Ba2YNbO6:Mn4+-based red phosphor for warm white light-emitting diodes (WLEDs): Photoluminescent and thermal characteristics

Optical Materials 70 (2017) 144e152

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Ba2YNbO6:Mn4þ-based red phosphor for warm white light-emitting diodes (WLEDs): Photoluminescent and thermal characteristics Anjie Fu, Qi Pang, Hong Yang, Liya Zhou* School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 April 2017 Received in revised form 7 May 2017 Accepted 16 May 2017

A commercial Mn4þ-doped phosphor with deep-red emission has attracted attention in recent years. This can provide red components in the fabrication of warm white light-emitting diodes (WLEDs). The Ba2YNbO6:Mn4þ phosphor is successfully synthesized through conventional solid-state reaction. The crystal structure, composition, and doping site were investigated using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectra, respectively. UVeVis absorption spectra with a broad band ranging from 275 nm to 600 nm demonstrate that it can be efficiently excited by UV/ blue light. The photoluminescence spectra exhibit a strong narrow red emission band peaking at 695 nm. The luminescence mechanism has been investigated by TanabeeSugano diagram. Nephelauxetic ratio, b1, crystal field strength, Dq, and Racah parameters, B and C, have been evaluated. Thermal characteristics in high and low temperatures have been studied, demonstrating its excellent thermal stability. The values of the activation energy, Ea, have been calculated to explain thermal quenching. © 2017 Elsevier B.V. All rights reserved.

Keywords: Mn4þ Red phosphor Luminescence Thermal stability

1. Introduction Rare earth-doped phosphors have been widely investigated and used in fluorescent lamps. However, the shortage in the supply of rare earth materials makes economical and practical alternative materials an urgent need. Commercial raw materials, especially nonrare-earth red phosphor, with high efficiency are desirable for the society [1e3]. Mn4þ ions can be the alternative activator in red phosphor and have been extensively studied. The non-rare-earth Mn4þ ions have drawn much attention, ascribing to its unique 3d3 electron configuration that can provide complicated optical spectra in different crystal field environments [4]. The 3d states of Mn4þ can split into threefold and twofold degenerate t2g and eg states; the larger energy gap of which is created by crystal field splitting [4]. Mn4þ ions can be doped into the host with an abundant octahedral conformation, such as [AlO6], [SiF6], and [TiO6], and replace the cation (Al, Si, Ti) to luminescence and stabilizing the þ4 oxidation states in these oxides [5]. The broad absorption region ranges from 280 nm to 550 nm, and the narrow red emission band ranges from 600 nm to 750 nm, which can compensate the insufficient red light of the LEDs based on the combination of indium gallium nitride blue

LED chips and yellow phosphors [6e8]. The emission wavelength of Mn4þ in oxides is longer than that in fluorides, and the variation of the emission energy in oxides is greater [4,9]. According to TanabeeSugano diagram [10,11], the emission band is ascribed to the spin- and parity-forbidden 2Eg/4A2g transition. The emission energy is independent of the crystal field splitting and highly affected by the Mneligand hybridization [4]. The first research on Mn4þdoped magnesium germanate red phosphors has been found by Williams in 1940s [12]. Afterward, 3.5MgO.0.5MgF2.GeO2:Mn4þ (in 1960), Sr2MgAl22O36:Mn4þ, CaAl2O4:Mn4þ, Y2Ti2O7:Mn4þ, 4þ 4þ 4þ Cs2TiF6:Mn , K2TiF6:Mn , and Sr4Al14O25:Mn have been reported [13e19]. Ba2YNbO6 with rock salt structure has abundant [NbO6] octahedrons that are very suitable for Mn4þ ions to incorporate into its structure. In this research, Ba2YNbO6:Mn4þ phosphors have been synthesized through high-temperature solid-state reaction. The structure of doped and undoped Ba2YNbO6 has been analyzed, and the optical properties of Mn4þ-doped Ba2YNbO6 phosphors are investigated. All these investigations demonstrate that the Ba2YNbO6:Mn4þ phosphors possess optimal qualities that meet the requirement for WLEDs. 2. Preparation and characterization

* Corresponding author. E-mail address: [email protected] (L. Zhou). http://dx.doi.org/10.1016/j.optmat.2017.05.028 0925-3467/© 2017 Elsevier B.V. All rights reserved.

