Synthesis and luminescence properties of Li2SnO3:Mn4+ red-emitting phosphor for solid-state lighting

Synthesis and luminescence properties of Li2SnO3:Mn4+ red-emitting phosphor for solid-state lighting

Journal of Alloys and Compounds 704 (2017) 124e130 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:...

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Journal of Alloys and Compounds 704 (2017) 124e130

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis and luminescence properties of Li2SnO3:Mn4þ red-emitting phosphor for solid-state lighting Renping Cao a, *, Wudi Wang a, Jinlong Zhang a, Shenhua Jiang b, Zhiquan Chen a, Wensheng Li c, Xiaoguang Yu a a b c

College of Mathematics and Physics, Jinggangshan University, Ji'an, 343009, China College of Pharmacology and Life Science, Jiujiang University, Jiujiang, 332000, China Personnel Office, Jinggangshan University, Ji'an, 343009, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 June 2016 Received in revised form 31 January 2017 Accepted 8 February 2017

Novel Li2SnO3:Mn4þ red-emitting phosphor is synthesized by high-temperature solid-state reaction method in air. Emission band of Li2SnO3:Mn4þ phosphor within the range 620e760 nm is attributed to the 2 E / 4A2 electron transition of Mn4þ ion. Excitation bands peaking at 330, 395, and 480 nm are attributed to O2- / Mn4þ charge transfer and 4A2 / (4T1, 2T1, and 4T2) transitions of Mn4þ ion, respectively. The optimal Mn4þ concentration is ~0.4 mol% and the corresponding quantum efficiency and lifetime are ~42.3% and 16.3 ms. The influences of temperature and Mn4þ concentration to emission intensity are discussed and analysized. Lifetime data and time resolved spectra confirm that there is only a single type of Mn4þ ion luminescent center in Li2SnO3:Mn4þ phosphor. The influences of the crystal field and the “Mn4þligand” bonding to luminescence properties of Mn4þ ion are summaried and analysized. The luminous mechanism is explained via Tanabe-Sugano energy level diagram and the configuration coordinate diagram of Mn4þ in the octahedron. The results indicate that Li2SnO3:Mn4þ phosphor has a hope to be used as red phosphor candidate for solid-state lighting. © 2017 Elsevier B.V. All rights reserved.

Keywords: Optical materials Phosphors Mn4þ ion Luminescence properties Red-emitting

1. Introduction Due to the continuous growth of population and fast development of industrialization, energy and environmental are becoming the serious challenge. Many countries are making many policies for energy saving and explore the new alternative and environmentfriendly energy. White light-emitting diodes (LEDs), which are recognized as the next-generation of solid-state lighting to replace the fluorescent lamps, have been attracted much attention because of energy-saving, mercury pollution-free, life-durable, low operating voltage, good stability, and environment friendly [1e5]. It is well known that LED chip-phosphor system is the mainstream preparation method of white LEDs, such as the blue LED chip (450e480 nm) with red and green phosphors, the combination of a near ultraviolet (UV) LED (380e420 nm) with three kind of phosphors (red, green, and blue), and the combination of a blue LED (450e480 nm) with a yellow Y3Al5O12:Ce3þ (YAG:Ce) phosphor

* Corresponding author. E-mail address: [email protected] (R. Cao). http://dx.doi.org/10.1016/j.jallcom.2017.02.079 0925-8388/© 2017 Elsevier B.V. All rights reserved.

[6e8]. In the first and two methods, red phosphor is one of important raw materials in white LEDs. In third method, white LEDs show a higher correlated color temperature and a lower color rendering index because of the lack of red color contribution. Therefore, novel efficient red-emitting phosphors for white LEDs are considered to be one of important research objects. Up to now, Rare-earth activators are most widely used in phosphors for energy efficient lighting and have been reported widely, such as Y2O3:Eu3þ, Y2O2S:Eu3þ, and Eu2þ or Ce3þ doped nitride [9e12]. However, their applications in white LEDs are limited because of some reasons, such as the sharp absorption peaks in UV and blue region, the instability of sulfides, and the very harsh synthesize conditions (e.g., at ~ 1800  C, 10 atm, and N2 atmosphere). Therefore, it is the urgency of developing rare-earth-free phosphors for lighting and other optoelectronic applications. Recently, Bi2þ doped red phosphors have also been reported [13,14], but the doping hosts are limited. Mn4þ ion (3d3) as a rare-earth-free activator belongs to one of transition metal ions and can be stabilized in an octahedral environment and give complicated optical spectra in different crystalline field [15,16]. Mn4þ doped materials, which have broad absorption

