Mn2+ codoped Ba9Lu2Si6O24 phosphors

Mn2+ codoped Ba9Lu2Si6O24 phosphors

Accepted Manuscript Title: Solid state synthesis, energy transfer and tunable luminescence of Ce3+ /Mn2+ codoped Ba9 Lu2 Si6 O24 phosphors Authors: Ye...

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Accepted Manuscript Title: Solid state synthesis, energy transfer and tunable luminescence of Ce3+ /Mn2+ codoped Ba9 Lu2 Si6 O24 phosphors Authors: Yeqiu Wu, Tao He, Liyong Lun PII: DOI: Reference:

S1010-6030(18)31827-6 https://doi.org/10.1016/j.jphotochem.2019.03.002 JPC 11739

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

15 December 2018 21 February 2019 4 March 2019

Please cite this article as: Wu Y, He T, Lun L, Solid state synthesis, energy transfer and tunable luminescence of Ce3+ /Mn2+ codoped Ba9 Lu2 Si6 O24 phosphors, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019), https://doi.org/10.1016/j.jphotochem.2019.03.002 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.

Solid state synthesis, energy transfer and tunable luminescence of Ce3+/Mn2+ codoped Ba9Lu2Si6O24 phosphors

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Yeqiu Wu1, Tao He2, , Liyong Lun3

1. Department of Building Engineering,Shanxi Datong University,Datong 037003,

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P.R. China

2. Department of Electrical and Electronic Engineering, Wenzhou Vocational &

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Technical College, Wenzhou 325035, P.R. China

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3. Resources & Environment Business Department, China International Engineering

[email protected] (Corresponding author: Tao He)

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Graphical abstract

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Consulting Corporation, Ltd., Beijing 100048, P.R. China

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Highlights •

Solid state synthesis of Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors



Tunable luminescence of Ba9Lu2Si6O24:Ce3+/Mn2+ by changing Mn2+ concentration

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Warm white light generated by Ba9Lu2Si6O24:0.10Ce3+/0.03Mn2+ phosphor

Abstract: A series of Ce3+ and/or Mn2+ doped Ba9Lu2Si6O24 phosphors were prepared by the

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solid state reaction. The phase and luminescent properties of the synthesized phosphors were investigated. The single phase of the synthesized phosphors indicates

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that Ce3+/Mn2+ ions doped into Ba9Lu2Si6O24 hosts entirely and formed solid state compounds. Mn2+ ions substitute Lu3+ sites in Ba9Lu2Si6O24 but Ce3+ ions substitute

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both Lu3+ and Ba2+ sites in Ba9Lu2Si6O24. The emission bands of Ba9Lu2Si6O24:Ce3+

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phosphors show dependence on excitation wavelength and Ce3+ concentration. Upon

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the excitation at 400 and 350 nm, Ba9Lu2Si6O24:Ce3+ phosphors show emission bands

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peaking at 495 and 422 nm, respectively. Upon the excitation at 350 nm,

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Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors show emission bands corresponding to Ce3+ and Mn2+ ions. Due to energy transfer from Ce3+ to Mn2+, tunable luminescence is

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obtained in Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors.

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Keywords: Ba9Lu2Si6O24:Ce3+/Mn2+; Energy transfer; Luminescence. 1. Introduction

Phosphors can convert a broad spectrum of light into photons of a particular

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wavelength, which have potential applications in a wide range of field, such as information displays, lighting, X-ray intensification, energy conversion and scintillation [1, 2]. In the field of lighting, phosphors are in focus because that phosphor is one of fundamental components in white light emitting diodes (WLEDs). 2

WLEDs have some advantages, such as longevity life, low energy-consumption and high efficiency, which benefit to the settlement of energy and environmental risks [3, 4]. Currently, the general fabrication of WLEDs is the assembly of the blue chip and yellow Y3Al5O12:Ce3+ phosphor and this assembly has an obvious disadvantage of

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high correlated color temperature (CCT) due to the absence of red light [5]. Researchers in the world have tried a number of ways to put the axe in the helve. At

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last, two ways are widely accepted: One is the red shift of Ce3+ emission or the

compensation of red light [4-9], the other is the use of white light phosphors with a

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single phase [10-12].

