Tb3+ phosphors for w-LEDs

Accepted Manuscript Tunable emission, thermal stability and energy-transfer properties of SrAl2Si2O8: 3+ 3+ Ce /Tb phosphors for w-LEDs Pingchuan Ma, Bo Yuan, Ye Sheng, Keyan Zheng, Yuexin Wang, Chengyi Xu, Haifeng Zou, Yanhua Song PII:

S0925-8388(17)31513-X

DOI:

10.1016/j.jallcom.2017.04.296

Reference:

JALCOM 41700

To appear in:

Journal of Alloys and Compounds

Received Date: 12 February 2017 Revised Date:

25 April 2017

Accepted Date: 27 April 2017

Please cite this article as: P. Ma, B. Yuan, Y. Sheng, K. Zheng, Y. Wang, C. Xu, H. Zou, Y. Song, 3+ 3+ Tunable emission, thermal stability and energy-transfer properties of SrAl2Si2O8: Ce /Tb phosphors for w-LEDs, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.296. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Tunable Emission, Thermal Stability and Energy-transfer Properties of SrAl2Si2O8: Ce3+/Tb3+ Phosphors for w-LEDs Pingchuan Maa, Bo Yuanb, Ye Shenga, Keyan Zhenga, Yuexin Wanga, Chengyi Xua,

College of Chemistry, Jilin University, Changchun 130012, PR China

b

Institute of Petrochemical Technology, Jilin Institute of Chemical Technology, Jilin

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a

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Haifeng Zoua,* , Yanhua Songa,*

Jilin 132022, PR China

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College of Chemistry, Jilin University, Changchun 130012, PR China E-mail address: [email protected] (H. Zou); [email protected] (Y. Song).

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Abstract

Ce3+/Tb3+ ions singly and co-doped SrAl2Si2O8 phosphors were synthesized by

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solid-state reaction method. The crystal structure, photoluminescence excitation and emission spectra, decay lifetime, energy transfer, quantum yield and thermal stability

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have been investigated in detail. The SrAl2Si2O8:0.04Ce3+ and SrAl2Si2O8:0.04Tb3+ phosphors can emit blue and green light, respectively. Under 365nm excitation, the emission color of SrAl2Si2O8:Ce3+, Tb3+ can be adjusted from blue to green by changing the proportion of Ce3+ and Tb3+ ions based on the energy transfer. With constantly increasing of Tb3+ ions concentration, the energy transfer efficiency from Ce3+ to Tb3+ in SrAl2Si2O8 host increased gradually and reached as high as 82.91%, the quantum yield was about 67.37%. The energy transfer mechanism is proved to be dipole-quadrupole interaction. Over 100 oC, 90% initial emission intensity can be still

ACCEPTED MANUSCRIPT remained. The above results indicate that SrAl2Si2O8:Ce3+, Tb3+ is a potential multicolor emission phosphor for the application in w-LEDs.

Key words: SrAl2Si2O8:Ce3+, Tb3+; Blue–Green Tunable Emission; Energy

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Transfer; Thermal Stability.

1. Introduction

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Nowadays, white lighting-emitting diodes (w-LEDs) have become an important light source due to their unique characteristics such as high luminescence efficiency,

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environment friendly, high brightness, long lifetime and low cost [1-4]. Rare earth ions in phosphors of w-LEDs play an important role since they emit variety of colors based on f-d and f-f transitions

[5-7]

. Tb3+ ions, as a well-known activators of green

light, the 5D4–7F5 transition located at 541nm leads to the predominantly green

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emission. However, the absorption of Tb3+ peaks in the near ultraviolet (NUV) region is rather weak and the width is very narrow due to the 4f-4f forbidden transition. Therefore, it cannot match well with LED light

[8]

. In order to enhance Tb3+

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absorption in the NUV region, one of the most feasible ways is to introduce sensitizer,

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which induces intense emission in the NUV region. As a good activator, the Ce3+ can be used as a highly efficient emission center

and emit blue, green and even red bands with wavelength of 400–700 nm by changing the substitution in the host lattices, such as blue for Ca2Al3O6F:Ce3+ [8], green for Ca3Sc2Si3O12:Ce3 + [9], yellow for YAG:Ce3+

[10]

, and red for α-(Y, Gd)FS:Ce3+ [11].