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spectrum in the range of 220e600 nm and show red-emitting within the range 600e780 nm, have been extensively studied and reported for lighting, holography, optical data storage, laser, and dosimetry [15,17]. In 1947, the emission of Mn4þ doped magnesium germanate red phosphor is reported by Williams [18]. Later, red emitting 3.5MgO$0.5MgF2$GeO2:Mn4þ phosphor used as commercial red phosphor is reported by Kemeny et al., in 1960 [19]. In recent years, Mn4þ-doped phosphors have received great interest and been reported widely because of the excellent luminescent properties and chemical stability, such as CaAl2O4:Mn4þ, CaYAlO4, SrAl4O7:Mn4þ, Rþ (Rþ ¼ Liþ, Naþ, and Kþ), Sr2MgAl22O36:Mn4þ, Sr4Al14O25:Mn4þ, K2TiF6:Mn4þ, KNaSiF6, Li2MgTiO4, Ca14Zn6M10O35:Mn4þ (M ¼ Al3þ and Ga3þ), Mg7Ga2GeO12, Li3Mg2NbO6, NaLaMgTeO6, Y2Ti2O7:Mn4þ, and Ba2LaNbO6:Mn4þ [20e33]. Mn4þ ion can usually substitute in the octahedral crystal system. In an octahedral environment, the Mn-3d states are split into three- and two-fold degenerate t2g and eg states, respectively [34]. So, the host including octahedron is one of important influence factors to luminescence properties of Mn4þ ion. Li2SnO3 can be synthesized easily with cheap raw materials and contains [SnO6] octahedron. Mn4þ ion can replace the Sn4þ ion site after it is doped. Up to now, the luminescence properties of Li2SnO3:Mn4þ phosphors have seldom been reported. Therefore, Li2SnO3:Mn4þ phosphor is selected as the research object in this work. In this work, novel Li2SnO3:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. The crystal structures, quantum efficiency (QE), fluorescence lifetimes, time resolved spectra, and luminescence properties are investigated. The influences of Mn4þ concentration and temperature to emission intensity are discussed and analysized. The influences of the crystal field and the “Mn4þ-ligand” bonding to luminescence properties of Mn4þ ion are summaried and analysized. The luminous mechanism is explained via Tanabe-Sugano energy level diagram and the configuration coordinate diagram of Mn4þ in the octahedron. The possible applied experiment of Li2SnO3:Mn4þ phosphor in LED device is demonstrated. 2. Experimental procedure 2.1. Experiment All the raw materials are purchased from the Aladdin Chemical Reagent Company in Shanghai, China, such as, Li2CO3 (A.R. 99.5%), MnCO3 (A.R. 99.9%), and SnO2 (99.95%). A series of Li2Sn(14þ (x ¼ 0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%) phosphors are x)O3:xMn synthesized by high temperature solid-state reaction method in air. The stoichiometric raw materials are well grounded in an agate mortar without further purification. The mixtures are placed in an alumina boat, heated to 650  C for 5 h, and allowed to cool to room temperature in air. The obtained product is ground again in order to improve the homogeneity and reheated for 10 h at 1100  C in air. All products are obtained after natural cooling to room temperature. In order to study the potential application, LED devices are fabricated by an encapsulation technology in the lab. The phosphors are dispersed into silicone resin and coated onto a blue (~460 nm) chip. 2.2. Characterization The crystal structures of all phosphors are checked at room temperature by X-Ray Powder Diffraction (XRD) (Philips Model PW1830) with Cu-Ka radiation at 40 kV and 40 mA. The XRD patterns data are collected in the 2q range of 10e90 at room temperature. The morphology of Li2SnO3:Mn4þ phosphor was characterized by using Field emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 430). Luminescence properties,

