Enhancement of red emission in Ce3+, RE3+, Mn2+ codoped Ca5(BO3)3F phosphors: Luminescent properties and structural refinement

Enhancement of red emission in Ce3+, RE3+, Mn2+ codoped Ca5(BO3)3F phosphors: Luminescent properties and structural refinement

Accepted Manuscript 3+ 3+ 2+ Enhancement of red emission in Ce , RE , Mn codoped Ca5(BO3)3F phosphors: Luminescent properties and structural refinemen...

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Accepted Manuscript 3+ 3+ 2+ Enhancement of red emission in Ce , RE , Mn codoped Ca5(BO3)3F phosphors: Luminescent properties and structural refinement† Liping Yi, Jilin Zhang, Fang Liu, Zhongxian Qiu, Wenli Zhou, Liping Yu, Shixun Lian PII:

S0925-8388(16)32105-3

DOI:

10.1016/j.jallcom.2016.07.072

Reference:

JALCOM 38240

To appear in:

Journal of Alloys and Compounds

Received Date: 23 April 2016 Revised Date:

23 June 2016

Accepted Date: 5 July 2016

Please cite this article as: L. Yi, J. Zhang, F. Liu, Z. Qiu, W. Zhou, L. Yu, S. Lian, Enhancement of red 3+ 3+ 2+ emission in Ce , RE , Mn codoped Ca5(BO3)3F phosphors: Luminescent properties and structural refinement†, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.07.072. 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.

ACCEPTED MANUSCRIPT

Enhancement of Red Emission in Ce3+, RE3+, Mn2+ Codoped Ca5(BO3)3F Phosphors: Luminescent Properties and Structural Refinement†

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Liping Yi,a, b Jilin Zhang,*a, b Fang Liu,a, b Zhongxian Qiu,a, b Wenli Zhou,a, b Liping Yua, b and Shixun

a

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Lian*a, b

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of

b

M AN U

Education of China), Hunan Normal University, Changsha 410081, China

Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province

Corresponding authors:

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College, Hunan Normal University, Changsha 410081, China

*E-mail: [email protected]

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*E-mail: [email protected], Tel/Fax: +86-731-88865345

ACCEPTED MANUSCRIPT Abstract: Ce3+, Mn2+ codoped and Ce3+, RE3+, Mn2+ (RE3+ = Tb3+, La3+, Gd3+, Lu3+) codoped Ca5(BO3)3F phosphors have been synthesized by a high-temperature solid-state reaction. Crystal structure and

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site occupancy of the doped ions were determined by Rietveld refinement. Photoluminescence (PL) spectra as well as fluorescence lifetimes were investigated. Under the excitation of 360 nm, Ca5(BO3)3F:Ce3+, Mn2+ phosphors show a strong emission band at 392 nm and a very weak emission

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band at ~630 nm, which belong to Ce3+ and Mn2+, respectively. The introduction of Tb3+ and even

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non-luminescent ions such as La3+, Gd3+ and Lu3+ to the Ce3+, Mn2+ codoped phosphors leads to the enhancement of Mn2+ emission. The analyses of fluorescence lifetimes suggest that there is energy transfer from Ce3+ and especially Tb3+ to Mn2+. However, Rietveld refinement results indicate that the introduction of RE3+ ions leads to the adjustment of Mn2+ ions from Ca(3) site to Ca(2) site,

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which is the key factor for the enhancement of Mn2+ emission. The crystal-site engineering is expected to provide a promising route to tune the luminescent properties of phosphors with more than one site for activators.

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Keywords: phosphor; luminescence; Ce3+; Mn2+; solid state reaction; inorganic materials.

1. Introduction

It is well known that the commercial white light-emitting diodes (LEDs) combined with a blue LED chip and Y3Al5O12:Ce3+ (YAG:Ce3+) yellow-emitting phosphor cannot achieve a warm white light with a color-rendering index (CRI) larger than 80 due to the lack of red-emitting component [1,2]. Regardless of new problems that may be encountered, several ways can be used to overcome this drawback, for example, codoping Pr3+ with YAG:Ce3+ [3], the combination of a near-UV/blue LED

ACCEPTED MANUSCRIPT chip with two or three phosphors [4-6], the combination of a near-UV LED chip with a single-phase white phosphor [7-11], etc. Mn2+ ion attracts much attention as activator in new phosphors, because it can emit a broad-band red emission in suitable hosts. However, the emission intensity of Mn2+