Ba2YNbO6:Mn4þ phosphors were prepared by temperature solid-state reaction. Analytical reagent

highgrade

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chemicals, such as Ba2CO3, Nb2O5, Mn2CO3, and Y2O3 (99.99%) (HWRK Chemistry Co., Ltd. Beijing China), were weighed in a certain stoichiometric ratio and thoroughly mixed together in a mortar. After grinding, the powder was calcined at 1573 K for 8.5 h in air atmosphere. Finally, the samples were cooled to room temperature and ground for characterization. The sample phase was recorded through X-ray powder diffraction (XRD) using Cu Ka radiation at 40 kV and 40 mA on a RIGAKU D/max 2200 vpc X-ray diffractometer. Step scan was conducted over an angle range of 10 e90 with a step size of 0.02 . The morphology was studied with scanning electron microscopy (SEM; S-3400N, HITACHI, Japan). Fourier transform infrared (FT-IR) spectrum of the sample was measured on a Nicolet-IR 200 spectrometer in the range of 400e4000 cm1 using a KBr pellet technique. Raman spectra were gathered on a Raman/PL spectrometer (Horiba Jobin-Yvon, LabRAM HR). The UVevisible absorption spectra of the powders were recorded on a Cary 5000 UVevisible spectrophotometer. The excitation and emission spectra of the samples were measured using a fluorescence spectrometer (Hitachi F-4600, Japan) equipped with a 150-W xenon lamp as excitation source. The luminescence decay curve was obtained using an FLS920, and a 350-W xenon lamp was used as excitation source. The temperature-dependent photoluminescence (PL) spectra were recorded in a FLS-980 Edinburgh fluorescence spectrometer and measured between 77 K and 498 K. QE is measured by using FLS980 spectrofluorimeter with integrating sphere at room temperature. All the measurements were performed at room temperature.

145

microstructure nature of the particle is very suitable for use in solid state device to enhance the luminescence efficiency. The FT-IR spectra of Ba2YNbO6:0.005Mn4þ phosphor is shown in Fig. 3(a). Three peaks that originated from the atmospheric moisture within the sample can be observed. The absorption peak signed at 3450 cm1 is from OeH stretching vibration. Another two peaks located at 1635 and 1424 cm1 are from HeOeH bending vibration [20,21]. The vibration position at 554 cm1 corresponds to the symmetric NbeO stretching mode. The peaks located at 858 and 713 cm1 are from the asymmetric NbeO vibration absorption of NbO6 groups. The Raman spectra of Ba2YNbO6:xMn4þ (x ¼ 0.005, 0.01, 0.015, 0.02, 0.025) are shown in Fig. 3(b). Several Raman active modes can be observed: T2g(1), T2g(2), and A1g modes. A1g observed at ~810 cm1 is from the symmetric and asymmetric stretching vibrations of oxygen octahedrons (herein, NbO6), which were mainly confined in the short-range correlation of atoms and were not affected by the radius. T2g(2) detected at 373 cm1 is from the corresponding symmetric and asymmetric bending vibrations. T2g(1) located at 102 cm1 is from the translational motion of A-site cations (herein, Ba2þ), which have a close relationship with radius. T2g(1) is sensitive to A-site substitution, whereas A1g is sensitive to B-site (herein, Nb5þ) substitution [22]. The positions of T2g(1) and T2g(2) were nearly unchanged with increasing Mn4þ content, indicating that dopant did not affect the A-sites. However, the A1g position shifts toward lower frequency with increasing substitution level. Therefore, Mn4þ ions entered the Nb sites instead of Ba sites [23].

3. Result and discussion 3.1. Crystal structure 3.2. Photoluminescence properties The XRD pattern of Ba2YNbO6:xMn4þ (x ¼ 0, 0.001, 0.005, 0.01, 0.02) is shown in Fig. 1(a), comparing with the standard JCPDS card of Ba2CO3 (no.41-0373), Y2O3 (no.65-3178), Nb2O5 (no.18-0911), and Ba2YNbO6 (no.24-1042). All the diffraction peaks of samples are matched well with the JCPDS card of Ba2YNbO6, and no identical diffraction peak with raw materials is observed, which demonstrates that the purity of the samples and Mn4þ ions are well-doped into the host with no significant influence on the structure of Ba2YNbO6. The magnification of the peaks located at 29 e30.75 is shown in Fig. 1(b); a shift toward the higher angle can be observed because of the substitution of Nb5þ by the smaller Mn4þ ions. When the Nb5þ (CN ¼ 6, 0.64 Å) is replaced by Mn4þ (CN ¼ 6, 0.53 Å), the ions with smaller radii will lead to the shrinkage of the lattice, according to Bragg's law, 2dsinq ¼ nl. The shift to the higher angle can be explained. The structure of Ba2YNbO6 cell unit is shown in Fig. 1(c). BYN belongs to double perovskite oxides and has a cubic system with space group Fm-3m. The lattice parameters are a ¼ 8.44 Å, V ¼ 601.2 Å3, and Z ¼ 4. Y3þ or Nb5þ ions with eight O2þ ions around them construct the [YO6] or [NbO6] octahedron. Ba2þ ions fill in the cubic vacancies that are formed by the octahedron chains. The radii of the ions of Ba2YNbO6:Mn4þ are as follows: Ba2þ (CN ¼ 6, 1.35 Å; CN ¼ 7, 1.38 Å), Y3þ (CN ¼ 6, 0.90 Å; CN ¼ 7, 0.96 Å), Nb5þ (CN ¼ 6, 0.64 Å; CN ¼ 7, 1.10 Å), and Mn4þ (CN ¼ 6, 0.53 Å). According to the radius similar principle, Nb5þ ions are substituted by the Mn4þ ions in Ba2YNbO6:Mn4þ phosphor. Scanning electron microscopy (SEM) is conducted to characterize the sample morphology. The SEM micrographs of Ba2YNbO6:xMn4þ (x ¼ 0.005, 0.01, 0.02) phosphor are shown in Fig. 2. The image shows that the particles are agglomerated, and the formation of uniform crystals has an average diameter of approximately 1e2 mm. The morphology of the phosphors exhibits no significant change with the increase in Mn4þ content. Thus, the