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The generation of white light through energy transfer between ion pairs is a good

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strategy and it has been extensively used in phosphors. In a host, activator/sensitizer

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ion pairs are added. The sensitizer absorbs the exciting energy and the excited photons

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fall to the ground state through two ways: one is the radiation transition with lighting; the other is non-radiate transition to the activator. Due to the energy transfer from the

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activator to the sensitizer, the emission intensities of them in the host can be changed

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by changing the doping concentrations. Thus, the light of the phosphors can be tuned by changing the doping concentrations of activator/sensitizer ion pairs. When the appropriate concentration of activator/sensitizer is added, the white light could be

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generated. A large number of white light phosphors have been synthesized and investigated, such as Ce3+/Dy3+ doped phosphors [10, 13], Dy3+/Eu3+ doped phosphors [11, 14], Ce3+/Tb3+/Sm3+ and Ce3+/Tb3+/Eu3+ doped phosphors [12, 15-17], Ce3+/Eu2+ doped phosphors [18, 19], Ce3+/Mn2+/Tb3+ doped phosphors [20, 21], Ce3+/Mn2+ 3

doped phosphors [22, 23] and Eu2+/Mn2+ doped phosphors [24, 25]. Ba9R2Si6O24 (R = Sc, Y, Lu) is a type of material with orthosilicate structure and crystallizing in rohombohedral system [26]. A series of rare earth ions doped Ba9R2Si6O24 doped phosphors have been fabricated and they show great potential in

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commercial LED devices [27-33]. In this work, we synthesized a series of Ce3+/Mn2+ doped Ba9Lu2Si6O24 phosphors by a solid state reaction. The work aims to confirm the

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energy transfer from Ce3+ to Mn2+ and tunes the generated light by changing the doping concentration of Ce3+/Mn2+. The phase and luminescent properties of the

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synthesized phosphors were investigated.

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2. Materials and method

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A series of Ba9Lu2Si6O24:xCe3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) and

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Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0.01, 0.02, 0.03, 0.04 and 0.05) were synthesized

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through a solid state reaction. Barium carbonate (BaCO3), silicon dioxide (SiO2), lutetium trioxide (Lu2O3), ceric oxide (CeO2) and manganese carbonate (MnCO3)

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were used as starting materials. In the synthesis, stoichiometric materials were

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weighted and mixed in an agate mortar for 60 min. Then the mixture was calcined at 1300 ºC under a reduction atmosphere (N2:H2=95%:5%). After the system cooled to room temperature naturally, the product was collected and ground for the

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measurements.

The X-ray diffraction (XRD) measurements were carried out by a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 0.15418 nm). Diffraction data were collected in the step-scan mode, with a step size of 2θ=0.033° and an 4

accumulation time of 1 s per step. The excitation and emission spectra were measured by an Edinburgh Instrument FLS920 spectrophotometer equipped with a 150W xenon lamp as the excitation source and a grating to select a suitable excitation wavelength with excitation and emission slit widths of 2.5 nm, a scan rate of 600 nm/min.

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Absolute photoluminescence quantum yield data were collected by an absolute PL quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K.). The

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temperature-dependence of emission spectra was obtained on a fluorescence spectrophotometer equipped with a 450 W xenon lamp as the excitation source

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(Edinburgh Instruments FLSP-920) with a temperature controller.

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3. Results and discussion

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Fig.1 gives the XRD patterns of Ba9Lu2Si6O24:0.10Ce3+, Ba9Lu2Si6O24:0.03Mn2+,

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Ba9Lu2Si6O24:0.01Ce3+/0.03Mn2+ phosphors and the standard data of JCPDs card no.

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82-1119 (Ba9Sc2Si6O24). The XRD results indicate that the synthesized phosphors crystallize in the rhombohedral structure. There are no other diffraction peaks

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corresponding to impurities, suggesting that the ions have doped into the host entirely.