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single-doped phosphors is usually very low due to the spin- and parity-forbidden 3d-3d transition. Therefore, Ce3+/Eu2+ ions with allowed 4f↔5d transitions are usually used to enhance the intensity of Mn2+ based on energy transfer, and to realize white emission on a single-phase phosphor [7-9]. A

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white-emitting LED with CRI value higher than 90 was achieved by the combination of a blue chip

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and a single-phase phosphor Ca3Sc2Si3O12:Ce3+, Mn2+ phosphor through charge compensation [12]. In some hosts, green-emitting Tb3+ ions were used to improve the CRI value of the white-emitting codoped phosphors, which also need needs energy transfer from Ce3+ or Eu2+ [7-9,13]. Generally, efficient energy transfer from a sensitizer to an activator needs not only spectral

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overlap between the photoluminescent excitation spectrum (PLE spectrum) or absorption spectrum of the activator and the emission spectrum (PL spectrum) of the sensitizer, but also an interaction between them, such as exchange interaction and multi-polar interactions [14]. Although there were

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many published papers focusing on energy transfer from Ce3+/Eu2+ to Tb3+ and Mn2+, little of them

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paid attention to site-sensitive energy transfer. In our previous work, crystal-site sensitive energy transfer from Ce3+ to Mn2+ was observed in Ca3Al2O6 host due to different situations for unlike emission bands of Ce3+ ions (sensitizer) on different Ca sites [15]. In the present work, we are going to report a different type of site-sensitive energy transfer in phosphors Ca5(BO3)3F:Ce3+, RE3+, Mn2+ (RE = Tb, La, Gd and Lu). Investigation suggests that site-sensitive energy transfer from Ce3+ to Mn2+ is due to Mn2+ ions (activator) on different Ca sites. Most interestingly, the introduction of rare earth ions can enhance the emission intensity of Mn2+

ACCEPTED MANUSCRIPT dramatically, regardless of the luminescent characteristics of RE3+ ions. Studies are performed in detail based on structural refinement.

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2. Experimental Section 2.1 Materials and Synthesis

The samples were prepared by a traditional high-temperature solid-state reaction under a weak

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reductive atmosphere. Typically, the stoichiometric amounts of CaCO3 (A.R.), CaF2 (A.R.), H3BO3

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(A.R.), CeO2 (99.99%), La2O3 (99.99%), Gd2O3 (99.99%), Tb4O7 (99.99%), Lu2O3 (99.99%), MnCO3 (A.R.) and Na2CO3 (A.R.) were mixed in an agate mortar for 30 min with the assistance of alcohol. Subsequently, the dried mixtures were calcined in a tube furnace at 1000 °C for 6 h under an atmosphere of H2 (5%) diluted with N2.

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2.2 Measurements and Characterization

The powder X-ray diffraction patterns were collected on a PANalytical X’Pert Pro diffractometer with Cu Kα radiation (λ = 1.54056 Å) operated at 40 kV and 40 mA. The scanning speed is 1 degree

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per minute. The Rietveld structure refinements were performed using the GSAS (general structure

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analysis system) program [16]. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp. The fluorescence lifetime measurements were performed on an EDINBURGH FLS920 combined Fluorescence Lifetime & Steady State Spectrometer equipped with a 450 W xenon lamp, a ns flash lamp and a µs flash lamp. For comparison, PLE and PL spectra of selected phosphors were collected on the FLS920 spectrometer. All the measurements were performed at room temperature.

ACCEPTED MANUSCRIPT 3. Results and Discussion 3.1. Phase Characterization and Structure Refinement The phase purity of the as-synthesized samples was checked by XRD analysis firstly. Figure 1 shows

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the XRD patterns of selected CBOF samples doped with different ions, which are consistent with the standard data for Ca5(BO3)3F (JCPDS card no. 80-1702) with a C1m1 space group and a monoclinic crystal system. This result suggests that the introduction of doping ions with other synthesis

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conditions constant does not change the phase of the samples. However, the magnified XRD patterns

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in 29.5-31.2° of 2 theta show that the diffraction peaks shift with the introduction of doping ions, which suggest that the cell parameters of the as-prepared phosphors are different with each other. The photoluminescent properties of the luminescent centers are sensitive to the crystallographic site, especially Ce3+ and Eu2+ ions with allowed d-f transition and Mn2+ with d-d transition, etc, which is

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because of that the 5d orbits of Ce3+/Eu2+ and the 3d orbits of Mn2+ are on the outer shell of the atoms. There are three Ca sites suitable for the doping ions in the unit cell of Ca5(BO3)3F, and the number of Ca(1), Ca(2) and Ca(3) sites are 4, 4 and 2 in a unit cell, respectively. Therefore, Rietveld

