Photoluminescence properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as a novel red emitting phosphor

Accepted Manuscript Research paper Photoluminescence properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as a novel red emitting phosphor Peng Chen, Dingming Yang, Wenyuan Hu, Jing Zhang, Yadong Wu PII: DOI: Reference:

S0009-2614(17)30923-5 https://doi.org/10.1016/j.cplett.2017.10.006 CPLETT 35152

To appear in:

Chemical Physics Letters

Received Date: Accepted Date:

3 August 2017 2 October 2017

Please cite this article as: P. Chen, D. Yang, W. Hu, J. Zhang, Y. Wu, Photoluminescence properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as a novel red emitting phosphor, Chemical Physics Letters (2017), doi: https://doi.org/10.1016/j.cplett.2017.10.006

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Photoluminescence properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as a novel red emitting phosphor Peng Chen, Dingming Yang*, Wenyuan Hu, Jing Zhang, Yadong Wu School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China *Corresponding author: Dingming Yang ([email protected])

Abstract: Novel red-emitting Ba2Zn1-x-yWO6:xEu3+, yLi+ phosphors were prepared using a high-temperature solid-state method, and the crystal structure, the photoluminescence properties and the doping concentrations of Eu3+ and Li+ were investigated. The results show that these phosphors can be excited by near-ultraviolet light (250-400 nm) and co-doped Li+ can significantly enhance their PL performance. An intense red emission peak at 598 nm (5D0-7F1 transitions) was observed with an excitation wavelength of 316 nm. The CIE chromaticity coordinates of the phosphors are located in the red region, indicating that the BZW:Eu3+, Li+ phosphor holds promise as a red phosphor for near-ultraviolet excited WLEDs.

Keywords: Photoluminescence, charge compensation, red-emitting phosphors, Ba2ZnWO6

1. Introduction As energy shortages and environmental pressures grow, white-light-emitting diodes (WLEDs) have attracted wide attention and much research, due to their energy-saving performance, long lifetime, high efficiency, environmental safety and other virtues [1-4]. At present, combining the yellow phosphor YAG:Ce3+ with blue LED chips is the main method for creating WLEDs [5,6]. However, because these WLEDs lack red light, the white light they generate has serious weaknesses, such as a high correlative color temperature (CCT) and a low color-rendering index (CRI). For those reasons, WLEDs that combine a near-ultraviolet (UV) InGaN-based LED chip with phosphors that emit the three primary colors (blue, green and red) have excited a great deal of interest. Obviously, a red phosphor is indispensable for these devices, but the most widely used commercial red phosphor, Y2O2S:Eu3+, unfortunately has a lower luminescence efficiency than the blue and green phosphors and is also chemically unstable. Its limitations have been the major obstacle to the development of better WLEDs [7-10]. Therefore, much effort has gone into the search for novel red phosphors with excellent optical properties. Tungstate double perovskites with the formula A2BWO6 (A = Ba, Sr; B = Ca, Mg, Zn;) have attracted a great deal of interest due to their broad excitation band, which reaches from the near UV into the region of visible light (200-450 nm) [11-13]. The WO6 groups efficiently absorb ultraviolet light through excitation of the W-O chargetransfer states (CTS) and also transfer the excitation energy to the activator [14-18]. In addition, tungstate double perovskites have excellent thermal and chemical stability, which make them an ideal host material. Extensive investigation of Eu3+ ion-activated tungstate double perovskite structures, such as Sr2ZnWO6:Eu3+ [17], Ca2ZnWO6:Eu3+ [19], Ba2MgWO6:Eu3+ [20] and LiLaMgWO6:Eu3+ [21], has shown that these phosphors also have excellent photoluminescence (PL) properties. Most of them involve A-site substitution, but it has been reported that compounds of this type with B-site substitution have better PL properties than those with A-site substitution. Since the new Eu-O-W bond angle produced by B-site substitution is close to 180°, B-site

substitution has a higher efficiency of energy transfer from host to Eu3+ ions [13,22,23]. And since the ionic radius of Zn2+ (r = 0.74 Å) is considerably closer to that of Eu3+ (r = 0.93 Å) than the ionic radius of Ba2+ (r = 1.35 Å) is, the doped Eu3+ ions tend to occupy the Zn2+ (B) site in Ba2ZnWO6. Therefore, one might have already supposed that Ba2ZnWO6 would be a superior host material, but the luminescent properties of Eu3+ doped Ba2ZnWO6 had not been reported till now. In our work, the Eu3+-activated double perovskite Ba2ZnWO6 was synthesized using a high temperature solid-state method. Alkali metals M+ (M = Li, Na and K) were used as the charge compensation agent to keep the charge balance and improve the luminescent performance of the phosphors. In this case, since the radius of an Li+ ion is close to that of a Zn2+ ion, we co-doped Li+ ions as the charge compensation agent. The crystalline structure and the PL properties of Ba2ZnWO6:Eu3+, Li+ phosphors, the optimal doping concentrations of Eu3+ and Li+ and the mechanism of energy transfer in Ba2ZnWO6:Eu3+, Li+, all of which are discussed in detail below.