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time resolved luminescence spectra, and fluorescence lifetimes are investigated by using a steady-state FLS980 spectrofluorimeter (Edinburgh Instruments, UK, Edinburgh) with a high spectral resolution (signal to noise ratio > 12000:1) at room temperature. The QE is measured directly by using FLS980 spectrofluorimeter with integrating sphere at room temperature. A 450 W ozone free xenon lamp is used for steady-state measurements. A microsecond pulsed xenon flash lamp mF900 with an average power of 60 W is available to record the emission decay curves for lifetimes. 3. Results and discussion 3.1. Crystal structure analysis The unit cell of Li2SnO3 drawn on the basis of the Inorganic Crystal Structure Database (ICSD) #21032 is shown in Fig. 1. Li2SnO3 is described as the monoclinic crystal system with space-group C2/c (no. 15) and the lattice parameters a ¼ 5.2950 Å, b ¼ 9.1840 Å, c ¼ 10.0320 Å, v ¼ 480.24 Å3, and z ¼ 8 [35]. In host Li2SnO3 lattice, there are two types of layers perpendicular to c*, one with Liþ only and one with Liþ and Sn4þ in the ratio 1:2 [36]. Mn4þ ion can usually substitute in the pure octahedral, distorted octahedral symmetry, and invariably oxygen coordinated with six nearest neighbor crystal system [15]. Sn atom is coordinated by six oxygen atoms and forms [SnO6] octahedron, which offers good chance to accept Mn4þ substitute. So, Mn4þ ions can replace the Sn4þ ions sites in host Li2SnO3 lattice owing to their similar ionic radii (Sn4þ: ~0.71 Å and Mn4þ: ~0.54 Å) [37]. XRD patterns of Joint Committee on Powder Diffraction Standards (JCPDS) card no. 31e761 (Li2SnO3), blank Li2SnO3, and Li2SnO3:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphors are shown in Fig. 2. The XRD patterns of these samples can all be matched well with the standard data of JCPDS card (no. 31e761). The XRD patterns of other Li2SnO3:xMn4þ (0  x  1 mol%) phosphors are not displayed in Fig. 2, but those patterns are also in line with those of JCPDS card (no. 31e761). No other crystalline phase is formed after Mn4þ ions are added. XRD patterns confirm that the doping of Mn4þ into host lattice does not cause significant influence on the Li2SnO3 crystal structure. However, after Mn4þ ions are doped and replace the Sn4þ ions sites in host Li2SnO3 lattice, the dominated XRD diffraction peaks of the Mn4þ-doped samples compared to that of host Li2SnO3 have a gradual shift towards higher 2 theta angles with increasing Mn4þ concentration, which are illustrated more clearly in Fig. 2. The change is caused by the different ionic radii between Sn4þ and Mn4þ ions (Sn4þ: ~0.71 Å and Mn4þ: ~0.54 Å). The result indicates that a contraction of the lattice cell according to Bragg equation (2dsinq ¼ l, where d is the spacing between the planes in the atomic lattice, l is the wavelength of the X-ray, and q is diffraction angle between the incident ray) owing to the substitution of Mn4þ ions for Sn4þ ions in host Li2SnO3 lattice. According to FE-SEM image in the inset, the particles size value is in the range of 400e600 nm. 3.2. Luminescence properties analysis Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of Li2SnO3:0.4%Mn4þ phosphor at room temperature and the Commission Internationale De I'eclairage (CIE) chromaticity coordinates are shown in Fig. 3. Host Li2SnO3 with excitation 330 nm does not show emission. PL bands of Mn2þ-doped materials usually are shown in the range of 450e610 nm, and Mn3þ, Mn5þ, or Mn6þ ion-doped materials emit near-infrared light [15]. So, we speculate that PL of Li2SnO3:0.4%Mn4þ phosphor is derived from the Mn4þ ion. Li2SnO3:0.4%Mn4þ phosphor with excitation 330 and 480 nm emits deep red light, the corresponding chromaticity

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Fig. 1. The unit cell of Li2SnO3 drawn on the basis of ICSD #21032.