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structure refinement for the as-prepared samples is significant to have a deep sight in the relationship

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between the luminescent properties and crystal structure information. The crystal structure of Ca5(BO3)3F (ICSD#65763) was used as the starting model for structure refinement. The site occupancy of Ce3+ is determined based the structure refinement of CBOF:0.03Ce3+, 0.20Tb3+, 0.23Na+ phosphor and our previous work [17]. While the site occupancy of Tb3+ is based on YCa4O(BO3)3 with similar crystal structure as Ca5(BO3)3F, where Y took the crystal sites similar as Ca(1) and Ca(3) in Ca5(BO3)3F [18]. The structure refinement results of CBOF:0.03Ce3+, 0.20Tb3+, 0.23Na+ with different occupancy models are summarized in Table S1 of

ACCEPTED MANUSCRIPT the Supporting Information. Results suggest that Ce3+ ions occupy Ca(1) and Ca(2) sites, Tb3+ ions are on Ca(1) and Ca(3) sites, while Na+ ions are on Ca(1) and Ca(2) sites. The preferred site occupancy of Mn2+ is obtained by the structure refinement of CBOF:0.03Ce3+, 0.20Tb3+, 0.05Mn2+,

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0.23Na+ as shown in Table S2 of the Supporting Information. Mn2+ ions prefer to occupy Ca(2) and Ca(3) sites according to the refinement results.

Figure 2 shows the observed, calculated and difference results for Rietveld refinement of

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representative CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ and CBOF:0.05Ce3+, xMn2+, 0.05Na+

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phosphors. Crystallographic and refinement parameters are summarized in Table 1. Results show that almost all the diffraction peaks can be indexed to Ca5(BO3)3F with a monoclinic cell (C1m1). The cell parameters are close to those of Ca5(BO3)3F (ICSD#65763). As a typical example, the atomic coordinates, isotropic displacement parameters and occupancy of Ca5(BO3)3F:0.03Ce3+, 0.20Tb3+,

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0.15Mn2+, 0.23Na+ phosphor are listed in Table 2. A unit cell with coordination polyhedra is illustrated in Figure 3 based on the refinement result of Ca5(BO3)3F:0.03Ce3+, 0.20Tb3+, 0.15Mn2+, 0.23Na+. Figure 4 illustrates the cell parameters and cell volumes as a function of Mn2+ content for

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CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ and CBOF:0.05Ce3+, xMn2+, 0.05Na+ series. The lengths

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and volumes for the unit cell of CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ series decrease with the increase of Mn2+ content, while those for CBOF:0.05Ce3+, xMn2+, 0.05Na+ series increase slightly at first and then decrease with increasing Mn2+ content. The former tendency can be understood understand easily because Ca2+ is substituted by Mn2+ with a smaller radius than Ca2+. The situation in CBOF:0.05Ce3+, xMn2+, 0.05Na+ is unusual. However, evidence to support this variation can be found in the luminescent properties, which will be discussed below. Table 1. Final crystallographic and refinement parameters of CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ and CBOF:0.05Ce3+, xMn2+, 0.05Na+ phosphors

ACCEPTED MANUSCRIPT CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+

CBOF:0.05Ce3+, xMn2+, 0.05Na+

x = 0.05

x = 0.15

x = 0.05

x = 0.15

x = 0.25

a/Å

8.1318(8)

8.1281(4)

8.1237(4)

8.1414(2)

8.1419(2)

8.1381(2)

b/Å

16.0686(16)

16.0640(8)

16.0516(8)

16.0738(3)

16.0764(3)

16.0690(3)

c/Å

3.5491(4)

3.5479(2)

3.5427(2)

3.5467(1)

3.5491(1)

3.5477(1)

β/°

101.025

101.024

101.022

100.96(0)

100.97(0)

100.97(0)

V/Å3

455.19(15)

454.70(10)

453.45(9)

455.67(12)

456.06(12)

455.46(11)

Z

2

2

2

2

2

2

dCa(1)-OF/Å

2.321

2.336

2.316

2.300

2.312

2.307

dCa(2)-O/Å

2.449

2.438

2.419

2.486

2.525

2.448

dCa(3)-OF/Å

2.331

2.308

2.354

2.318

2.324

2.361

Rp/%

5.22

4.50

Rwp/%

7.07

6.52

χ2

3.373

2.912

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x=0

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phosphor

4.22

6.25

5.56

5.05

5.60

9.89

8.40

7.67

2.120

6.604

4.739

3.860

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Table 2. Atomic coordinates, isotropic displacement parameters (Uiso/Å2) and occupancy of Ca5(BO3)3F:0.03Ce3+, 0.20Tb3+, 0.15Mn2+, 0.23Na+ phosphor. Wyck.