2. Experimental 2.1 Synthesis A series of powder samples of Ba2Zn1-x-yWO6:xEu3+, yLi+ (x = 0.08, 0.09, 0.10, 0.11 and 0.12) was synthesized using a high-temperature solid-state reaction. BaCO3 (AR), ZnO (AR), WO3 (AR), LiOH (AR) and Eu2O3 (AR) were employed as the raw materials. They were thoroughly mixed with ethanol as a lubricant and ground for 2 h using a planetary ball mill. The mixture was preheated at 850℃ for 6 h in a muffle furnace, reground, and finally sintered at 1200℃ for 12 h in air. The reaction products were then allowed to slowly cool to room temperature inside the furnace and ground into powder for characterizations. 2.2 Characterization The phase structure of the powders was examined using a PANalytical X’Pert Pro Xray diffractometer with Cu Ka radiation (k = 1.5406). The diffraction patterns were scanned over an angle of 3° to 80° (2θ) at a scan rate of 10°/min. The morphology and

size of the phosphor powders were determined via a TM-1000 scanning electron microscopy. The Raman spectra were collected using a In Via spectrometer with 514 nm laser and the background of Roman date were subtracted using Origin 9.0 software. The photoluminescence emission and excitation spectra were measured using a Hitachi F-4600 Spectral spectrophotometer equipped with a xenon lamp. All the measurements were taken at room temperature.

3. Results and discussion 3.1 Crystal structure and morphology The X-ray diffraction (XRD) patterns for Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0, 0.05, 0.10, 0.15 and 0.20) are shown in Fig. 1(a). All the diffraction peaks of the samples, indexed to the (111), (220), (400), (422), (440) and (620) planes appearing at 2θ of 18.92, 31.13, 44.61, 55.40, 64.92 and 73.75º, respectively, are consistent with a cubic double perovskite structure Ba2ZnWO6 (JCPDS 17-0239), indicating that the phase is pure in all the samples. The diffraction angle for all the peaks gradually diminishes, however, as x increases; enlarged details of the diffraction peaks are given in Fig. 1(b). This shift is due mainly to the substitution of the larger Eu3+ (r = 0.93 Å, CN = 6) for Zn2+ (r = 0.74 Å, CN = 4) in the same coordination environment, causing the diffraction angle to decrease as the interplanar distance increases, which can be explained with the following Bragg equation: (1) where d is the interplanar distance, θ is the peak diffraction angle, and λ is the X-ray wavelength (0.15406 nm). When Eu3+ with its larger radius replaces Zn2+ in a Ba2ZnWO6 crystal cell, the interplanar distance (d) is larger than in pure Ba2ZnWO6. If the incident wavelength of 0.15406 nm stays the same, the peak diffraction angle necessarily decreases as d increases. This shift confirms that the Eu3+ has indeed been incorporated into the Ba2ZnWO6 molecules at the Zn2+ sites. Fig. 2 shows the Raman spectra of Ba2ZnWO6 and Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.05, 0.10, 0.15 and 0.20). Two kinds of Raman modes can be observed: T2g(2) modes and A1g modes. The T2g(2) modes at ~431 cm-1 can be attributed to W-O-W

bending vibrations, and the A1g modes at ~818 cm-1 to W-O stretching modes. Generally, the A1g modes are sensitive to B-site substitution [24,25]. As Fig. 2 shows, as the amount of Eu3+ increases, the A1g modes gradually produce a blue shift and split into two main peaks and broadening , which suggests that the crystal field has changed due to the changed quantity of electrons in the outermost energy level and the valence of the Zn2+ ions and Eu3+ ions. The peak in A1g modes with a shift to low wavenumber proves that the doped Eu3+ has successfully occupied the Zn2+ (B) site in the lattice, which is consistent with the XRD results. Representative SEM micrographs of Ba2ZnWO6 (a) and BZW:0.1Eu3+, 0.1Li+ (b) appear in Fig. 3. The particles of Ba2ZnWO6 are irregular, ranging in size from 0.12.0 μm (Fig. 3a). The morphology of the BZW:0.1Eu3+, 0.1Li+ particles (Fig. 3b) is mostly tetrahedral, with the larger particles ranging from 0.2-3.0 μm. A partial agglomeration between crystalline grains appears in all the samples, but the more regular grain shape of BZW:0.1Eu3+, 0.1Li+ indicates that the crystallinity of the sample is improved by doping with Eu3+ ions. Due to their compatible shape and size, such phosphors are used in the construction of displays and illumination. 3.2 Photoluminescence properties The room-temperature emission and excitation spectra of Ba2Zn0.8WO6:0.1Eu3+, 0.1Li+ are shows in Fig. 4. The PL excitation spectra were recorded by monitoring the characteristic emission of Eu3+ ions at 598 nm. As Fig. 4(a) shows, many excitation peaks appear over the wide range of excitation spectra from 250 to 550 nm, including a broad excitation peak from 250 to 350 nm centered at 316 nm that corresponds to the charge transfer transition, as well as the characteristic excitation peaks of Eu3+ ions at 394 nm, 468 nm and 530 nm that correspond to the 7F0-5L6, 7F0-5D2 and 7F05