Meanwhile, Ce3+ ion can also be used as an efficient sensitizer that strongly absorbs the excitation energy and then transfers to Eu2+, Tb3+, Dy3+, and Mn2+ ions, such as

ACCEPTED MANUSCRIPT CaSi2O2N2:Ce3+, Eu2+ [14]

[12]

, Ca2Al3O6F:Ce3+, Tb3+

, Ca9Y(PO4)7:Ce3+, Mn2+

[15]

[13]

, Ca20Al26Mg3Si3O68:Ce3+, Dy3+

, Y2SiO5:Ce3+, Tb3+ [16], Ca8MgLu(PO4)7:Ce3+, Tb3+,

Mn2+ [17]. Therefore, it is very feasible to enhance the Tb3+ absorption in the NUV

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region by using Ce3+. Aluminosilicate compounds are good candidates as host structures due to several advantages, such as excellent physical and chemical properties, high efficiency and [14, 18, 19]

. The host SrAl2Si2O8 was first reported by A.F. Reid and

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thermal stability

co-workers[20]. In 2007s,. Zhiyu Wang et al. reported the luminescence of Eu2+-Dy3+ [21]

. Nevertheless, little attention in literature has

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co-doping into the SrAl2Si2O8 host

been drawn towards SrAl2Si2O8: Ce3+, Tb3+ in the applications of NUV exciting for solid state lighting. Beyond that, the luminescent properties, the energy transfer and thermal stability of Ce3+ to Tb3+ in SrAl2Si2O8 have not been yet investigated up to

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now.

In this paper, the crystal structure, luminescence properties, quantum yield, decay lifetime, energy transfer, color chromaticity and thermal stability characteristics

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of SrAl2Si2O8:Ce3+, Tb3+ phosphors are reported. SrAl2Si2O8: Ce3+, Tb3+ phosphors can be effectively excited by NUV light and emit visible light from blue to green by

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changing the concentration ratio of Ce3+ and Tb3+. SrAl2Si2O8: Ce3+, Tb3+ is a potential blue to green emitting phosphor in white light-emitting phosphor for display and lighting.

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2. Experimental details 2.1. Synthesis of SrAl2Si2O8:Ce3+/Tb3+ phosphors The phosphors SrAl2Si2O8:Ce3+/Tb3+ (SASO:Ce3+/Tb3+) were synthesized by the

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solid-state reaction technique. The raw materials including SrCO3 (A.R.), α-Al2O3 (99.99%), SiO2 (A.R.), Li2CO3 (A.R.) were obtained from the Beijing Chemical Reagent Research Institute of China, and CeO2 (99.99%), Tb4O7 (99.99%) were

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supplied by Shanghai Yuelong Non-Ferrous Metals Limited of China. Stoichiometric amounts of above reagent were mixed together and thoroughly mixed in an agate

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mortar by grinding for 30min. The Li2CO3 was added as the charge compensator for Ce3+ and Tb3+. The doped concentrations of rare earth ions and Li+ ions are same. All samples doped with Ce3+/Tb3+ ions in our system were co-doped with Li+ ions but not shown in the chemical formula for briefness and convenience. Then the mixture

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samples were transferred to alundum crucible and calcined successively at 1400 oC for 3h in reductive atmosphere (N2:H2=90:10) in a tube furnace. After that, the samples

powder.

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were cooled down to room temperature in the tube furnace, grinding, obtain the

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2.2. Characterization

The powder samples were characterized by X-ray diffraction (XRD) with a

Rigaku D/max-II B X-ray diffractometer with monochromatic Cu K α radiation. The photoluminescence (PL) excitation and emission spectra were measured with a Hitachi F-7000 spectrophotometer equipped with a 150W xenon lamp as the excitation source. Luminescence lifetime was measured by a Lecroy Wave Runner 6100 Digital Oscilloscope using a tunable laser as the excitation. The decay curves

ACCEPTED MANUSCRIPT were measured with excitation wavelength at 365 nm. (Shutter: closed, detector HV: 950 V, measurement range: 200 ns, peak preset: 10000 counts). Quantum yields were measured by an integrating sphere in the FLS920 spectrophotometer. The thermal stability of luminescence was measured using a FLS920 spectrophotometer connected

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with heating equipment (TAP-02). All the measurements were performed at room temperature.

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3. Results and discussion

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3.1 Phase identification

In order to determine the real structure of the synthesized samples, ICSD-106117 was used as the standard data to refine SASO . The structural refinement of XRD was performed using GSAS. The observed, measured, difference and positions of SASO:

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0.04Ce3+, 0.04Tb3+ phosphor was shown in Figure 1(a). The calculated residual factors is
 = 6.74% and
 = 9.20%, Table 1 lists the refinement data of SASO:0.04Ce3+, 0.04Tb3+. According to our Rietveld refinement studies, the doping

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ions (Ce3+/Tb3+) preferably occupy the Sr2+ lattice site since the ion radii of Ce3+ (0.114 nm) and Tb3+ (0.104 nm) are similar to that of Sr2+ (0.126 nm), and they are

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much larger than that of Al3+ (0.039nm), Si4+ (0.026 nm). Figure 1(b) shows the crystal structure of SASO host, its lattice parameters are a=8.3933(1) Å, b=12.8934(1) Å, c=7.1223(2) Å, α=90°, β=115.72°, γ=90°, V=694.4(7) Å3. There is only one Sr2+ crystallographic sites in the monoclinic SASO unit cell, which is bonded to eight oxygen atoms forming an irregular hexahedron. The Al3+ and Si4+ connect four O atoms to form three-dimensional (Al/Si)O4 frameworks, which creates an infinite net structure by corner-sharking.