Ca1 Ce1 Tb1 Na1 Ca2 Ce2 Na2 Mn2 Ca3 Tb3 Mn3 F1 O1 O2 O3 O4 O5

4b 4b 4b 4b 4b 4b 4b 4b 2a 2a 2a 2a 4b 4b 2a 4b 4b

x

y

z

Uiso

S.O.F.

0.66234 0.66234 0.66234 0.66234 0.04323 0.04323 0.04323 0.04323 0.28418 0.28418 0.28418 0.48412 0.83919 0.02215 0.09679 0.84136 0.21863

0.11583 0.11583 0.11583 0.11583 0.17840 0.17840 0.17840 0.17840 0 0 0 0 0.07660 0.32852 0 0.22796 0.14344

0.78193 0.78193 0.78193 0.78193 0.45557 0.45557 0.45557 0.45557 0.08384 0.08384 0.08384 -0.29366 0.36741 0.25241 0.50287 0.84548 0.02768

0.0125 0.0125 0.0125 0.0125 0.0154 0.0154 0.0154 0.0154 0.0195 0.0195 0.0195 0.0009 0.0342 0.0240 0.0436 0.0099 0.0271

0.89195 0.01005 0.04071 0.05729 0.87547 0.00495 0.05771 0.06187 0.85516 0.11858 0.02626 1 1 1 1 1 1

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atom

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4b 2a

0.86686 0.92782

0.30797 0

0.05107 0.36841

0.0184 0.0184

1 1

3.2. Photoluminescence Properties of CBOF:Ce3+, xMn2+, Na+ and CBOF:Ce3+, Tb3+, xMn2+,

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Na+ Phosphors PLE and PL spectra of CBOF:0.05Ce3+, xMn2+, 0.05Na+ phosphors are illustrated in Figure 5. Upon excitation at 360 nm, the PL spectra contain a strong asymmetric emission band peaking at 392 nm

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and a very weak band at ~630 nm (Figure 5). The emission band at 392 nm belongs to the 5d-4f

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allowed transition transitions in Ce3+, detail discussion of which can be found in our previous work [17]. The weak emission band at ~630 nm originates from the forbidden 3d-3d transition in Mn2+. The emission intensity of blue-emitting Ce3+ decreases with the increase of Mn2+ content. The PLE spectrum monitored at 392 nm contains a strong band at 360 nm, plus several weak bands at shorter

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wavelength region. When monitoring the emission from Mn2+, the PLE spectrum still contains the 360 nm band. The insert in Figure 5 shows the magnified emission band of Mn2+, in which the emission intensity tends to increase with Mn2+ content. Furthermore, the emission peak of Mn2+

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shifts firstly to shorter wavelength and then to longer wavelength with the increase of Mn2+ content.

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The reason will be discussed below.

The PLE spectrum monitored at 645 nm supports the existence of energy transfer from blue-emitting Ce3+ to Mn2+. The energy transfer efficiency (η) can be presented as the following equation [19]

η =1-

IS I S0

(1)

where IS0 and IS are the emission intensity of Ce3+ in the absence and presence of Mn2+, respectively. The calculated value of η is 71.5% for CBOF:0.05Ce3+, 0.30Mn2+, 0.05Na+. The energy transfer can

ACCEPTED MANUSCRIPT also be proven by the luminescence decay curves of Ce3+ in Ce3+, Mn2+ codoped samples, which are shown in Figure 6. The decay curves monitored at 392 nm change from single exponential form for Ce3+ single doped one to non-exponential form for Ce3+, Mn2+ codoped samples. And the PL

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intensity decays faster with larger Mn2+ content. However, the emission intensity of Mn2+ is very low compared to that of Ce3+. When only Mn2+ ion is doped in CBOF, there is even no light under excitation of a near-UV or a blue light. Two reasons are found for the ineffective emission of Mn2+.

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Firstly, the spectra are collected on a Hitachi F-4500 spectrophotometer, in which spectral correction

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was not performed for the low sensitivity on red light of photomultiplier tube (PMT). This problem can be solved technically, and example is shown in the following part of CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors. The second reason is that the real amount of Mn2+ that can accept the excited energy of Ce3+ is low in CBOF:0.05Ce3+, xMn2+, 0.05Na+, which will be discussed in detail below. In order

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to show the variation of emission intensity clearly, the PL and PLE spectra in the main text are all collected from F-4500 spectrophotometer.