The UVevisible absorption spectra of Ba2YNbO6: xMn4þ(x ¼ 0, 0.005, 0.015, 0.025) are shown in Fig. 4. The doped sample exhibited a broad absorption band ranging from 280 nm to 600 nm. With increasing Mn4þ concentration, the absorption band intensities also increase. The absorption edges between 280 and 320 nm are attributed to the charge transfer (CT) transition of Mn4þ. Two noticeable and one weaker absorption band centered at 365, 415, and 530 nm are from 4A2g/4T1g, 4A1g/4T2g, and 4 A2g/2T2g transition of Mn4þ, respectively [24]. Photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra are shown in Fig. 5(a). Monitored at 695 nm, the PLE spectrum exhibited a broad band ranging from 260 nm to 580 nm, indicating that the phosphor can be excited by UV (310e420 nm) and blue (420e480 nm) LED chip [25]. The band fitted by Gaussian curves can be divided into four peaks: the weakest band peaking at ~523 nm (19120 cm1) is from 4A2g/4T2g spin-allowed transition. Another two stronger peaks located at ~361 (27701 cm1) and ~381 nm (26247 cm1) are produced from the 4A2g/4T1g and 4A2g/2T2g transitions. Besides these three peaks, another one located at ~327 nm (30581 cm1) is ascribed to Mn4þ/O2þ CT transitions [7,26,27]. When excited at 350 nm, the sample presents a red emission band with two maximum peaks at 668 (14970 cm1) and 695 nm (14388 cm1), which are from spin-forbidden transition of Mn4þ: 2 Eg/4A2g in [MnO6]8- octahedral environment. Serious emission bands of Ba2YNbO6:xMn4þ (x ¼ 0.001, 0.005, 0.01, 0.015, 0.02, 0.025) are depicted in Fig. 5(b). The optimal concentration that emits the strongest emission peak is 0.005. With further increase in Mn4þ content, concentration quenching can be observed, which is attributed to the exchange interaction or multipoleemultipole interaction mechanism. To analyze this point, critical distance (Rc) must be calculated by the following equation [28]:

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Fig. 1. (a) X-ray powder diffraction patterns of Ba2YNbO6:xMn4þ (x ¼ 0.001, 0.005, 0.01, 0.015, 0.02) phosphors and the Ba2YNbO6 JCPDS standard pattern. (b) Magnified XRD curves in the range of 29e30.75 (c) Projection view of crystal structure of Ba2YNbO6 unit cell.

1
3V 3 Rc ¼ 2 ; 4pCN

(1)

where C is the critical doped concentration (C ¼ 0.005), V is the volume of the host lattice (V ¼ 601.2 Å), and N is the number of sites available for the dopant in the unit cell (N ¼ 4). Thus, Rc is ~19.3 Å. Exchange interaction is considered only when Rc is less than 5 Å. Thus, multipoleemultipole interaction is responsible for the

concentration quenching. According to Dexter theory, the type of interaction mechanism can be decided by the following equation [29]:

logðI=xÞ ¼ C  ðq=3ÞlogðxÞ;

(2)

where C is a constant, I is the luminescence intensity, and x is the Mn4þ ion concentration. q is an index of the electric multipolar character, where q ¼ 6, 8, and 10 and represent dipoleedipole (d-d),

A. Fu et al. / Optical Materials 70 (2017) 144e152

147

Fig. 2. SEM micrographs of Ba2YNbO6:Mn4þ phosphor particles at different Mn concentrations. (a) 0.005, (b) 0.01, (c) 0.02.