ACCEPTED MANUSCRIPT The crystal structure of SASO: xCe3+, yTb3+ samples with different Ce3+ and Tb3+ concentration were analyzed by XRD, typical results are shown in Figure 2. The results show that all the diffraction peaks are indexed by SASO (JCPDS card No. 38–1454) with the space group C2/m (12), all these samples are pure phase without any

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impurities. This indicates that doping of Ce3+/Tb3+ in the SASO with such a small concentration does not influence the crystal structure of the host. According to Bragg’s formula

[22]

, 2dsinθ=nλ, as such, θ and d are inversely relationship. When

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Ce3+ and Tb3+ with small ion radius substituting Sr2+ with lager ion radius, the XRD

3.2 Photoluminescence properties

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peaks shift to right as shown in Figure 2.

The excitation and emission spectra of SASO: 0.04Ce3+ phosphors are shown in Figure 3. It can be clearly observed that the excitation spectrum (monitored at 446nm)

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contains two distinct excitation banks centered at 290 and 341nm, respectively. These could be ascribed to the transitions from the ground 4f state to the different crystal field splitting bands of excited 5d states of Ce3+ ions. Under the excitation of 365 nm,

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the emission spectrum has an asymmetric emission broad band in the range of 380–600 nm with a maximum at 446 nm. The emission band can be decomposed into

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two well-separated Gaussian components with maxima at 439 and 486 nm. They can be attributed to the transitions from the lowest 5d excited state to the 2F7/2 ground state and 2F5/2 state of Ce3+, respectively. The energy difference between two sub-bands peaks is about 2129 cm-1, which is in agreement with the theoretical energy difference between 2F7/2 and 2F5/2 levels (~2000 cm-1)

[23]

. Figure 3(c) shows the relative

photoluminescence intensity of Ce3+ as a function of its doping concentration in SASO:xCe3+ samples. It can be observed that the optimal doping concentration of

ACCEPTED MANUSCRIPT Ce3+ is 0.04. The peak position also occurs red shift from 438 to 450 nm. So the energy difference is about 609 cm−1, which indicates that higher Ce3+ concentration will enhance the energy transfer probability between Ce3+ ions and reduce the high-energy portion. Then the position of peak occurs red shift [24-26].

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Figure 4 shows the excitation and emission spectra of SASO:0.04Tb3+ phosphor. Monitored at 541 nm, the excitation spectrum contains a band centered at 232 nm originating from the 4f-5d transition of Tb3+ and many weak peaks located at 313 (7F6-

[27]

. 377 nm was chosen as the excitation

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377 nm (7F6 → 5D3), respectively

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→ 5H6), 337 (7F6 → 5L8), 353 (7F6 → 5L9), 358 (7F6 → 5G6), 366 (7F6 → 5L10), and

wavelength due to the 232 nm cannot match well with LED light. Upon excitation with 377 nm, the peaks of emission spectra appear at 487, 541, 584 and 619 nm, which are corresponding to the 5D4 → 7Fj (j= 6, 5, 4, 3) transitions of Tb3+ , respectively

[28]

. In addition, the transitions all split into several emissions which are

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resulted from crystal field splitting [29]. Moreover, the emissions 5D3→7Fj (j = 3, 4, 5, 6) of Tb3+ are too weak to be seen, which may be due to the cross relaxation between 5

D3 and 5D4 in higher concentration. In order to study the influence of Tb3+ ions

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concentration on the phosphor properties, a series of SASO:yTb3+ (y=0.01 - 0.06)

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samples were prepared to determine the optimal concentration of Tb3+. Figure 4(c) illustrated the emission spectra of the samples with the excitation at 377 nm. With the doping concentration of Tb3+ ions increased, the luminescent intensity increased firstly, and then decreased due to the concentration quenching, which can be explained that when the Tb3+ concentration exceeds the critical value, non-radiative relaxation and energy transfer between adjacent Tb3+ ions would weaken the fluorescence. So the optimum concentration of Tb3+ in this host is 0.04.

ACCEPTED MANUSCRIPT Figure 5(a) illustrates the spectral overlap between the excitation spectrum of Tb3+ and the emission spectrum of Ce3+. The spectral overlap indicates that the sensitizer and the activator may be essential to have an effective resonance-type [30]

. Figure. 5(b) presents the excitation spectra of SASO:0.04Ce3+,

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energy transfer

0.04Tb3+. When monitored at 541 nm (5D4 → 7F5 transition of Tb3+), a broad band ranging from 300 – 400 nm can be observed in the excitation spectra of

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SASO:0.04Ce3+, 0.04Tb3+ (Figure 5b), and it correspond to electron transitions from