Interesting phenomenon is found when Tb3+ ions are introduced to Ce3+, Mn2+, Na+ codoped

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CBOF phosphors, namely, the emission intensity of Mn2+ in Ce3+, Tb3+, Mn2+, Na+ codoped CBOF

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phosphors increases dramatically when compared with the Ce3+, Mn2+ codoped one with same Mn2+ content. The PL spectra of two series of CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors under 360 nm excitation are given in Figure 7a and 7b. The content of Ce3+ ions is fixed at 0.03, while those of Tb3+ ions are set at 0.15 and 0.20 for the two series, and those of Mn2+ ions change from 0 to 0.30. The emission intensities from Mn2+ increase firstly with its content, then decrease after reaching a maximum value due to concentration quenching. While the emission intensities from Ce3+ and Tb3+ decrease with increasing Mn2+ content. The PLE spectra of CBOF:0.03Ce3+, 0.20Tb3+, 0.10Mn2+,

ACCEPTED MANUSCRIPT 0.23Na+ phosphors are shown in Figure 7c. The PLE spectrum monitored at 390 nm exhibits the typical excitation bands of Ce3+ which has a strongest band peaking at 360 nm. The PLE spectrum monitored at 541 nm that originates from Tb3+ emission contains not only the excitation bands of

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Tb3+ (e.g.: band at 272 nm), but also those belong to Ce3+. The luminescent properties of Ce3+, Tb3+, and energy transfer in Ce3+, Tb3+ codoped CBOF phosphors have been investigated deeply in our previous work [17]. When monitoring Mn2+ emission at 645 nm (avoiding the interference of Tb3+

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emission in red emission region), the PLE spectrum contains mainly three bands at 272, 360 and 400

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nm, which belong to the transition in Tb3+, blue- and green-emitting Ce3+, respectively. The PL spectra of selected CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ phosphors under excitation at 400 nm are shown in Figure 7d, which are very weak compared with those under excitation at 360 nm. Corresponding PLE spectra monitored at 520 and 645 nm are show in Figure S1 in Supporting

emission of Ce3+.

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Information. To simplify the discussion, we focus on the luminescent properties based on the 392 nm

The PL spectra of CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors measured on an FLS920

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Fluorescence Spectrometer with 360 nm excitation and spectral correction are shown in Figure S2.

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The CIE coordinates calculated from these PL data are pointed out in Figure 8. The photos of corresponding phosphors under a 365 nm UV lamp are illustrated, too. It can be found that the emission color can be tuned from blue-green to red with increasing the content of Mn2+ from 0 to 0.30.

The result suggests that the introduction of Tb3+ plays a key role in the enhancement of Mn2+ emission. To find out the mechanism of Tb3+ on the enhancement of PL intensity of Mn2+, the PLE spectra of a Ce3+, Tb3+, Mn2+, Na+ codoped CBOF phosphor and the luminescence decay curves are

ACCEPTED MANUSCRIPT studied firstly. The decay curves for Ce3+ and Tb3+ are illustrated in Figure 9, monitored at 392 and 541 nm, respectively. The decay rate of the Ce3+ emission becomes faster than the Ce3+, Tb3+ codoped one with the increase of Mn2+ content, which indicates the existence of energy transfer from

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Ce3+ to Mn2+. The decay curves of Tb3+ also change from single exponential to non-exponential, and the decay time shortens with the increase of Mn2+ content. These results suggest the existence of energy transfer from Tb3+ to Mn2+, besides that from Ce3+ to Mn2+.

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It seems that Tb3+ ions act as a bridge between Ce3+ and Mn2+. However, it worth noticing that

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the PLE band at 272 nm is weaker than that situated at 360 nm when monitoring Mn2+ emission as shown in Figure 7c. This phenomenon suggests that the energy transfer from Ce3+ to Mn2+ is more efficient than that from Tb3+ to Mn2+. Furthermore, the decay curves of Ce3+, Mn2+, Na+ codoped CBOF phosphors suggest that there is already strong energy transfer from Ce3+ to Mn2+, however, the

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emission of Mn2+ is very low. Therefore, there should be other reason for the increase of Mn2+ emission intensity besides the bridging effect of Tb3+. It is well known that the emission intensity of an activator will increase with its concentration before it reaches concentration quenching. This law