Fig. 2. XRD patterns of JCPDS card no. 31e761 (Li2SnO3), blank Li2SnO3, and Li2SnO3:xMn4þ (x ¼ 0.2, 0.6, and 1.0 mol%) phosphors (left), and the enlarged figure in the 2q range of 17.75e18.25 (right). The inset: FE-SEM image of the Li2SnO3:0.4%Mn4þ phosphor.

of Mn4þ ion, respectively [41e43], which indicate that Li2SnO3:Mn4þ phosphor may be excited by (near) UV and blue LED chips. According to the inset in Fig. 3(c), Li2SnO3:0.4%Mn4þ phosphor under 365 nm UV lamp emits red light, can be excited by blue (~460 nm) LED chip, and has a potential application prospect as a red phosphor candidate for white LEDs based on blue chip. PL spectra of Li2SnO3:xMn4þ (0  x  1.0 mol%) phosphors with excitation 330 nm at room temperature are shown in Fig. 4. All PL spectra shapes and peak positions are the same with changing Mn4þ concentration except PL intensity. PL intensity increases with increasing Mn4þ concentration in the range of 0.2e0.4 mol%, and decreases with further increasing Mn4þ concentration. The former observation could be attributed to the distance between Mn4þ ions, and the intensity is proportional to the content of Mn4þ ion. The latter observation is presumably due to the concentration quenching of Mn4þ ions. Therefore, the optimal Mn4þ concentration in Li2SnO3:Mn4þ phosphor is about 0.4 mol%. The critical transfer distance (Rc) can be calculated via using the formula suggested by Blasse and Grabmaier [15]: Rc z 2[3V/(4pXcN)]1/3, where

Fig. 3. (a) PLE and (b) PL spectra of Li2SnO3:0.4%Mn4þ phosphor (lex ¼ 330 and 480 nm; lex ¼ 658 and 672 nm) at room temperature and (c) the corresponding CIE chromaticity coordinates. The inset: (1) The picture of Li2SnO3:0.4%Mn4þ phosphor under 365 nm UV lamp, (2) LED device based on blue (~460 nm) chip þ Li2SnO3:0.4%Mn4þ, and (3) LED device based on blue chip þ YAG:Ce þ Li2SnO3:0.4%Mn4þ. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

coordinates are (0.7239, 0.2761). PL band within the range 620e760 nm is assigned to the 2E / 4A2 electron transition of Mn4þ ion, and two PL band peaks (658 and 672 nm) are caused by the different lattice vibrations [38e40]. Monitored at 658 and 672 nm, PLE spectra cover the region from 220 to 600 nm and PLE bands peaking at 330, 395, and 480 nm are attributed to O2- / Mn4þ charge transfer (CT) and 4A2 / (4T1, 2T1, and 4T2) transitions

V is the unit cell volume of host lattice, Xc is the critical concentration, N is the number of sites available for the dopant in the unit cell. By taking the experimental and analytic values of V, Xc, and N (480.24, 0.004, and 8), the critical transfer distance of Mn4þ ions in Li2SnO3:Mn4þ phosphor is ~30.6 Å. So, we infer that the concentration quenching mainly occure via electric multipolar interactions between Mn4þ ions because the value of Rc is lager than

R. Cao et al. / Journal of Alloys and Compounds 704 (2017) 124e130

IðtÞ ¼ Ið0Þ expð  t=tÞ þ A

Fig. 4. PL spectra of Li2SnO3:xMn4þ (0  x  1.0 mol%) phosphors (lex ¼ 330 nm) at room temperature.

the typical distance 5 Å [44]. The QE of Li2SnO3:0.4%Mn4þ phosphor is about 43.3%. So, its QE and luminescence properties should be further improved in order to find the practical application in LEDs. The QE is measured directly via using FLS-980 spectrofluorimeter with integrating sphere. The QE can be calculated as follows [45]:



 Z Z Ldirect = Ewithout  Edirect

(1)

where, h is the quantum yield. Ldirect is the complete emission spectrum of the sample being measured, collected using the sphere. Edirect is the emission spectra of the excitation light, recorded with the sample in place, and collected using the sphere. Ewithout is the emission spectra of the excitation light, recorded with the equipment blank sample in place, and collected using the sphere. Decay curve of Li2SnO3:0.4%Mn4þ phosphor at room temperature is shown in Fig. 5. The monitoring wavelength is 658 nm with excitation 330 nm. The red curve is a fit of the experimental data to a first order exponential decay equation. The luminescence decay curve is well fitted by a first-order exponential function [15].