Fig. 4. UVevis absorption spectra of Ba2YNbO6: xMn4þ(x ¼ 0, 0.005, 0.015, 0.025).

Ba2GdNbO6:0.005Mn4þ, and Ba2YNbO6:0.005Mn4þ phosphors are depicted in Fig. 6 for comparison. These three kinds of phosphors show many similarities in luminescence properties due to the similar structure of the molecules. The excitation spectra are nearly ranging from 250 nm to 580 nm. The crystal field is distorted, and the main emission peak positions of different phosphors vary from each other because of the different radii among the Gd, La, and Y ions. With the same excitation at 360 nm, the main emission peak positions of these three kinds of phosphors exhibit a red shift. Moreover, the Mn4þ-doped Ba2LaNbO6 and Ba2GdNbO6 phosphors have been reported by Srivastava et al. [30,31]. The emission band of Ba2GdNbO6:Mn4þ phosphor is peaking at 676 nm. Besides, the emission spectra of Ba2LaNbO6:Mn4þ exhibit an emission band ranging from 635 nm to 700 nm and peaking at 681 nm. Herein, the main peak position of Ba2YNbO6:Mn4þ phosphor is located at 695 nm, which is in a longer range relative to another two phosphors. Fig. 3. (a) FT-IR spectra of Ba2YNbO6:0.005Mn4þ (b) Raman Ba2YNbO6:xMn4þ (x ¼ 0.005, 0.01, 0.015, 0.02, 0.025) phosphors.

spectras

of

dipoleequadrupole (d-q), and quadrupoleequadrupole (q-q) interaction, respectively. Fig. 5(c) shows the linear relationship between lg(I/x) and lg(x) in which the slope (q/3) ¼ ~1.83 is evaluated and q is approaching 6, thereby indicating that the dipoleedipole interaction is the main mechanism for concentration quenching. The luminescence spectras of Ba2LaNbO6:0.005Mn4þ,

3.3. Decay properties of Ba2YNbO6:Mn4þ phosphor The fluorescence decay curves of Ba2YNbO6:xMn4þ(x ¼ 0, 0.001, 0.005, 0.01, 0.015, 0.02) phosphors are shown in Fig. 7. The decay curve is well-fitted by the following double-exponential equation [32,33]:

I ¼ A1 expð  t=t1 Þ þ A2 expð  t=t2 Þ;

(3)

where I is the luminescence intensity, A1 and A2 are constants, t is the lifetime, and t1 and t2 are the lifetimes for exponential

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Fig. 6. Luminescence spectras and CIE coordinates of Ba2LaNbO6:0.005Mn4þ, Ba2GdNbO6:0.005Mn4þ, and Ba2YNbO6:0.005Mn4þ phosphors.

Fig. 7. The decay curves of Ba2YNbO6:xMn4þ (x ¼ 0.001, 0.005, 0.01, 0.015, 0.02) at room temperature.

Mn4þ ions [36]. With the excitation at 360 nm, the lifetimes decrease from 0.539 ms to 0.390 ms when the content of Mn4þ ions increase from 0.001 to 0.02, indicating the presence of a single type of Mn4þ ions; the concentration quenching is a contributing factor [37]. Fig. 5. (a) PLE spectra (lem ¼ 695 nm) and PL spectra (lex ¼ 350 nm) of Ba2YNbO6:Mn4þ (b) PL spectra of Ba2YNbO6:xMn4þ (x ¼ 0.001, 0.005, 0.01, 0.015, 0.02, 0.025) phosphors under 350 nm UV excitation (b) The relationship between lg(x) and lg(I/x) according to Eq. (2).

components. Thus, the average lifetimes (ts) can be calculated [34,35] as follows:



ts ¼ A1 t21 þ A2 t22

. ðA1 t1 þ A2 t2 Þ:

(4)

The lifetimes of Mn4þ-doped phosphors are in a microsecond range, which can be attributed to the intra-d-shell transitions of

3.4. Crystal field analysis and nephelauxetic effect As shown in Fig. 8(a), the TanabeeSugano diagram illustrates the corresponding electron transitions between the different energy levels of Mn4þ. By combining with the values of mean peak energy of 4A2g/4T1g, 4A2g/4T2g, and 2Eg/4A2g transitions, the crystal field parameter of Dq, B, and C for Mn4þ in octahedral field can be evaluated by the following equation [38]: Dq ¼ E(4A2g/4T2g)/10.