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should be available in the present situation. The site occupancy of Mn2+ as a function of Mn2+

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content based on the Rietveld refinement of Ce3+, Mn2+, Na+ codoped and Ce3+, Tb3+, Mn2+, Na+ codoped phosphors is illustrated in Figure 10a. For CBOF:Ce3+, Mn2+, Na+ phosphors, occupancy of Mn2+ on Ca(2) site is much lower than that on Ca(3) site, and the occupancy of Mn2+ on both sites increase with total Mn2+ content. On the contrary, occupancy of Mn2+ on Ca(2) site is higher than that on Ca(3) site in CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors. Mn2+ ions on a site that has F atom coordinated [20-23] usually exhibit an emission band with a peak shorter than those of Mn2+ ions on a site coordinated with only O atoms [24-31], on condition that the coordination number is equal or

ACCEPTED MANUSCRIPT larger than six. This phenomenon is due to the fact that crystal field splitting (Dq) produced by Fligand is smaller that produced by O2-. Crystal field splitting (Dq) can be determined by the following

1 2 r4 Dq = Ze 5 6 R

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equation [21,32]

(2)

where Dq is the energy level separation, Z is the charge of anion, e is the electron charge, r

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corresponds to the radius of the d wave function, and R is the bond length. Mn2+ on Ca(2) coordinated with only O atoms in Ca5(BO3)3F host should have has an emission band peaking at

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longer wavelength when compared with Ca(1) and Ca(3) sites that both coordinate with F atom. That is to say, the emission band beyond 600 nm originates from Mn2+ on Ca(2) site in CBOF. The Mn2+ content on Ca(2) site increases by introducing Tb3+ ions, which results in the enhancement of Mn2+ emission. A smaller longer average bond length corresponds to a larger Dq according to equation 2,

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resulting in the decrease of energy difference between the lowest excited state 4T1 (4G) and the ground state 6A1 of Mn2+ and consequently a longer emission wavelength according to equation 2.

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The shift of emission peak for Mn2+ in both Ce3+, Mn2+ codoped and Ce3+, Tb3+, Mn2+ codoped samples described above is in accordance with the variation of average Ca(2)-O distance as

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illustrated in Table 1.

3.3. Enhancing Mn2+ Emission by Introducing La3+, Gd3+ or Lu3+ Ions in CBOF:Ce3+, Mn2+ Phosphors

The refinement results and discussion on Dq value suggest that the key factor for the enhancement of Mn2+ emission is not the energy transfer from Tb3+ to Mn2+, but the induction of Tb3+ ions resulting in the increasing content of Mn2+ on Ca(2) site. Other RE3+ ions such as La3+, Gd3+ and Lu3+ are introduced instead of Tb3+ ion to further verify the fact that the increase of Mn2+ amount on Ca(2)

ACCEPTED MANUSCRIPT site is the main reason for the enhancement of Mn2+emission. La3+ ion has no 4f electrons, and the 4f orbits of Lu3+ ion are full of 4f electrons, while the energy difference between the lowest excited state and ground state of 4f levels for Gd3+ ion (32000 cm-1) is larger than that of 5d-4f for Ce3+ ion

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in CBOF. Therefore, La3+, Gd3+ and Lu3+ ions cannot act as bridge to transfer energy from Ce3+ to Mn2+. The emission spectra of CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, (0.03+x)Na+ (RE3+ = La3+, Gd3+ and Lu3+) are shown in Figure 11. Results indicate that the emission intensity of Mn2+ enhances

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indeed with the increase of RE3+ amount.

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Figure S3 shows the experimental and fitted patterns of representative CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, (0.03+x)Na+ (RE = La, Gd, Lu) phosphors. Refinement parameters for these phosphors are listed in Table S3-S5. According to the refinement results, Figure 10b depicts the site occupancy of Mn2+ as a function of RE3+ content in CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, (0.03+x)Na+ phosphors.