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4f state to 5d level of Ce3+. That is same to the Wang's reports [31], which demonstrates the existence of energy transfer from Ce3+ to Tb3+ in SASO host. The PL spectra and emission intensities of Ce3+ and Tb3+ in the SASO:0.04Ce3+, zTb3+ phosphors with the increase of Tb3+ - doping concentration from 0 to 6 at. % have been presented in Figure 6. Upon 365 nm excitation, the emission spectrum

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presents both Ce3+ emission band around 446 nm and many sharp peaks ranging from 450 to 700 nm corresponding to 5D4 → 7Fj of Tb3+. The Tb3+ transition of 5D4 → 7F5 (541 nm) becomes stronger than that of Ce3+, which makes the sample present a green

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color under excitation by a 365 nm UV lamp. The PL spectra and emission intensity

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of Ce3+ and Tb3+ in the SASO:0.04Ce3+, zTb3+ phosphors with the increase of Tb3+ doping concentration from 0 to 6 at.%. With the increasing Tb3+ concentration, the Ce3+ emission section becomes weaker although the content of Ce3+ was fixed, and the Tb3+ emission area becomes stronger until the z = 0.04, then decreases because of concentration quenching effect. For w-LEDs applications, the quantum yield (η) is an important factors for LED phosphors, which can be calculated by [32]:

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(1)

    

where I represents emission spectrum of sample, I and I are the spectra of the excitation light of the samples and blank solid, respectively. All spectra

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are collected by the integrating sphere. Figure 7 shows the quantum yield emission spectrum of SASO:0.04Ce3+, 0.04Tb3+ under excitation of 365 nm, the results of SASO:0.04Ce3+, zTb3+ phosphors are given in Table 2. The η values of

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SASO:0.04Ce3+, zTb3+ can reach 67.37% at z=0.04.

To study the luminescence dynamics of the phosphor, the decay lifetime of Ce3+

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was measured. The decay curves of the phosphors monitored at 438 nm with excitation at 365 nm are shown in Figure 8(a). One can see that the decay curves of Ce3+ deviate from a single exponential and this deviation is more evident with an increase in the Tb3+ doping concentration, and the decay becomes faster and faster.

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Because of the nonexponential decay of Ce3+ in all samples, we define the average fluorescence lifetime of Ce3+ as follows [33]: &

&

(2)

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 = ' !"(!)%!(' "(!)%!

where "(!) is the fluorescence intensity at time t. On the basis of above equation, the

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average lifetimes of Ce3+ were calculated and are shown in Fig. 8(b). With increasing the concentration of doped Tb3+, the lifetime of Ce3+ decreased from 65.19 to 12.44 ns. The energy - transfer efficiencies from Ce3+ to Tb3+ can be estimated by : ,

) = 1 − , -

.

(3)

where / and 0 are the decay time of Ce3+ with and without the Tb3+ ions, . The energy-transfer efficiencies can be calculated as a function of Tb3+ doping

ACCEPTED MANUSCRIPT concentration (z) and is shown in Figure 8(b). With the doping concentration of Tb3+ increasing, the value of ) is gradually increases to 80.92% at z=0.06. It is well known that the energy transfer between activator ions and sensitizer

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ions strongly depend on the distance. The crystallographic sites occupied by the activator ions in the unit cell. There are two possible aspects that are responsible for the resonant energy transfer mechanism [34]. The first one is exchange interaction and the other one is multipolar interaction. The exchange interaction from Ce3+ prevails

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when
1 > R and multipolar interaction from Ce3+ to Tb3+ dominates when
1 < R.

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In this case, the critical distance for the activators is restricted in the range of 5 – 8 Å. When the value reaches the critical transfer distance (
1 ), the concentration quench will take place. The critical distance
1 between Ce3+ and Tb3+ in the SASO host can be estimated by the following equation [35]:

45

6789 :


;

(4)

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1 ≈ 2 3

where >1 is the total critical concentration of Ce3+ and Tb3+, N is the number available crystallographic sites occupied by the activator ions in the unit cell and V is

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the volume of the unit cell. By taking the experimental and analytical values of >1 , N and V (0.08, 4 and 696.6 Å3 respectively), the critical transfer distance (
1 ) of Ce3+

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and Tb3+ in SASO is calculated as 16.08Å. Apparently, the value is much larger than 8 Å, indicating little possibility of energy transfer via the exchange interaction mechanism. Consequently, the transfer between the Ce3+ ions and Tb3+ ions mainly takes place via electric multipolar interactions. According to Dexter's energy transfer expressions of multipolar interaction and Reisfeld's approximation, the following relationship can be obtained to analyze the potential mechanism [36] [37]

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∝ B C/4

(5)

where D0 and D/ stand for the luminescence intensity of the Tb3+ ions with the absence and the presence of the Ce3+ ions, respectively. C is the total doping

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concentration of the Ce3+ and Tb3+ ions. The values for n= 6, 8 and 10 correspond to dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. The relationship of D0 /D/ ∝ B C/4 is plotted in Figure 9. By

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comparing the fitting factors R2, the relation D0 /D/ ∝ B E/4 is the best-fitting,

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indicating that the dipole-quadrupole mechanism contributes to the energy transfer from Ce3+ to Tb3+.