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Ba9Lu2Si6O24 exhibits a single layer of rhombohedral structure in which larger octahedral LuO6 units are linked by smaller tetrahedral SiO4 units and the LuO6 octahedra are arranged in a nearly hexagonal array sharing corners with the SiO4

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tetrahedra [32]. There are one site of Lu coordinated with 6 oxygen atoms, one site of Si coordinated with 4 oxygen atoms and three sites of Ba coordinated with 12, 9, 10 oxygen atoms. The ionic radii are 0.861 Å for Lu3+ (CN = 6) and 0.26 Å for Si4+ (CN = 4). The ionic radii of Ba2+ are 1.48 Å (CN = 9), 1.52 Å (CN = 10) and 1.61 Å (CN = 5

12), respectively. The ionic radii of Ce3+ are 1.01 Å (CN = 6), 1.196 Å (CN = 9), 1.25 Å (CN = 10) and 1.34 Å (CN = 12), respectively. The ionic radius of Mn2+ is 0.83 Å (CN = 6). On the basis of ionic radii of the doped ions, Mn2+ ions substitute Lu3+

sites in Ba9Lu2Si6O24 host. But it is difficult to confirm the substitution of Ce3+ ions.

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Ce3+ ions possibly substitute Lu3+ and Ba2+ sites in Ba9Lu2Si6O24 host. Fig.2 exhibits the excitation and emission spectra of Ba9Lu2Si6O24:0.10Ce3+ and

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Ba9Lu2Si6O24:0.03Mn2+ phosphors. As shown in Fig.2A, the excitation spectrum of

Ba9Lu2Si6O24:0.10Ce3+ monitoring at 495 nm consists of several excitation bands in

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the range of 200-450 nm. The strongest excitation peaks at about 400 nm. Under the

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excitation at 400 nm, Ba9Lu2Si6O24:0.10Ce3+ gives green emission band peaking at

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about 495 nm. The excitation spectrum of Ba9Lu2Si6O24:0.10Ce3+ monitoring at 422

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nm shows two obvious excitation bands, as shown in Fig.2B. The stronger excitation

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peaks at about 350 nm. Under the excitation at 350 nm, Ba9Lu2Si6O24:0.10Ce3+ shows an emission band with a peak at about 422 nm. The luminescence of

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Ba9Lu2Si6O24:0.10Ce3+ suggests that Ce3+ ions substitute both Lu3+ and Ba2+ in

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Ba9Lu2Si6O24 host. The Ce3+ located in Lu3+ sites should have a larger crystal field strength and the longer wavelength emission of 495 nm due to the shorter Lu–O bond length, while Ce3+ located in Ba2+ sites should have the shorter wavelength emission

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of 422 nm [33]. Fig.3 presents the emission spectra of Ba9Lu2Si6O24:xCe3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10 and 0.12) phosphors under the excitation at 350 nm. The Ce3+ emission intensity shows the obvious dependence on Ce3+ concentration. The emission intensity increases with the increasing Ce3+ concentration up to x = 0.10. 6

The emission intensity decreases if the Ce3+ concentration is further increased. The decrease of emission is induced by the concentration quenching. Fig.2C shows the excitation and emission spectra of Ba9Lu2Si6O24:0.03Mn2+. The d-d transitions of Mn2+ ions are spin and parity forbidden, which induces the

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weak intensities of excitation and emission bands [34]. As shown in Fig.2, the excitation spectrum of Ba9Lu2Si6O24:0.03Mn2+ overlaps with the emission spectrum

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of Ba9Lu2Si6O24:0.10Ce3+ excited by the light with the wavelength of 350 nm but

does not overlap with the emission spectrum of Ba9Lu2Si6O24:0.10Ce3+ excited by the

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light with the wavelength of 400 nm. For phosphors with sensitizer and activator, the

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effective energy transfer occurs when the emission spectrum of sensitizer overlaps

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with the excitation spectrum of activator [35]. These results suggest the energy

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transfer from Ce3+ to Mn2+ possibly occurs only when the Ce3+/Mn2+ codopded

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Ba9Lu2Si6O24 phosphors are excited by the light with the wavelength of 350 nm. To confirm the energy transfer from Ce3+ to Mn2+ in Ce3+/Mn2+ codopded

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Ba9Lu2Si6O24 phosphors, a series of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0.01, 0.02,

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0.03, 0.04 and 0.05) phosphors were synthesized. The emission spectra of the synthesized phosphors are provided in Fig.4. Under the excitation at 350 nm, the Ce3+/Mn2+ codopded Ba9Lu2Si6O24 phosphors show emission bands corresponding to

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Ce3+ and Mn2+ ions. The Ce3+ emission intensity decreases continuously with the increasing Mn2+ concentration, which indicates the energy transfer from Ce3+ to Mn2+ in Ba9Lu2Si6O24:0.10Ce3+/yMn2+ phosphors. Due to the energy transfer, the Mn2+ emission

intensity

increases

continuously. 7

The

emission

intensity

of

Ba9Lu2Si6O24:0.10Ce3+/0.05Mn2+ is lower than Ba9Lu2Si6O24:0.10Ce3+/0.04Mn2+, which is induced by the concentration quenching. The energy transfer was also confirmed by the decay curves of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03,

0.04

and

0.05)

phosphors.