Fig. 5. Decay curve of Li2SnO3:0.4%Mn4þ phosphor at room temperature (lex ¼ 330 nm and lex ¼ 658 nm). The red curve is a fit of the experimental data to a first order exponential decay equation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(2)

where, I(t) is the luminescence intensity at time t, I(0) is the initial luminescence intensity, A is the value for different fitting, and t is the decay time for the exponential components. All decay curves of Li2SnO3:xMn4þ phosphors (0.2  x  1.0 mol%) can be fitted by first-order exponential function and their luminescence lifetimes are ~ 16.5, 16.3, 16.1, 15.9, and 15.8 ms, respectively. The result shows that lifetime has little change with increasing Mn4þ ion concentration from 0.2 to 1.0 mol%. Fig. 6 shows time resolved spectra of Li2SnO3:0.4%Mn4þ phosphor at room temperature. The excitation wavelength is 330 nm and the PL scanned area is from 620 to 750 nm. PL spectra shapes and peak positions are the same except their PL intensity with changing delay time. According to lifetime data in Fig. 5 and time resolved spectra in Fig. 6, we infer that there is only a single type of Mn4þ ion luminescent center in Li2SnO3:Mn4þ phosphor. In order to evaluate the potential application in solid-state lighting, it is important to measure the thermal quenching (TQ) behavior. The relation between temperature and PL intensity of Li2SnO3:0.4%Mn4þ phosphor with excitation 330 nm is shown in Fig. 7. PL intensity decreases with increasing temperature from 25 to 250  C. To further understand the TQ characteristics, the activation energy DE may be calculated by the Arrhenius equation [27,46].

i h IT ¼ I0 = 1 þ AeðDE=kTÞ Z

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(3)

where I0 is the initial PL intensity, IT is the PL intensity at temperature T, k is the Boltzmann constant (8.629  105 eVK1), and A is a constant. It is calculated that the value of DE is about 0.32 eV. The TQ mechanism may be analyzed by the configuration coordinate diagram shown in Fig. 8. 3.3. Luminous mechanism analysis Now we analysize the luminous mechanism of Li2SnO3:Mn4þ phosphor via Tanabe-Sugano energy level diagram and the configuration coordinate diagram of Mn4þ in Li2SnO3:Mn4þ phosphor shown in Fig. 8. The energetic structure of Mn4þ ion with 3d3 electronic configuration may be well described by standard crystalfield theory. The most important free ion states in Mn4þ ion are 4F ground level and 2H excited level. It is well known tha Mn4þ ion can substitute in the octahedral symmetry. The 4F level can split into

Fig. 6. Time resolved spectra of Li2SnO3:0.4%Mn4þ phosphor at room temperature (lex ¼ 330 nm).

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R. Cao et al. / Journal of Alloys and Compounds 704 (2017) 124e130 Table 1 The main PL and PLE band peak of Mn4þ ion in different host lattices.

Fig. 7. PL spectra of Li2SnO3:0.4%Mn4þ phosphor with excitation 330 nm above room temperature (25e250  C).

the ground state 4A2 and the excited states 4T2 and 4T1 because of the influence of octahedral field [15]. The 2E, 2T1, 2T2 and 4A2 levels are located in the t32 electronic orbital and the 4T1 and 4T2 levels are situated in the t22e orbital. The broad PLE or absorption bands of Mn4þ ions are derived from the spin-allowed transitions 4A2 / 4 T1,2 of Mn4þ ion. The 2E / 4A2 transition is spin-forbidden, however, luminescence can appear due to the admixture of the 4 T2 level state to the 2E level state through spinorbital interaction [47,48]. PL band contain several broad emission lines originating from the 2E / 4A2 transition aided by lattice vibrations and PL band peaks can be shown in the region from 620 to 750 nm. The energy of the 2E / 4A2 transition is strongly dependent on the covalence of the “Mn4þ - ligand” bonding and crystal field strength, which can be changed by changing the constitution (cations and anions) of the host lattice [17]. Table 1 shows PL and PLE band peak positions of Mn4þ ion in different host lattices, which are helpful to analysize the influences of the crystal field and the “Mn4þ-ligand” bonding to luminescence properties of Mn4þ ion. According to Table 1, PL band peak appears in the region from 630 to 730 nm and PLE band peaks are shown in the range 270e510 nm. The polyhedral around central atoms, (e.g., Ti, Sn, Al, Si, Zn, Te, Ta, and Ge) are octahedral structure, which is conducive to Mn4þ ions instead of those cations. In the Mn4þ-doped materials, PL and PLE band peak positions are