If the donors and acceptors are distributed uniformly in SASO host, the energy migration process can be negligible when it compared with the energy transfer from acceptors (Ce3+) to acceptors (Tb3+). The normalized intensity of the donor

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fluorescence decay curves following the Inokuti–Hirayama model equation for multipolar interactions as following [33]:

6

4

P P

(6)

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F(!) = GHI J− 4 KL 31 − ; MN O - ! - Q

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nA is the number of activator ions per unit volume; α is the rate constant for energy transfer; S=6, 8, 10 corresponding to the d–d, d–q, and q–q interactions, respectively. From equation 7, it can be found that log{ln[I0(t)/I(t)]} acts as a linear function of log(t), and the slope is 3/S. The accurate value of S can be got by plotting the log{ln[I0(t)/I(t)]} as a function of log(t) for SASO:0.04Ce3+,zTb3+, and shown in Figure 10. The slope values of the fitted lines are approximately 0.399, 0.358, 0.353 for the SASO:0.04Ce3+,zTb3+ (z=0.03, 0.04, 0.05) samples. The calculated results of

ACCEPTED MANUSCRIPT S are close to 8. Thus, it further indicates that the energy transfer from Ce3+ to Tb3+ is the dipole–quadrupole interaction in the SASO host. The energy level scheme of SASO: Ce3+, Tb3+ is presented in Figure 11. Firstly, the Ce3+ ion was excited by the NUV light, electrons are excited to a higher state and

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then non-radiatively relaxes to a lower level of 5d. Partial of the absorption energy has been released to the 2F5/2 and 2F7/2, and blue light emitted; the other part was contributed by the energy-transfer process from Ce3+ to Tb3+, which can enhance the

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green emitting efficiently.

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Figure 12 depicts the CIE chromaticity diagram of SASO:0.04Ce3+, zTb3+ (z = 0.01-0.06) calculated according to the corresponding emission band excited at 365 nm. The color and CIE chromaticity diagram shifted gradually from blue (446 nm) to green (541 nm) through adjusting concentration of the Tb3+ ions. Based on these

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findings, it is clear that SASO: 0.04Ce3+, zTb3+ phosphors can be efficiently excited in the NUV range, which indicates that the tuned luminous color with co-doped Ce3+ provides a promising application of Tb3+ activated SASO phosphors in w-LEDs and

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some other optical fields.

3.3 Thermal stability of SASO:Ce3+, Tb3+

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The thermal stability is an important parameter for the application of phosphors

in LEDs, due to operating temperature of the device is beyond 100 oC

[38, 39]

. Figure

13(a) shows the emission spectra of SASO:0.04Ce3+, 0.04Tb3+ measured at temperature range from 25 oC to 225 oC. The activation energy RS can be calculated by Arrhenius formula[40]: 

V

TM ( U − 1) = − X)W

(7)

ACCEPTED MANUSCRIPT where I0 is the initial luminescence at room temperature and I is the intensity at a given testing temperature, respectively; c is a constant; k is Boltzmann’s constant ( k = 8.617 × 10[ eV · K < ).

The

ln (I' ⁄I − 1)

values

versus

1⁄T

of

SASO:0.04Ce3+, 0.04Tb3+ is plotted in Figure 13(b). According to the above

rate at the same temperature

[41]

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Arrhenius formula, Ee was 0.2352 eV. A lower Ea indicates a rapid non-radiative . The relatively‐high‐Ea of SASO:0.04 Ce3+,

0.04Tb3+ indicate that the sample is less sensitive to the thermal effect. Moreover,

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from the Figure 13(a), only 9.7% quench for SASO:0.04 Ce3+, 0.04Tb3+ at 100 oC, which is lower than 26% quench for commercial Sr2SiO4:Eu2+ [42, 43]. This indicates

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that the green phosphor of SASO:0.04Ce3+, 0.04Tb3+ has good thermal stability for w-LEDs.

4. Conclusions

Ce3+/Tb3+ singly doped and co-doped SrAl2Si2O8 phosphors were synthesized by reaction

method.

The

phosphors

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solid-state

of

SrAl2Si2O8:0.04Ce3+

and

SrAl2Si2O8:0.04Tb3+ can emit blue and green light, respectively. Under 365nm excitation, the emission color of SrAl2Si2O8:Ce3+, Tb3+ can be adjusted from blue to

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green by changing the proportion of Ce3+ and Tb3+ based on the energy transfer. With

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constantly increasing Tb3+ concentration, the energy transfer efficiency from Ce3+ to Tb3+ in SrAl2Si2O8 host increased gradually and reached as high as 80.92%, the quantum yield was about 67.37%. The energy transfer mechanism is illustrated to be dipole-quadrupole interaction. Over 100 oC, 90% initial emission intensity can still be remained and the activation energy is 0.2352eV for SrAl2Si2O8. The above results indicate that SrAl2Si2O8:Ce3+, Tb3+ is a potential multicolor emission phosphor for the application in w-LEDs.

