaks at 1635 cm−1 and around 3415 cm-1 ascribed to OH-
groups are decreased by increasing the calcination temperature while the intensities of the other bands almost
unchanged.
Fig. 4 shows the emission spectra of the samples annealed at different temperatures. As presented in Fig.
4, under the excitation of 222 nm light, the emission spectra of the samples are shown with similar sharp
bands in the 350-650 nm region. In addition, a weak broad peak for all the samples at about 250 to 530 nm
can be observed, which corresponds to intermediate band emission within the band gap of the YPO4 host [30].
The four emission peaks located at 382, 415, 440, and 472 nm arise from the 5D3-7FJ (J = 6, 5, 4, 3) transitions
of Tb3+ ions, and the other emission lines at 491, 546, 588, and 622 nm are assigned to 5D4-7FJ (J = 6, 5, 4, 3)
5
transitions of Tb3+ ions, respectively. The D3-7F6 band exhibits stronger emission intensity than that of other
5D3-7FJ (J = 5, 4, 3) peaks, and is the same as the 5D4-7F5 line compared with 5D4-7FJ (J = 6, 4, 3) transitions.
It is remarkable that the intensities of the emission bands increase with increasing annealing temperature,
indicating that the samples prepared at higher annealing temperature display improved luminescence
5
5
properties. Interestingly, while the emission intensities of D3-7FJ and D4-7FJ bands all increase with rising
annealing temperature, the growth speed is different for 5D3-7FJ and 5D4-7FJ bands. As it can be observed, the
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5
samples obtained at the relatively low temperature (no more than 850 C) show the higher intensities of D4-
5
7FJ emission bands than those of D3-7FJ bands. However, the emission spectra of YPT05-950 and YPT05-
1050 samples exhibit the opposite behavior. The result implies that the luminescence color of samples may
change with annealing temperature. The emission color is dependent on the intensity ratio of 5D3–7FJ to 5D4–
5
5
7FJ emissions [31]. The dependence of the ratio of the relative intensity of D3-7F6 (382 nm) to D4-7F5 (546
nm) on the annealing temperatures is shown as an inset of Fig. 4. The inset of Fig. 4 shows that the intensity
ratio of 382 nm to 546 nm increased from 0.38 to 2.74 with increasing annealing temperature, implying that
the emission colors may turn from green to bluish.
The excitation spectra of the samples obtained by monitoring the emission bands at 382 nm and 546 nm
are described in Fig. 5(A) and Fig. 5(B), respectively. As presented in Fig. 5(A), the excitation spectra of the
samples consist of a strong band centered at 222 nm, which is attributed to charge-transfer band (CTB)
absorption between O2- and Tb3+. Meanwhile, some sharp bands in the 250-350 nm region can be observed,

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which can be assigned to the 4f-5d transition of Tb3+ [32]. The excitation spectra obtained by monitoring the
5D4-7F5 transition (546 nm) are composed of a strong CTB band (222 nm), and a series of weak sharp bands in
the region 250-520 nm (Fig. 5(B)). The excitation peaks in the range 250-300 nm and 300-520 nm correspond
to the 4f-5d and 4f-4f transitions of Tb3+ ions, respectively. It should be noted that the intensity of the CTB
for all the samples is much stronger than those of the 4f-5d and 4f-4f transitions, indicating that the CTB
plays a key role in the luminescence intensity of samples. It is clear that the intensity of the CTB increase with
rising annealing temperature, revealing that high-temperature treatment improves the luminescence
properties, in accordance with the emission result mentioned above.
The CIE coordinates were calculated from the emission spectra, as shown in Fig. 6. The CIE coordinate
of the YPT05-650 sample is (0.247, 0.343), indicating that it emits bluish-green light due to the broad
luminescence peak of the YPO4 matrix at blue-green areas, and the weak emission bands of the Tb3+ ions,
5
5
even though the emission intensity of the D4-7F5 peak is higher than that of the D3-7F6 band (Fig. 4). With
the increase of annealing temperature to 850 oC, the CIE coordinate is changed to (0.271, 0.428), meaning that
the colors were shifted from bluish-green to green due to the growing Tb3+ emission with higher temperature.
Interestingly, by raising the temperature from 850 oC to 1050 oC, the colors are transferred back from green to
blue-green, and the CIE coordinate is varied from (0.271, 0.428) to (0.254, 0.374), corresponding to the
intensity ratio of 382 nm to 546 nm increased from 0.63 to 2.74.