The host

PLE peak (nm)

PL peak (nm)

Ref

CaAl2O4 CaYAlO4 LaAlO3 SrMgAl10O17 Sr2MgAl22O36 K2TiF6 KNaSiF6 Li2TiO3 Li2MgTiO4 LiGaTiO4 MgO$GeO2 Li2MgGeO4 Mg7Ga2GeO12 Ca14Al10Zn6O35 Ba2LaNbO6 Li3Mg2NbO6 NaLaMgTeO6 La2LiTaO6 Y2Sn2O7 Li2SnO3

325 335, 275, 323 312 350, 350, 350 330, 330, 325 323 310, 310, 380, 310, 320, 330, 361 330,

658 710 730 661 658 630 630 680 676 668 660 671 660 700 683 670 700 707 670 658

[20] [21] [50] [42] [23] [25] [26] [51] [27] [52] [18] [53] [30] [28] [33] [29] [31] [54] [32] This work

470 325

460 460 476 480

420 460 507 470 500 495 480

different owing to the influences of their ionic radii and ionic banding force [32,49]. In Li2SnO3:Mn4þ phosphor, electrons absorb energy and are raised from the ground state 4A2 to the excited states 4T2 and 4T1, the excited electrons then relax to the lower level states and populate the excited state 2E by nonradiative process, finally the 2E / 4A2 electron transition can occur, photon energy is released, thus the phosphor emits red light. If temperature is increased, thermal excitation may lead to part electrons reach the O by paths 1, 2 and 3 in Fig. 8(b). So, PL intensity decreases with decreasing photon energy. The result may be used to explain the relation between temperature and PL intensity in Fig. 7. In addition, the energy between valence band and conduction band in host lattice may transfer to the electrons in high excited states in Li2SnO3:Mn4þ phosphor. 4. Conclusions In summary, novel Li2SnO3:Mn4þ phosphor is synthesized by high-temperature solid-state reaction method in air. XRD patterns confirm that all samples have a single pure phase Li2SnO3. PL band

Fig. 8. (a) Tanabe-Sugano energy level diagram of Mn4þ in the octahedron, (b) The configuration coordinate diagram of Mn4þ in Li2SnO3:Mn4þ phosphor.