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It is obvious that the content of Mn2+ on Ca(2) site increases dramatically with the introduction of RE3+ ions, accompanying with the decrease of Mn2+ content on Ca(3) site. This is because of that the RE3+ ions prefer to occupy Ca(3) site, as indicated by the variation of site occupancy for RE3+ on

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Ca(3) site (see Figure S4). For Gd3+ (x = 0.25) and Lu3+ (x ≥ 0.15), all Mn2+ ions substitute on Ca(2)

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site according to the Rieveld refinement. The decay curves of Ce3+ in CBOF:0.03Ce3+, xGd3+, 0.25Mn2+, (0.03+x)Na+ are illustrated in Figure 12. The luminescence of Ce3+ decays faster with the increase of Gd3+ content, which suggests that more Mn2+ can accept the excited energy of Ce3+. Therefore, it is no doubt that the introduction of RE3+ ions (including Tb3+) increases the amount of Mn2+ on Ca(2), which is the key factor for the enhancement of Mn2+ emission. Furthermore, the maximum intensity of Mn2+ emission band exhibits a blue shift from 628 nm to 624 nm in CBOF:Ce3+, xLa3+, Mn2+, Na+ with the increase of La3+ amount. However, the maximum

ACCEPTED MANUSCRIPT intensity of Mn2+ emission band exhibits a red shift from 628 nm to 643 nm in the other two CBOF:Ce3+, xRE3+, Mn2+, Na+ series with the increase of RE3+ amount (RE3+ = Gd3+ and Lu3+) and in CBOF:Ce3+, Tb3+, xMn2+, Na+ with the increase of Mn2+ amount. These results may be due to the

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change in the size of MnO6 polyhedron (or bond length), which will result in different crystal field around Mn2+. According to equation 2 on Dq, the larger the bond length, the smaller the crystal filed splitting. Generally, the expansion of unit cell corresponds to the increase of bond length. Cell

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parameters and cell volumes as a function of RE3+ content are illustrated in Figure 13 according to

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the structure refinement results that are listed in tables S3-S5. The introduction of La3+ ions and the increase of its amount enlarge enlarges the unit cell, while Gd3+ and Lu3+ ions compress the unit cells in CBOF:Ce3+, xRE3+, Mn2+, Na+ series. The increase of Mn2+ content also compresses the unit cell in CBOF:Ce3+, Tb3+, xMn2+, Na+ series. One can find evidence for the evolution of unit cell volume

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simply by comparing the radii of these ions. The radii of La3+, Na+, Ce3+, Ca2+, Gd3+, Tb3+, Lu3+ and Mn2+ ions with a coordination number of six are 1.031, 1.02, 1.01, 1.00, 0.938, 0.923, 0.861 and 0.830 Å, respectively [33]. Therefore, the crystal field splitting of Mn2+ decreases with the increase

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of x in CBOF:Ce3+, xLa3+, Mn2+, Na+ series, resulting in the blue shift of Mn2+ emission due to the

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increase of energy difference between 4T1(4G) and 6A1 levels. And on the other hand, red shift of Mn2+ emission is observed with the increase of x in CBOF:Ce3+, xRE3+, Mn2+, Na+ series (RE = Gd, Lu) and in CBOF:Ce3+, Tb3+, xMn2+, Na+ series due to the decrease of energy difference between 4

T1(4G) and 6A1 levels. In summary, the variation of Mn2+ emission peak is in good agreement with

cell parameters in the CBOF:Ce3+, xRE3+, Mn2+, Na+ series in the present work. However, the variation of Mn2+ emission peak is not quite in agreement with the average bond length of Ca(2)-O, especially in the series with La3+ codoped. These may be due to the fact that RE3+ ions (La3+, Gd3+,

ACCEPTED MANUSCRIPT Lu3+) do not occupy Ca(2) site. Therefore, the influence of increasing RE3+ content on the size of Ca(2) site is indirect, which is different from the Ce3+, (Tb3+), xMn2+ codoped ones by increasing Mn2+ content and consequently changing the size of Ca(2) site directly. Furthermore, bond length (R)

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in equation 2 is an average value. Equation 2 does not deal with the site symmetry and single bond length, which may limit its application, such as the bond lengths in a coordination polyhedron

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changing in different trends.

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Conclusions

In conclusion, several series of multi-activator codoped CBOF phosphors were successfully synthesized by a high-temperature solid-state reaction. Ce3+, Mn2+ codoped CBOF phosphors showed low intensities of Mn2+ emission band. However, Ce3+, Tb3+, Mn2+ codoped CBOF

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phosphors showed strong emission from form Mn2+ ions, and the emission color of the phosphors can be tuned from blue-green to red. Furthermore, Ce3+, RE3+, Mn2+ codoped CBOF phosphors with non-emitting RE3+ ions, such as La3+, Gd3+ and Lu3+ ions, also enhanced the intensities of Mn2+

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emission band. Analyses indicated that the key factor for the enhancement of Mn2+ emission is the

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adjustment of Mn2+ ions on Ca crystallographic sites, on condition that there is energy transfer from Ce3+ to Mn2+. The crystal-site engineering may provide a promising route to tune the luminescent properties of phosphors, in which more than one site can be substituted by activators.