Commonly in Tb-doped phosphors, the Tb3+ (4f8) electrons can be excited to higher 4f75d1 levels, and
5
5
7
then rapidly relaxes to D3 and D4 excited states by non-radiative processes. It further relaxes to FJ ground
states via radiative decay and the blue, and green emissions appear due to the 5D3-7FJ, and 5D4-7FJ transitions,
respectively. However, in most cases the emission spectra of Tb-doped luminescent materials are dominated
5
by the D4-7F5 transition at about 546 nm, meaning that the phosphors emit green light. This is because the
energy difference between 5D3 and 5D4 states is approximately equal to that of between 7F0 and 7F6 levels, and
the excited electrons from the 5D3 level populate the 5D4 state through the 5D3-5D4 and 7F0-7F6 cross-relaxation
5
processes [33]. Usually, the D3 level of Tb3+ ions is highly sensitive to doping concentration. These cross-
5
5
relaxation processes increase the population of the D4 level at the expense of D3, so that the intensities of
5
emission bands originating from the D3 level decrease, especially for the phosphors with a heavy doping
5
concentration [22]. On the other hand, if the concentration quenching of D3 level of Tb3+ ions is slow, the
Tb3+ ions can emit blue light [34].

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Interestingly, the samples obtained at various annealing temperatures emit different colors (Fig.4, Fig. 6)
even though the samples have the same concentration of Tb ions doped in the YPO4 matrix. In nature,
Terbium exists in two valence states, +3 and +4. Tb3+ and Tb4+ can be converted to each other under certain
conditions. The Tb4+ ion in any host does not yield emission but acts as a quenching center [22]. If some Tb3+
ions in the YPO4 lattice are oxidized to Tb4+, the content of Tb3+ ions might decrease, resulting in improved
blue emission at 382 nm. It is in accordance with Fig. 4. Therefore, one might suppose that a part of the
doping Tb3+ ions were transformed into Tb4+ ions at high-temperature annealing in atmospheres.
XPS analysis was used to characterize the valence states of the Tb component as is shown in Fig. 7. The
high-resolution XPS spectrum of Tb 3d for YPT05-650 shows two very weak peaks due to the low Tb doping
concentration, from which it is hard to identify the valence states of Tb in YPT05-650 (Fig. 7(a)). To enhance
the peaks of Tb 3d, the YPT5-650 and YPT5-1050 samples were synthesized with the same procedure except
for the Tb doping content. The XPS spectra of YPT5-1050 and YPT5-650 are presented in Fig. 7(b) and Fig.
7(c), respectively. As described in the Fig. 7(c), the two strong XPS peaks located at 1277.50 eV and 1242.96
eV are due to Tb 3d3/2 and Tb 3d5/2 levels of Tb3+ ions, respectively. No other XPS signals can be observed,
indicating that the Tb ions in YPT5-650 are present in the trivalent state [35]. It is obvious that the two similar
peaks were observed in Fig. 7(b), although the peak intensities were weaker than those of YPT5-650,
revealing that the YPT5-1050 sample also contains Tb3+ ions. In addition, two new peaks at 1251.3 eV and
1288.6 eV (arrows in Fig. 7(b)) appeared owing to the Tb 3d5/2 and Tb 3d3/2 of Tb4+ ions, respectively. A
similar phenomenon was reported in TbOx oxides [36]. It is demonstrated that there are some Tb4+ ions in the
YPT5-1050 sample. The XPS results reveal the simultaneous presence of Tb3+ and Tb4+ ions in YPT5-1050.
Similarly, for the YPT05-1050 sample, it can be inferred that a part of the doping Tb3+ ions were also
converted to Tb4+ ions on heating at 1050 oC due to the same preparation procedure.
Fig. 8 presents the emission spectra of the YPT5-650 and YPT5-1050 samples. Under the 222 nm light
excitation, all samples can emit characteristic green light of Tb3+ ions. The 5D4-7F5 band located at 546 nm is
split into two sharp peaks located at 545 and 548 nm, respectively, attributing to the high Tb3+ ions doped in
YPO4 matrix (compare with Fig.4) [33]. The emission color cannot be changed obviously by increasing the
annealing temperature for the high doping samples. Although the XPS results reveal a part of doping Tb3+
ions have been oxidized to Tb4+ in the high-temperature thermal treatment process, the content of Tb3+ ions is

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still far higher than that of Tb4+ ions in the YTP5-1050 sample (Fig. 7), which could be the reason for the
greenish color of YTP5-1050 sample.