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Acknowledgements This work is financially supported by National Natural Science Foundation of China

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(Grant No. 51272085 and 21671078).

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Luminescence Properties of Ca8Mg3Al2Si7O28:Eu2+ for WLEDs, Advanced Optical

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[22] R. Rüdel, F. Zite-Ferenczy, Interpretation of light diffraction by cross-striated

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muscle as Bragg reflexion of light by the lattice of contractile proteins, Journal of Physiology, 290 (1979) 317-330. [23] C.H. Hsu, B.M. Cheng, C.H. Lu, Photoluminescent Properties and Energy Transfer Mechanism of Color-Tunable CaSi2O2N2:Ce3+,Eu2+ Phosphors, Journal of the American Ceramic Society, 94 (2011) 2878–2883. [24] Q. Wu, X. Wang, Z. Zhao, C. Wang, Y. Li, A. Mao, Y. Wang, Synthesis and luminescence characteristics of nitride Ca1.4Al2.8Si9.2N16:Ce3+,Li+ for light-emitting devices and field emission displays, Journal of Materials Chemistry C, 2 (2014) 7731.

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Synthetic Route to Sr2Si5N8 :Eu2+ -Based Red Phosphors for White Light-Emitting

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Tunable luminescence and energy transfer process between Tb3+ and Eu3+ in GYAG:Bi3+,Tb3+, Eu3+ phosphors, Solid State Sciences, 12 (2010) 719-724.

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[28] M. Xu, L. Wang, D. Jia, F. Le, Luminescence properties and energy transfer investigations of Zn2P2O7:Ce3+,Tb3+ phosphor, Journal of Luminescence, 158 (2015) 125-129.

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earth ions in Y2O3. II. Non

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levels, and crystal

Kramers ions in C2 sites, The Journal of Chemical Physics, 76 (1982) 4775-4788. [30] W. Lv, W. Lü, N. Guo, Y. Jia, Q. Zhao, M. Jiao, B. Shao, H. You, Efficient

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sensitization of Mn2+ emission by Eu2+ in Ca12Al14O33Cl2 host under UV excitation,

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RSC Advances, 3 (2013) 16034. [31] G.-G. Wang, X.-F. Wang, L.-W. Dong, Q. Yang, Synthesis and photoluminescence of green-emitting Ce3+,Tb3+co-doped Al6Si2O13 phosphors with high thermal stability for white LEDs, RSC Adv., 6 (2016) 42770-42777. [32] P. Ma, Y. Song, Y. Sheng, B. Yuan, H. Guan, C. Xu, H. Zou, Single-component and white light-emitting phosphor BaAl2Si2O8:Dy3+,Eu3+ synthesis, luminescence, energy transfer, and tunable color, Optical Materials, 60 (2016) 196-203.

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refinement and luminescence properties of Ce3+ singly doped and Ce3+/Mn2+ co-doped KBaY(BO3)2 for n-UV pumped white-light-emitting diodes, RSC Advances, 3 (2013) 16534-16541.

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generation, Journal of Materials Chemistry, 22 (2012) 15146-15152. [38] L. Lv, X. Jiang, S. Huang, X.a. Chen, Y. Pan, The formation mechanism, improved photoluminescence and LED applications of red phosphor K2SiF6:Mn4+,

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Journal of Materials Chemistry C, 2 (2014) 3879-3884.

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[39] C.-H. Huang, Y.-C. Chiu, W.-R. Liu, Ca3Si2O4N2:Ce3+,Li+ Phosphor for the Generation of White-Light-Emitting Diodes with Excellent Color Rendering Index Values, European Journal of Inorganic Chemistry, 2014 (2014) 3674-3680. [40] X. Zhang, L. Huang, F. Pan, M. Wu, J. Wang, Y. Chen, Q. Su, Highly Thermally Stable Single-Component White-Emitting Silicate Glass for Organic-Resin-Free White-Light-Emitting Diodes, Acs Applied Materials & Interfaces, 6 (2014) 2708-2716.

ACCEPTED MANUSCRIPT [41] Y.C. Fang, S.Y. Chu, P.C. Kao, Y.M. Chuang, Z.L. Zeng, Energy Transfer and Thermal Quenching Behaviors of CaLa2(MoO4)4:Sm3+ , Eu3+ Red Phosphors, Journal of the Electrochemical Society, 158 (2011) J1-J5. [42] J.S. Kim, H.P. Yun, M.K. Sun, C.C. Jin, L.P. Hong, Temperature-dependent

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[43] W. Lv, Y. Jia, Q. Zhao, L. Wei, M. Jiao, B. Shao, H. You, Synthesis, Structure,

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and Luminescence Properties of K2Ba7Si16O40:Eu2+ for White Light Emitting Diodes,

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Journal of Physical Chemistry C, 118 (2014) 4649-4655.