(5)

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149

3.5. Temperature dependence of the PL spectra To further investigate the vibronic structure of Mn4þ ions in the host, a series of PL spectra at low and high temperatures has been

Fig. 8. (a) TanabeeSugano diagram of Mn4þ ion (b) The energy dependence of Mn4þ Eg level on the b1 value according to E(2Eg) ¼ 8650.64 b1 þ 6487.21.

2

Dq 15ðx  8Þ ¼ 2 ; B x  10x     E 4 A2g /4 T 1g  E 4 A2g /4 T 2g : x¼ Dq   E 2 E/4 A2g B

¼

3:05C 1:8C þ 7:9  ; B Dq

(6)

(7)

(8)

where the values of Dq, B, and C are 1912, 869, and 3314 cm1, respectively. The 2Eg state is barely affected by the crystal field, and the corresponding 2Eg/4A2g transition is highly dependent on the nephelauxetic effect in a different host [39]. Thus, the nephelauxetic ratio (b1) can be determined by the following equation [40]:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2 B C ; b1 ¼ þ B0 C0

(9)

where B0 and C0 are the Racah parameters for free ions (B0 ¼ 1160 cm1, C0 ¼ 4303 cm1). b1 is 1.0744. According to the different values of 2Eg energy level in different hosts and the corresponding b1, the linear relationship and the equation E(2Eg) ¼ 8650.64 b1 þ 6487.21 can be obtained (Fig. 8(b)), and the emission peak positions of different Mn4þ-doped phosphors can be determined.

Fig. 9. (a) Temperature-dependent PL spectra of Ba2YNbO6:Mn4þ at low temperature (77e298 K) (b) The PL spectra of Ba2YNbO6:Mn4þ above room temperature (298e498 K) (c) linear relationship between ln(I0/I1) and 1/kT according to Eq. (11) and the configurational coordinate diagram of Mn4þ.

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Fig. 10. The CIE coordinates profile of Ba2YNbO6:0.005Mn4þ.

A. Fu et al. / Optical Materials 70 (2017) 144e152

analyzed in Fig. 9. As shown in Fig. 9(a), with the excitation at 355 nm and increase in temperature from 77 K to 298 K, the emission intensities decrease gradually, which is ascribed from the predominance of the non-radiative transition in higher temperatures. The bands also exhibit a red shift and become broader, which results from the unit cell expansion and vibration mode enhancement [41]. With increasing temperature, the anti-Stokes emission intensities increase dramatically because of the population and vibration transitions of the absorbed photons previously restricted at low temperatures. Thus, the thermal quenching effect has a slight influence on anti-Stokes sideband [5]. The analysis of thermal stability beyond 298 K is necessary before taking into practical use because the operating temperature of LEDs is much higher than room temperature. The temperature dependence of PL spectra is shown in Fig. 9(b). With an increase in temperature from 298 K to 498 K, emission intensities also decrease gradually. The emission peak position exhibits a red shift and becomes broader, which corresponds to the low temperature. The decrease is attributed to thermal quenching; the mechanism can be explained by evaluating the activation energy, DE, according to the Arrhenius equation [34]:

I¼

I0 ; 1 þ AeðEa =KB TÞ

(10)

Rearranging this equation results in

ln


I0 Ea  1 ¼ ln A  ; I KB T

(11)

where I and I0 are the emission intensity at temperature T and initial temperature, respectively. KB is the Boltzmann constant (8.629  105 ev K1), and A is a constant. As shown in Fig. 6(b), the linear relationship between ln[(I0/I)-1] and 1/KBT is obtained, from which the slope, Ea (acquired from the equation) is calculated to be ~0.315 eV, which is higher than that of nitride compounds (~0.23 eV) [35]. The thermal quenching can be explained by the configuration coordinate scheme of Mn4þ ions in the inset of Fig. 9(c). With the absorption of UV energy, the electrons of 4A2g ground states transit to the 4T1g, 4T2g excited states. After the nonradiative relaxation to the lowest 4E2g excited states, the phosphors emit the red light through 4E2g/4A2g radiative transition. When elevating the temperature, the electrons can be thermally excited to the crossover between 4E2g and 4A2g states and transferred to the ground through non-radiative decay, which leads to the decrease in emission intensity [5]. The CIE chromaticity coordinate and quantum efficiency of Ba2YNbO6:0.005Mn4þ is depicted in Fig. 10(a). Upon 350 nm excitation, the coordinate and quantum yield are (0.705, 0.295) and 29.2%, respectively. The quantum efficiency calculated according to the following equation:

Table 1 Chromaticity parameters for the Ba2YNbO6:xMn4þ (x ¼ 0.001, 0.005, 0.01, 0.015, 0.02, 0.025). Series