The fluorescence decay curves for the 5D3-7F6 (382 nm) transition, and the 5D4-7F5 (546 nm) transition of
5
5
Tb3+ ions in samples have been plotted in Fig. 9. The decay curves of D3-7F6 and D4-7F5 for different
annealing temperature in YPO4:Tb3+ show a single exponential decay. The decay lifetimes of the 5D3 (τ382 nm
)
and 5D4 (τ546 nm) were calculated and listed in Fig. 9. It should be noted that the lifetime of 5D3 increases from
o
0.484 to 1.181 ms as the annealing temperature rises from 650 to 1050 C. In general, the increase of the
average lifetime of the 5D3 transition should result from the decrease of the nonradiative relaxation (5D3-5D4)
rate at low Tb3+ concentrations [33]. The results imply that the sample obtained at 1050 oC may have less Tb3+
o
5
ions in the matrix than that prepared at 650 C. Meanwhile, the lifetime of D4 also increases from 3.437 to
4.117 ms, attributing to the decrease of the cross-relaxation rate at high annealing temperature samples with
less Tb3+ doping. The present lifetime result agrees with the report of Liu et al [33].
4. Conclusions
Tb-doped YPO4 phosphors with xenotime structure were successfully synthesized via a polyacrylamide
gel route. The XRD results revealed that pure phase Tb-doped YPO4 crystals can be obtained at 650 - 1050
oC. The PL spectra results demonstrated that the annealing temperature plays a key role in the luminescence
properties of samples. The samples obtained under 850 °C exhibit a strong green emission at 546 nm,
5
o
attributing to the D4-7F5 transition of Tb3+. While the samples produced above 850 C emit dominating blue
5
5
lines at 382 and 415 nm, corresponding to the D3-7F6 and D3-7F5 transitions, respectively. The XPS results
imply that a part of the doping Tb3+ ions may be oxidized to Tb4+ on the high-temperature thermal treatment
process, which could be the reason for the bluish color of Tb-doped YPO4 phosphors.
Acknowledgements
The authors acknowledge the financial supports of the National Natural Science Foundation of China
(No. 21361007), the International Cooperation Foundation of Guizhou Province (No. G-[2013]7015), the
Natural Science Foundation of Guizhou Province (No. J-[2012]2269). The authors thank Dr. Anthony W.
Stapleton and Dr. Da Wei for their linguistic assistance.
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Figure captions
Fig. 1. XRD patterns of the samples annealed at different temperatures (a: YPT05-650; b: YPT05-750; c:
YPT05-850; d: YPT05-950; e: YPT05-1050). Inset: Crystallite size of the powders, as a function of
calcination temperatures.
Fig. 2. TG-DTA curves of the as-synthesized precursor.
Fig. 3. FT-IR spectra of the as-prepared samples (a: YPT05-1050; b: YPT05-650).
Fig. 4. Emission spectra (λexc = 222 nm) of the as-synthesized samples (a: YPT05-650; b: YPT05-750; c:
YPT05-850; d: YPT05-950; e: YPT05-1050). Inset: The dependence of the intensity ratio of 5D3-7F6 (382 nm)
to 5D4-7F5 (546 nm) on temperatures.
Fig. 5. Excitation spectra ((A) λem = 382 nm and (B) λem = 546 nm) of the as-synthesized samples (a: YPT05-
650; b: YPT05-750; c: YPT05-850; d: YPT05-950; e: YPT05-1050). The inset in (A) shows a magnification
of the fragment of excitation spectra in the range of 230-360 nm. Two representative zoomed ranges 200-250
nm and 230-520 nm of the excitation spectra (λem = 546 nm) shown as insets: I-1 and I-2, respectively.
Fig. 6. The CIE coordinates of the as-synthesized samples (a: YPT05-650; b: YPT05-750; c: YPT05-850; d:
YPT05-950; e: YPT05-1050).
Fig. 7. XPS spectra of the high resolution of the Tb 3d in (a) YPT05-650, (b) YPT5-1050, and (c) YPT5-650.
Fig. 8. Emission spectra (λexc = 222 nm) of the as-synthesized samples (a: YPT5-650; b: YPT5-1050).
5
5
Fig. 9. Fluorescence decay curves for D3-7F6 (382 nm) transition, and D4-7F5 (546 nm) transition of Tb3+
ions in the as-synthesized samples (a: YPT05-650; b: YPT05-750; c: YPT05-850; d: YPT05-950; e: YPT05-
1050).

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Highlights
1. YPO4:Tb samples with the same dopant content exhibit various optical properties.
2. The annealing temperature plays a key role in the emission properties of products.
3. A part of doping Tb3+ ions may be oxidized to Tb4+ on the thermal treatment process.