ACCEPTED MANUSCRIPT Figureure captions Figure 1 Measured (black crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for the Rietveld fit of SASO:0.04Ce3+,0.04Tb3+ XRD

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pattern using the GSAS program, and crystal structure of SASO (b). Figure 2 XRD patterns of (a) SASO:0.04Ce3+, (b) SASO:0.04Tb3+, (c) SASO:0.04Ce3+, 0.01Tb3+, (d) SASO:0.04Ce3+, 0.04Tb3+

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Figure 3 Excitation spectrum (a), Gaussian deconvolution of SASO:0.04Ce3+ emission (b) and the emission spectra of SASO: xCe3+ (x=0.01–0.08) phosphors

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excited at 365 nm (c).

Figure 4 (a, b) Photoluminescence excitation and emission spectra of SASO: 0.04Tb3+, and (c) emission spectra of SASO: yTb3+ (y=0.01–0.09) phosphors with

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various Tb3+ concentration (inset is changing curve of the Tb3+ emission intensity). Figure 5 (a) Spectra overlap between the excitation spectrum of Tb3+ and the emission spectrum of Ce3+ in SASO host. (b) Excitation spectra of SASO: 0.04Ce3+,

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0.04Tb3+, and SASO: 0.04Tb3+ monitored at 541nm.

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Figure 6 (a) (a) PL spectra for SASO: 0.04Ce3+, zTb3+ (z = 0 – 0.06) under 365 nm excitation. (b) The relative emission intensity of Ce3+ and Tb3+ as a function of Tb3+ concentration.

Figure 7 Emission spectrum of BaSO4 and SASO:0.04Ce3+, 0.04Tb3+ phosphor collected by an integrating sphere.

ACCEPTED MANUSCRIPT Figure 8 (a) Decay curves of Ce3+ in SASO: 0.04Ce3+, zTb3+ phosphors; (b) Dependence of the energy transfer efficiency ) and the fluorescence lifetime of Ce3+ on the Tb3+ concentration (z).

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Figure 9 Dependence of If /Ig of Ce3+ on (a) C6/3, (b) C8/3, and (c) C10/3 Figure 10 Experimental data plots of log{ln[I(t)/I0(t)]} versus log(t) of Ce3+ in the SASO:0.04Ce3+,zTb3+ phosphors. The red lines indicate the fitting behaviors.

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mechanism of SASO: Ce3+, Tb3+ phosphors.

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Figure 11 Schematic energy-level diagram showing the excitation and emission

Figure 12 CIE coordinates of SASO:0.04Ce3+, zTb3+ (z=0–0.06) under and the corresponding photographs under 365 nm UV lamp excitation. Figure 13(a) Temperature-dependent PL spectra of SASO:0.04Ce3+, 0.04Tb3+

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phosphor excited at 365 nm. Inset: the changing curve of the phosphor emission intensity with the temperature variation. (b) the activation energy Ee of

Tables

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SASO:0.04Ce3+, 0.04Tb3+ phosphors.

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Table 1 Crystal Structural Data and Lattice constants of SASO:0.04Ce3+, 0.04Tb3+ as the results of Structure Refinement. Table 2 Luminescence quantum yield of SASO:0.04Ce3+, zTb3+ samples under the excitation of 365 nm Table 3 Luminescence Quantum Yield of SASO:0.04Ce3+, zTb3+ samples under the excitation of 365 nm.

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Figure 1 Measured (black crosses) and calculated (red solid line) XRD patterns and their difference (blue solid line) for the Rietveld fit of SASO:0.04Ce3+,0.04Tb3+ XRD

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pattern using the GSAS program, and crystal structure of SASO (b).

Figure 2 XRD patterns of SASO:xCe3+/yTb3+.

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Figure 3 Excitation spectrum (a), Gaussian deconvolution of SASO:0.04Ce3+ emission (b), and the emission spectra of SASO: xCe3+ (x=0.01–0.08) phosphors

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excited at 365 nm (c).

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Figure 4 (a, b) Photoluminescence excitation and emission spectra of SASO: 0.04Tb3+,

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and (c) emission spectra of SASO: yTb3+ (y=0.01–0.06) phosphors with various Tb3+

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concentration (inset is changing curve of the Tb3+ emission intensity).

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Figure 5(a) Spectra overlap between the excitation spectrum of Tb3+ and the emission spectrum of Ce3+ in SASO host. (b) Excitation spectra of SASO: 0.04Ce3+, 0.04Tb3+,

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and SASO: 0.04Tb3+ monitored at 541nm.

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Figure 6(a) PL spectra for SASO: 0.04Ce3+, zTb3+ (z = 0 – 0.06) under 365 nm excitation. (b) The relative emission intensity of Ce3+ and Tb3+ as a function of Tb3+ concentration.