BYN: Mn4þ content(wt%)

a b c d e f

BYN:0.001Mn4þ BYN:0.05Mn4þ BYN:0.01Mn4þ BYN:0.015Mn4þ BYN:0.02Mn4þ BYN:0.025Mn4þ

CIE x

y

0.705 0.702 0.704 0.703 0.705 0.704

0.295 0.298 0.296 0.297 0.295 0.296

151

Z

h¼Z

Lemission Z Eblank  Esample

(12)

where h is the quantum yield. The Lemission is the integrated value of the emission spectrum, Eblank and Esample are integrated value upon the “excitation” band of the blank and the integrated value upon the excitation band of the sample. The coordinate value is closer to the standard red color coordinates (0.670, 0.330) relative to many other Mn4þ-doped phosphors. The CIE values of different Mn4þdoped concentration of Ba2YNbO6:xMn4þ(x ¼ 0.001, 0.005, 0.01, 0.015, 0.02, 0.025) phosphors are listed in Table 1. To further investigate its applications, Ba2YNbO6:Mn4þ phosphor was coated onto the UV chip (lpeak ¼ 380 nm) to fabricate a red-emitting LED. When operating at 20 mA and 30 V, the LED exhibit a good performance. As is shown in Fig. 10(b), the spectra of the LED is composed of two main peaks: The one peaking at ~380 nm is ascribe to the emission of the UV chip. The another one located at ~694 nm is from the emission of the BYN:Mn4þ phosphor after the absorption of UV light. The luminous efficiency, CRT, and CCT of the LED are 35 lm/W, 80.7, and 4131 K, respectively. 4. Conclusion The Ba2YNbO6:Mn4þ phosphor was successfully synthesized through high-temperature solid-state reaction. The crystal structure was characterized by XRD, SEM, and FT-IR spectra. The blue shift of A1g mode in Raman spectra also indicates the site occupancy preferences of Mn4þ in Nb5þ sites. The absorption band of UVeVis spectra ranges from 275 nm to 600 nm, which can be efficiently excited by near-UV/blue light. The emission spectrum shows a band peaking at 698 nm, which arises from spin-forbidden 2Eg/4A2g transition. The experimental optimal concentration is x ¼ 0.005, and the concentration quenching is due to dipoleedipole interaction among Mn4þ ions. Based on TanabeeSugano diagram, the crystal field strength, Dq, and Racah parameters, B and C, are calculated to be 1912, 869, and 3314 cm1, respectively. A linear relationship equation E(2Eg) ¼ 8650.64 b1 þ 6487.21 was obtained to determine the emission positions of Mn4þ-doped phosphors according to b1. The temperature dependence of PL spectra has been investigated, and the thermal quenching is due to the predominant role of non-radiative transition at high temperatures. A red-emitting LED has been fabricated by combining with the phosphor and the UV chip. Under 20 mA current, the LED with a luminous efficiency of 35 lm/W exhibit a good performance. All these results reveal that Ba2YNbO6:Mn4þ phosphors possess a desirable potential for the application of warm WLEDs. Acknowledgements This work was supported by the Science Foundation of Guangxi Province (No. 2015GXNSFAA139025, 2016GXNSFDA380036). References [1] D. Ravichandran, R. Roy, W.B. White, S. Erdei, Synthesis and characterization of sol-gel derived hexa-aluminate phosphors, J. Mater. Res. 12 (1997) 819e824. [2] D.Y. Wang, C.H. Huang, Y.C. Wu, T.M. Chen, BaZrSi3O9:Eu2þ: a cyan-emitting phosphor with high quantum efficiency for white light-emitting diodes, J. Mater. Chem. 21 (2011) 10818e10822. [3] W.R. Liu, C.H. Huang, C.W. Yeh, J.C. Tsai, Y.C. Chiu, Y.T. Yeh, R.S. Liu, A study on the luminescence and energy transfer of single-phase and color-tunable KCaY(PO4)2:Eu2þ, Mn2þ phosphor for application in white-light LEDs, Inorg. Chem. 51 (2012) 9636e9641. [4] M.H. Du, Chemical trends of Mn4þ emission in solids, J. Mater. Chem. C 2