Fig.5

shows

the

decay

curves

of

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Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors monitored at 422 nm upon the 350 nm excitation. It can be seen that the decay curves

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fit well with a second-order exponential decay mode of I = A1 exp⁡(−t⁄τ1 ) +

A2 (exp⁡−t⁄τ2 ), where I is the emission intensity, A1 and A2 are constants, t is the time,

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τ1 and τ2 are the rapid and slow lifetimes. The average lifetime (τ) can be calculated

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by the formula of τ = (A1 τ12 + A2 τ22 )⁄(A1 τ1 + A2 τ2 ) . The calculated average

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lifetimes of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.5)

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phosphors are shown in Table 1. The decay times of Ce3+ emission decreases

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continuously with the increasing Mn2+ concentration. This strongly demonstrates the occurrence of energy transfer from Ce3+ to Mn2+. Energy transfer efficiency (η) can be

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obtained from the decay lifetime by using the equation of η = 1 − τ⁄τ0 , where τ

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and τ0 are the lifetimes of sensitizer (Ce3+) with and without the presence of activator (Mn2+). The η values of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0.01, 0.02, 0.03,

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0.04 and 0.05) phosphors are shown in Table 1. On the basis of luminescent properties of Ba9Lu2Si6O24:Ce3+/Mn2+ phosphor, the

possible energy transfer process was speculated. Under the excitation at 350 nm, electrons of Ce3+ were pumped to a higher component of 5d level. Then, the pumped electrons relaxed to the lowest 5d crystal field splitting state by a nonradiative way. 8

Due to similar energy values between the lowest 5d level of Ce3+ with the 4T2(4D) level of Mn2+, part of energy of Ce3+ electrons was transferred to Mn2+. In a nutshell, part of Ce3+ electrons relaxed to the 4f ground state with a result of emission band peaking at about 442 nm and the excited Mn2+ by the transferred energy from part of

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Ce3+ electrons relaxed to the 6A1(6S) ground state with a result of emission band peaking at about 613 nm. The energy transfer process and relevant optical transitions

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are shown in Fig.6.

The increasing energy transfer efficiency from Ce3+ to Mn2+ leads to the changes

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of Ce3+ and Mn2+ emission intensities. Thus, tunable luminescence was obtained in

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Ce3+/Mn2+ codopded Ba9Lu2Si6O24 phosphors. Fig.7 and Table 1 give the Commission

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International de I’Eclairage (CIE) coordinates of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0,

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0.01, 0.02, 0.03, 0.04 and 0.05) phosphors. As shown in Fig.7, the CIE coordinates of

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Ba9Lu2Si6O24:0.10Ce3+/yMn2+ phosphors move from blue region through white region to orange region with the increasing Mn2+ concentration. This is induced by the

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decrease of Ce3+ emission intensity and the increase of Mn2+ emission intensity as we

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vary the Mn2+ concentration from y = 0 to 0.05. The CIE coordinates of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ phosphors locate in the white region when y = 0.02, 0.03 and 0.04. This indicates that white light can be generated by controlling

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Ce3+/Mn2+ concentrations in Ba9Lu2Si6O24 phosphors. The absolute quantum efficiencies of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors are also provided, as shown in Table 1. The fast decrease of quantum efficiency for Ba9Lu2Si6O24:0.10Ce3+/0.05Mn2+ is induced by the concentration 9

quenching of Mn2+ [36]. The thermal stability of phosphor is one of the most important technological parameters for the applications. Temperature dependence of the integrated emission intensity for Ca3La6(SiO4)6:0.10Ce3+/0.03Mn2+ phosphor is shown in Fig.8. The

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integrated emission intensity gradually decreases as the temperature increases from 25 ºC to 250 ºC. The thermal quenching originates from the temperature-dependent

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electron–phonon interaction. It can be observed in Fig.8 that the emission intensity at

175 ºC drops to 55.2% of the initial intensity at 25 ºC. This suggests the excellently

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thermal stability of the phosphors.