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Figure 7 Emission spectrum of blank and SASO:0.04Ce3+, 0.04Tb3+ phosphor

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collected by an integrating sphere.

Figure 8(a) Decay curves of Ce3+ in SASO: 0.04Ce3+, zTb3+ phosphors; (b)

Dependence of the energy transfer efficiency ) and the fluorescence lifetime of Ce3+ on the Tb3+ concentration (z).

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Figure 9 Dependence of If /Ig of Ce3+ on (a) C6/3, (b) C8/3, and (c) C10/3.

Figure 10 Experimental data plots of log{ln[I(t)/I0(t)]} versus log(t) of Ce3+ in the SASO:0.04Ce3+,zTb3+ phosphors. The red lines indicate the fitting behaviors.

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Figure 11 Schematic energy-level diagram showing the excitation and emission

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mechanism of SASO: Ce3+, Tb3+ phosphors.

Figure 12 CIE coordinates of SASO:0.04Ce3+, zTb3+ (z=0–0.06) under and the corresponding photographs under 365 nm UV lamp excitation.

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Figure 13(a) Temperature-dependent PL spectra of SASO:0.04Ce3+, 0.04Tb3+

phosphor excited at 365 nm. Inset: the changing curve of the phosphor emission

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intensity with the temperature variation. (b) The activation energy Ee of

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SASO:0.04Ce3+, 0.04Tb3+ phosphors.

ACCEPTED MANUSCRIPT Table 1 Crystal Structural Data and Lattice constants of SASO:0.04Ce3+, 0.04Tb3+ as the results of Structure Refinement.

SrAl2Si2O8:0.04Ce3+,0.04Tb3+

Space group

C2/m (12) monoclinic

Crystal system

monoclinic

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Formula

a=8.3933(1) Å, b=12.8934(1) Å, c=7.1223(2) Å, α=γ=90°, Cell parameters

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β=115.72°, V=694.4(7) Å3


 = 6.74%,
 = 9.20%, χ2=1.825%

Reliablity factors x

Al1

8

0.01452(5)

Si1

8

-0.00428(1)

Al2

8

0.16681(3)

Si2

8

0.19730(2)

Sr

4

O1

8

O2 O3

y

z

Occupancy

0.17386(0)

0.21826(3)

0.5000

0.18201(7)

0.23284(5)

0.5000

0.40120(2)

0.32792(4)

0.5000

0.37373(4)

0.34590(2)

0.5000

0.27047(9)

0.000000

0.13092(5)

0.9210

0.01791(1)

0.30135(4)

0.25207(1)

1.0000

8

0.19828(5)

0.12521(9)

0.40083(6)

1.0000

8

0.31307(5)

0.37071(1)

0.21881(0)

1.0000

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Mult

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Atom

O4

4

0.59441(8)

0.000000

0.30993(2)

1.0000

O5

4

0.000000

0.12779(5)

0.000000

1.0000

Ce

4

0.27047(9)

0.000000

0.13092(5)

0.0389

Tb

4

0.27047(9)

0.000000

0.13092(5)

0.0391

ACCEPTED MANUSCRIPT Table 2 Luminescence Quantum Yield of SASO:0.04Ce3+, zTb3+ samples under the excitation of 365 nm. Samples

Quantum Yield (%) 33.63

SASO:0.04Ce3+, 0.01Tb3+

43.73

SASO:0.04Ce3+, 0.03Tb3+

64.25

SASO:0.04Ce3+, 0.04Tb3+

67.37

SASO:0.04Tb3+

53.02 39.31

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SASO:0.04Ce3+, 0.06Tb3+

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SASO:0.04Ce3+, 0.05Tb3+

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SASO:0.04Ce3+

23.46

Samples

τ1 (ns)

τ2 (ns)

A1

A2

τav (ns)

10.108

44.258

2.177E7

43836.86

12.87

SASO:0.04Ce,0.01Tb

8.025

40.902

1.049E7

45614.44

8.77

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Table 3 The lifetime detailed parameters of SASO:0.04Ce3+, zTb3+.

SASO:0.04Ce,0.03Tb

5.761

34.461

1.694E7

51649.09

5.81

SASO:0.04Ce,0.04Tb

4.973

31.907

8.069E7

52022.93

4.98

SASO:0.04Ce,0.05Tb

4.391

30.857

3.483E8

52167.65

4.39

SASO:0.04Ce,0.06Tb

2.203

27.837

5.344E14

62534.94

2.20

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Highlights 1.SrAl2Si2O8:Ce3+,Tb3+ phosphors are synthesized by solid state reaction method. 2. The SrAl2Si2O8:Ce3+,Tb3+ phosphors can emit tunable blue to green light under the

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NUV excitation. 3. The energy transfer from Ce3+ to Tb3+ in SrAl2Si2O8 host is efficient.

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4. SrAl2Si2O8:Ce3+,Tb3+ phosphors have good thermostability.