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(2014) 2475e2481. [5] Y.H. Jin, Y.H. Hu, H.Y. Wu, H. Duan, L. Chen, Y.R. Fu, G.F. Ju, Z.F. Mu, A deep red phosphor Li2MgTiO4:Mn4þ exhibiting abnormal emission: potential application as color converter for warm w-LEDs, Chem. Eng. J. 288 (2016) 596e607. [6] R. Zhang, H. Lin, Y.L. Yu, D.Q. Chen, J. Xu, Y.S. Wang, A new-generation color converter for high-power white LED: transparent Ce3þ:YAG phosphor-inglass, Laser Photonics Rev. 8 (2014) 158e164. [7] B. Wang, H. Lin, J. Xu, H. Chen, Y.S. Wang, CaMg2Al16O27:Mn4þ-based red phosphor: a potential color converter for high-powered warm W-LED, Acs Appl. Mater. Interfaces 6 (2015) 22905e22913. [8] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372e1386. [9] M.G. Brik, A.M. Srivastava, On the optical properties of the Mn4þ ion in solids, J. Luminescence 133 (2013) 69e72. [10] Y. Tanabe, S. Sugano, On the absorption spectra of complex ions IV, J. Phys. Soc. Jpn. 9 (1954) 766e779. [11] Y. Tanabe, S. Sugano, On the absorption spectra of complex ions, III the calculation of the crystalline field strength, J. Phys. Soc. Jpn. 11 (1956) 864e877. [12] F.E. Williams, Some new aspects of germanate and fluoride phosphors, J. Opt. Soc. Am. 37 (1947) 302e307. [13] G. Kemeny, C.H. Hakke, Activator center in magnesium fluorogermanate phosphors, J. Chem. Phys. 33 (1960) 783e789. [14] R.P. Cao, M.Y. Peng, E.H. Song, J.R. Qiu, High efficiency Mn4þ doped Sr2MgAl22O36 red emitting phosphor for white LED, ECS J. Solid State. Sci. Technol. 1 (4) (2012) R123eR126. [15] X.G. Zhang, Y.M. Huang, M.L. Gong, Dual-emitting Ce3þ, Tb3þ co-doped LaOBr phosphor: luminescence, energy transfer and ratiometric temperature sensing, Chem. Eng. J. 307 (2017) 291e299. [16] X.G. Zhang, M. Gong, Single-phased white-light-emitting NaCaBO3: Ce3þ, Tb3þ, Mn2þ phosphors for LED applications, Dalton Trans. 43 (2014) 2465e2472. [17] Q. Zhou, Y.Y. Zhou, Y. Liu, Z.L. Wang, G. Chen, J.H. Peng, J. Yan, M.M. Wu, A new and efficient red phosphor for solid-state lighting: Cs2TiF6:Mn4þ, J. Mater. Chem. C 3 (2015) 9615e9619. [18] H.M. Zhu, C.C. Lin, W.Q. Luo, S.T. Shu, Z.G. Liu, Y.S. Liu, J.T. Kong, E. Ma, Y.G. Cao, R.S. Liu, X.Y. Chen, Highly efficient non-rare-earth red emitting phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014), 4312e4312. [19] M. Peng, X. Yin, P.A. Tanner, M.G. Brik, P. Li, Site occupancy preference, enhancement mechanism, and thermal resistance of Mn4þ red luminescence in Sr4Al14O25:Mn4þ for warm WLEDs, Chem. Mater. 27 (2015) 2938e2945. [20] A. Ouasri, A. Rhandour, M.C. Dhamelincourt, P. Dhamelincourt, A. Mazzah, Structural and vibrational study of hydrazinium hexafluorsilicate, J. Raman Spectrosc. 33 (2002) 726e729. [21] N. Du, H. Wu, H. Zhang, C. Zhai, P. Wu, L. Wang, D. Yang, Large-scale synthesis of water-soluble nanowires as versatile templates for nanotubesw, Chem. Commun. 47 (2011) 1006e1008. [22] Q. Liu, J.L. Shen, T. Xu, L.X. Wang, J. Zhang, Q.T. Zhang, L. Zhang, Enhanced luminescence of a Eu3þ-activated double perovskite (Na,Li) LaMgWO6 phosphor based on A site in ducing energy transfer, Ceram. Int. 42 (2016) 13855e13862. [23] L.L. Wang, H.M. Noh, B.K. Moon, S.H. Park, K.H. Kim, J.S. Shi, J.H. Jeong, Dualmode luminescence with broad near UV and blue excitation band from

[24]

[25]

[26]

[27]

[28] [29] [30] [31]

[32]

[33]

[34]

[35]

[36]

[37]

[38] [39]

[40]

[41]