Acknowledgments This work is financially supported by the National Natural Science Foundation of China (NSFC 51402105, 21471055), Specialized Research Fund for the Doctoral Program of Higher Education

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Electronic Supplementary Information (ESI) available:

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Structure refinement results of CBOF:0.03Ce3+, 0.20Tb3+, 0.23Na+ and CBOF:0.03Ce3+, 0.20Tb3+, 0.05Mn2+, 0.23Na+ with different occupancy models (Tables S1 and S2); PLE spectra of CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ phosphors monitored at 520 and 645 nm (Figure S1); PL

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spectra of CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors excited at 360 nm on an FLS920 Fluorescence

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Spectrometer with spectral corrections (Figure S2); Rietveld refinement results of powder XRD patterns of representative CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, (0.03+x)Na+ (RE = La, Gd, Lu) phosphors (Figure S3 and Table S3-S5); variation of site occupancy for RE3+ on Ca(3) site with the

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ACCEPTED MANUSCRIPT Figure Captions: Figure 1. XRD patterns of selected CBOF:Ce3+, RE3+, Mn2+, Na+ phosphors. Figure 2. Rietveld refinement of powder XRD patterns of representative CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ and CBOF:0.05Ce3+, xMn2+, 0.05Na+ phosphors.

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Figure 3. Crystal structure of Ca5(BO3)3F:0.03Ce3+, 0.20Tb3+, 0.15Mn2+, 0.23Na+ and coordination polyhedra are depicted according to the refinement result.

Figure 4. Dependence of cell parameters on Mn2+ content in CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ and CBOF:0.05Ce3+, xMn2+, 0.05Na+ phosphors.

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Figure 5. PLE and PL spectra of CBOF:0.05Ce3+, xMn2+, 0.05Na+.

Figure 6. Decay curves of Ce3+ in CBOF:0.05Ce3+, xMn2+, 0.05Na+ (x = 0, 0.05, 0.15,

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0.25, monitored at 392 nm).

Figure 7. PL spectra of (a) CBOF:0.03Ce3+, 0.15Tb3+, xMn2+, 0.18Na+ and (b) CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ excited at 360 nm, (c) PLE spectra of CBOF:0.03Ce3+, 0.20Tb3+, 0.10Mn2+, 0.23Na+ monitored at different emission peaks, (d) PL spectra of CBOF:0.03Ce3+, 0.20Tb3+,

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xMn2+, 0.23Na+ excited at 400 nm.

Figure 8. CIE coordinates of CBOF:Ce3+, Tb3+, Mn2+, Na+ phosphors. Inserts are the corresponding photos of the phosphors under a 365 nm UV lamp. Figure 9. Decay curves of (a) Ce3+ (monitored at 392 nm) and (b) Tb3+ (monitored at

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541 nm) in CBOF:0.03Ce3+, 0.20Tb3+, xMn2+, 0.23Na+ (x = 0, 0.05, 0.15, 0.25, monitored at 541 nm) compared with Ce3+ single-doped one.

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Figure 10. Site occupancy of Mn2+ (a) as a function of (a) Mn2+ content and (b) as a function of RE3+ contents in related phosphors from Rietveld refinement of powder XRD patterns.

Figure 11. Normalized PL spectra (λex = 360 nm) of CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, 0.03+xNa+. (a) RE3+ = La3+, (b) RE3+ = Gd3+, (c) RE3+ = Lu3+.

Figure 12. Decay curves of Ce3+ in CBOF:0.03Ce3+, CBOF:0.05Ce3+, 0.25Mn2+ and CBOF:0.03Ce3+, xGd3+, 0.25Mn2+, 0.03+xNa+ (monitored at 392 nm). Figure 13. Dependence of cell parameters on RE3+ content in CBOF:0.03Ce3+, xRE3+, 0.25Mn2+, (0.03+x)Na+ (RE = La, Gd, Lu) phosphors (right image).

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ACCEPTED MANUSCRIPT Highlights: ▶ Emission of Mn2+ in Ca5(BO3)3F:Ce3+, Mn2+ is very weak under excitation at 360 nm. ▶ Introduction of RE3+ ions increases the amount of Mn2+ on Ca(2) site with 6 O atoms.

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▶ Introduction of RE3+ ions into Ca5(BO3)3F:Ce3+, Mn2+ enhances the emission of Mn2+.