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4. Conclusion

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We prepared a series of Ce3+ and/or Mn2+ doped Ba9Lu2Si6O24 phosphors by the

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solid state reaction. From the excitation and emission spectra of Ba9Lu2Si6O24:Ce3+

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and Ba9Lu2Si6O24:0Mn2+ phosphors, we speculated the energy transfer from Ce3+ to Mn2+ when Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors were excited at 350 nm. The

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luminescent properties of Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors confirm the energy

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transfer. The emission intensities of Ce3+ and Mn2+ ions change with the changing Ce2+/Mn2+ concentrations due to the energy transfer. This leads to the tunable luminescence of Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors. The white light with the CIE

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coordinates of (0.335, 0.259) and the CCT of 5088 K is generated by Ba9Lu2Si6O24:0.10Ce3+/0.03Mn2+ phosphor. The tunable luminescence of the Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors indicate that they have potential applications in light emitting diodes. 10

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white LED applications, J. Mater. Sci. 52 (2017) 10927-10937.

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single-phase full-color emitting Ba9Lu2Si6O24:Ce3+/Mn2+/Tb3+ phosphors for

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34. M. Zhao, Z. Zhao, L. Yu, L. Yang, J. Jiang, X. Li, G. Li, Single phased Sr3La(PO4)3:Eu2+/Mn2+ phosphors: solid state synthesis, tunable luminescence and

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potential applications in white light LEDs, J. Mater. Sci.: Mater. Electron. 29

N

(2018) 1832-1836.

A

35. Y. Yang, B. Liu, Y. Zhang, X. Lv, L. Wei, X. Wang, Fabrication and luminescence

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of BiPO4:Tb3+/Ce3+ nanofibers by electrospinning, Superlattices Microstruct. 90

ED

(2016) 227-235.

36. D. Zeng, Y. Chen, Y. Cai, C. Peng, S. Peng, Synthesis and luminescence of Pr3+

A

CC E

PT

doped Lu2MoO6 phosphors, J. Lumin. 206 (2019) 376-379.

15

Figure

1

XRD

patterns

Ba9Lu2Si6O24:0.10Ce3+,

of

Ba9Lu2Si6O24:0.03Mn2+,

Ba9Lu2Si6O24:0.01Ce3+/0.03Mn2+ phosphors and the standard data of JCPDs card no.

M

A

N

U

SC R

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82-1119

Figure 2 Excitation and emission spectra of Ba9Lu2Si6O24:0.10Ce3+ and

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CC E

PT

ED

Ba9Lu2Si6O24:0.03Mn2+ phosphors

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Figure 3 Emission spectra of Ba9Lu2Si6O24:xCe3+ (x = 0.02, 0.04, 0.06, 0.08, 0.10 and

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U

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0.12) phosphors

A

Figure 4 Emission spectra of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0.01, 0.02, 0.03,

A

CC E

PT

ED

M

0.04 and 0.05) phosphors

Figure 5 Decay curves of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors

17

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N

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Figure 6 Energy levels and energy transfer process from Ce3+ to Mn2+ in

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ED

M

A

Ba9Lu2Si6O24:Ce3+/Mn2+ phosphors

Figure 7 CIE coordinates of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors 18

IP T integrated

Ca3La6(SiO4)6:0.10Ce3+/0.03Mn2+ phosphor

M ED PT CC E A

19

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Temperature-dependence

emission

U

of

N

8

A

Figure

intensity

for

Table 1 CIE coordinates, lifetimes, CCT values, energy transfer efficiencies (η) and quantum efficiencies of Ba9Lu2Si6O24:0.10Ce3+/yMn2+ (y = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) phosphors

0.162 0.246 0.301 0.335 0.371 0.403

0.031 0.143 0.232 0.259 0.265 0.272

122 115 101 76 58 39

CCT (K) η (%) Quantum efficiencies (%) 1882 / 41.7 7557 5.74 40.2 11626 18.19 38.8 5088 37.58 35.6 2766 55.24 31.3 1934 68.88 21.5

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0 0.01 0.02 0.03 0.04 0.05

Lifetimes (ns)

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CIE (x, y)

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CC E

PT

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A

N

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y values

20