Sr2CaMoO6:Sm3þ phosphor for white LEDs, J. Phys. Chem. C 119 (2015) 15517e15525. L. Lv, X. Jiang, S. Huang, X.A. Chen, Y. Pan, The formation mechanism, improved photoluminescence and LED applications of red phosphor K2SiF6: Mn4þ, J. Mater. Chem. C 2 (2014) 3879e3884. R.P. Cao, J.J. Huang, X.F. Ceng, Z.Y. Luo, W. Ruan, Q.L. Hu, LiGaTiO4:Mn4þ red phosphor: synthesis, luminescence properties and emission enhancement by Mg2þ and Al3þ ions, Ceram. Int. 42 (2016) 13296e13300. M.S. Kishore, S. Marinel, V. Pralong, V. Caignaert, S. D'Astorg, B. Raveau, The rock salt oxide Li2MgTiO4: type I dielectric and ionic conductor, Mater. Res. Bull. 41 (2006) 1378e1384. L. Wang, Y. Xu, D. Wang, R. Zhou, N. Ding, M. Shi, Y. Chen, Y. Jiang, Y. Wang, Deep red phosphors SrAl12O19:Mn4þ, M (M ¼ Liþ, Naþ, Kþ, Mg2þ) for high colour rendering white LEDs, Phys. Status Solidi A 210 (2013) 1433e1437. G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (1969) 131e144. L. Van Uitert, Characterization of energy transfer interactions between rare earth ions, J. Electrochem. Soc. 114 (1967) 1048e1053. A.M. Srivastava, M.G. Brik, Ab initio and crystal field studies of the Mn4þdoped Ba2LaNbO6 double-perovskite, J. Lumin. 132 (2012) 579e584. A.J. Fu, C.Y. Zhou, Q. Chen, Z.Z. Lu, T.J. Huang, H. Wang, L.Y. Zhou, Preparation and optical properties of a novel double-perovskite phosphor Ba2GdNbO6: Mn4þ for light-emitting diodes, Ceram. Int. 43 (2017) 6353e6362. F. Lei, X. Zou, N. Jiang, Q. Zheng, K.H. Lam, L. Luo, Z. Ning, D. Lin, Regulated morphology/phase structure and enhanced fluorescence in YF3:Eu3þ, Bi3þ via a facile method, Cryst. Eng. Comm. 17 (2015) 6207e6218. H.X. Guan, G.X. Liu, J.X. Wang, X.T. Dong, Multicolor tunable luminescence and paramagnetic properties of NaGdF4:Tb3þ/Sm3þ multifunctional nanomaterials, Dalton Trans. 43 (2014) 10801e10808. W. Lü, W. Lv, Q. Zhao, M. Jiao, B. Shao, H. You, A novel efficient Mn(4þ) activated Ca14Al10Zn6O35 phosphor: application in red-emitting and white LEDs, Inorg. Chem. 53 (2014) 11985e11990. S.S. Wang, W.T. Chen, Y. Li, J. Wang, H.S. Sheu, R.S. Liu, Neighboring-cation substitution tuning of photoluminescence by remote-controlled activator in phosphor lattice, J. Am. Chem. Soc. 135 (2013) 12504e12507. C. Yang, Z.F. Zhang, G.C. Hu, R. Cao, X.J. Liang, W.D. Xiang, A novel deep red phosphor Ca14Zn6Ga10O35:Mn4þ as color converter for warm W-LEDs: structure and luminescence properties, J. Alloys Compd. 694 (2017) 1201e1208. C.Y. Xu, Y. Sheng, B. Yuan, H.X. Guan, P.C. Ma, Y.H. Song, H.F. Zou, K.Y. Zheng, Luminescence properties, energy transfer and multisite luminescence of Bi3þ/ Sm3þ/Eu3þ-coactivated Ca20Al26Mg3Si3O68 as a potential phosphor for whitelight LEDs, RSC Adv. 6 (2016) 89984e89993. M.J. Reisfeld, N.A. Matwiyof, L.B. Aspery, Electronic spectrum of cesium hexafluoromanganese (IV), J. Mol. Spectrosc. 39 (1971) 8e20. M.G. Brik, S.J. Camardello, A.M. Srivastava, Influence of covalency on the Mn4þ 2 Eg/4A2g emission energy in crystals, ECS J. Solid State Sci. Technol. 4 (2015) R39eR43. M.G. Brik, S.J. Camardello, A.M. Srivastava, N.M. Avram, A. Suchocki, Spinforbidden transitions in the spectra of transition metal ions and nephelauxetic effect, ECS J. Solid State Sci. Technol. 5 (2016) R3067eR3077. L.L. Wei, C.C. Lin, M.H. Fang, M.G. Brik, S.F. Hu, H. Jiao, R.S. Liu, A low-temperature co-precipitation approach to synthesize fluoride phosphors K2MF6: Mn4þ (M ¼ Ge, Si) for white LED applications, J. Mater. Chem. C 3 (2015) 1655e1660.