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of Li2SnO3:Mn4þ phosphor with excitation 330 and 480 nm is attributed to the 2E / 4A2 electron transition of Mn4þ ion within the range 620e760 nm. PLE bands peaking at 330, 395, and 480 nm are attributed to O2- / Mn4þ CT and 4A2 / (4T1, 2T1, and 4T2) transitions of Mn4þ ion, respectively. The optimal Mn4þ concentration is ~0.4 mol% and the corresponding QE and lifetime are ~42.3% and 16.3 ms, respectively. The influences of Mn4þ concentration and temperature to PL intensity are discussed and analysized. Lifetime data and time resolved spectra confirm that there is only a single type of Mn4þ ion luminescent center in Li2SnO3:Mn4þ phosphor. The influences of the crystal field and the “Mn4þ-ligand” bonding to luminescence properties of Mn4þ ion are summaried and analysized. The luminous mechanism is explained via TanabeSugano energy level diagram and the configuration coordinate diagram of Mn4þ in the octahedron. The experimental results indicate that Li2SnO3:Mn4þ phosphor has a potential application as red phosphor candidate for solid-state lighting. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 11464021) and Natural Science Foundation of Jiangxi Province of China (No. 20151BAB202008). References [1] Z. Zhang, C. Li, C. Han, J. Li, Y. Jia, D. Wang, High-brightness Sm3þ-doped La0.67Mg0.5W0.5O3 red phosphor for NUV light-emitting diodes application, J. Alloys Compd. 654 (2016) 146e150. [2] Z. Yang, P.C. Lin, C.F. Guo, W.R. Liu, Color-tunable luminescence and energy transfer properties of Eu2þ- and Mn2þ-activated BaCa2MgSi2O8 phosphor for ultraviolet light-emitting diodes, RSC Adv. 5 (2015) 13184e13191. [3] F. Xiong, D. Guo, H. Lin, L. Wang, H. Shen, W. Zhu, High-color-purity redemitting phosphors RE2WO6:Pr3þ (RE ¼ Y, Gd) for blue LED, J. Alloys Compd. 647 (2015) 1121e1127. [4] C. Zeng, Y. Hu, Z. Xia, H. Huang, A novel apatite-based warm white emitting phosphor Ba3GdK(PO4)3F: Tb3þ, Eu3þwith efficient energy transfer for w-LEDs, RSC Adv. 5 (2015) 68099e68108. [5] W. Zhou, J. Han, X. Zhang, Z. Qiu, Q. Xie, H. Liang, S. Lian, J. Wang, Synthesis and photoluminescence properties of a cyan-emitting phosphor Ca3(PO4)2: Eu2þ for white light-emitting diodes, Opt. Mater 39 (2015) 173e177. [6] T. Murata, T. Tanoue, M. Iwasaki, K. Morinaga, T. Hase, Fluorescence properties of Mn4þ in CaAl12O19 compounds as red-emitting phosphor for white LED, J. Lumin 114 (3e4) (2005) 207e212. [7] T. Wang, X. Xu, D. Zhou, J. Qiu, X. Yu, Red phosphor Ca2Ge7O16:Eu3þ for potential application in field emission displays and white light-emitting diodes, Mater. Res. Bull. 60 (2014) 876e881. [8] L. Wang, H. Mi Noh, B. Kee Moon, B. Chun Choi, J. Hyun Jeong, J. Shi, Luminescent properties and energy transfer of Sm3þ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs, J. Alloys Compd. 663 (2016) 808e817. [9] S. Som, S. Das, S. Dutta, H.G. Visser, M.K. Pandey, P. Kumar, R.K. Dubey, S.K. Sharma, Synthesis of strong red emitting Y2O3:Eu3þ phosphor by potential chemical routes: comparative investigations on the structural evolutions, photometric properties and JuddeOfelt analysis, RSC Adv. 5 (2015) 70887e70898. [10] L. Yang, J. Wang, X. Dong, G. Liu, W. Yu, Synthesis of Y2O2S: Eu3þ luminescent nanobelts via electrospinning combined with sulfurization technique, J. Mater. Sci. 48 (2) (2013) 644e650. [11] H.S. Kim, K. Machida, T. Horikawa, H. Hanzawa, Luminescence properties of CaAlSiN3:Eu2þ phosphor prepared by direct-nitriding method using fine metal hydride powders, J. Alloys Compd. 633 (2015) 97e103. [12] J. Ruan, R. Xie, S. Funahashi, Y. Tanaka, T. Takeda, T. Suehiro, N. Hirosaki, Y. Li, A novel yellow-emitting SrAlSi4N7:Ce3þ phosphor for solid state lighting: synthesis, electronic structure and photoluminescence properties, J. Solid State Chem. 208 (2013) 50e57. [13] R. Cao, Y. Cao, T. Fu, S. Jiang, W. Li, Z. Luo, J. Fu, Synthesis and luminescence properties of novel red-emitting R3P4O13:Bi2þ (R ¼ Sr and Ba) phosphors, J. Alloys Compd. 661 (2016) 77e81. [14] L. Li, M. Peng, B. Viana, J. Wang, B. Lei, Y. Liu, Q. Zhang, J. Qiu, Unusual concentration induced antithermal quenching of the Bi2þ emission from Sr2P2O7: Bi2þ, Inorg. Chem. 54 (12) (2015) 6028e6034. [15] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag. BerlinHeidelberg, 1994. [16] T.M. Chen, J.T. Lou, Highly Saturated Red-emitting Mn(IV) Activated Phosphors and Method of Fabricating the Same, US 7,846,350 B2, Dec, 7, 2010. [17] M.H. Du, Chemical trends of Mn4þ emission in solids, J. Mater. Chem. C 2 (2014) 2475e2481.

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