D1 transitions, respectively. The charge transfer band (CTB) excitation peak reflects

the electronic excitation of O2-→Eu3+ and O2-→W6+, whose intensity was the highest among all the excitation peaks. Furthermore, the f-f transitions show that energy is efficiently transferred from the host to the Eu3+ ions. The corresponding wavelength of the excitation peaks of Eu3+ 4f levels is the same as that found in the literature [26,27].

The Ba2Zn0.8 WO6:0.1Eu3+, 0.1Li+ phosphor shows red luminescence in Fig. 4(b) at the excitation wavelength of 316 nm, and the emission spectra are composed of the characteristic emission peaks of Eu3+, including 5D0-7F0 (586 nm), 5D0-7F1 (598 nm) and 5D0-7F2 (620 nm), respectively. Of these emission peaks, the 5D0-7F1 (598 nm) transition is the dominant one. It can be assigned to the magnetic dipole (MD) transition and the phosphor has great potential as a red phosphor for applications in near UV chip excited WLEDs. Remarkably, the emissions from the Eu3+ ion are significantly affected by the site it occupies. When it occupies the centrosymmetric site, its emission mainly derives from the MD transition. However, if it occupies the non-centrosymmetric site, the electric dipole (ED) transition is dominant. In Fig. 4(b), the intensity of the 5D0-7F1 transition (598 nm, MD) is greater than that of the 5D0-7F2 transition (620 nm, ED), because of the compound pseudo-cubic structure and the fact that the W-O-Eu angle is close to 180°. This reflects the crystal structure of Eu3+substituted Zn2+ sites in Ba2ZnWO6. Several studies indicate that the doping concentration of rare earth (RE) ions in phosphors directly affects their luminescence properties. In order to determine the optimum doping concentration, the excitation spectra of tungstates doped with different Eu3+ concentrations at an excitation wavelength of 316 nm are given in Fig. 5. The form of the spectrum at different Eu3+ doping concentrations does not change, but the intensity of the emission peaks changes greatly as the Eu3+ doping concentration increases. Fig. 5 shows a rapid increase in the emission intensity as the Eu3+ doping concentration rises. The emission intensity reaches a maximum at 10 mol% and decreases quickly as the Eu3+ doping concentration rises further. The influence of the concentration on the excitation intensity of Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.08, 0.09, 0.1, 0.11 and 0.12) phosphors is shown in the graph embedded in Fig. 5, and the optimal doping concentration of Eu3+ is about 10 mol% (x = 0.1). The PL excitation and emission intensity decrease at high Eu3+ ion concentrations mainly due to the effects of concentration quenching. This concentration quenching could be caused by either radiative energy transfer or non-radiative energy transfer among Eu3+ ions. Radiative energy transfer between

Eu3+ ions increases as the concentration of Eu3+increases, eventually causing concentration quenching. In additional, non-radiative energy transfer, including exchange interaction, radiation re-absorption and multipole-multipole interaction, can also lead to concentration quenching [28,29]. Usually, the mechanism of exchange interaction comes into effect only when the typical critical distance (Rc) is 5 Å. According to Blasse's theory [30], Rc can be estimated by using the following equation: (2) where Xc is the critical concentration, Z is the number of host cationic sites in the unit cell and V is the volume of the unit cell. The values of Xc, Z and V in the BZW:Eu3+, Li+ phosphor in question were 10 mol%, 4, and 544.98 Å, respectively. Given these parameters, Rc is about 13.75 Å, which is much larger than 5 Å, so exchange interaction is ruled out. The second possibility, radiation re-absorption, produces a broad overlap between the emission peaks of the sensitizer and the activator. But no broad overlapping peak appears in Fig. 4, meaning that it too can be ruled out [31]. That leaves multipole-multipole interaction energy transfer as probably the dominant mechanism of concentration quenching in the present case. In a BZW:Eu3+ phosphor, as described earlier, the Eu3+ ion is incorporated into a host lattice and substituted for a Zn2+ ion, creating a charge imbalance that affects the PL properties of the phosphor. To preserve electrical neutrality and improve PL intensity, Li+ was used as a charge compensation agent. Fig. 6 shows the PL spectra of samples at different Li+ doping concentrations. The optimal doping concentration of Li+ turned out to be about 10 mol% (y = 0.1). As Fig. 6 demonstrates, co-doped Li+ significantly increases the PL intensity of the phosphor. The main reason is that the radius of Li+ (r = 0.76 Å) is small--almost the same as the radius of Zn2+ (r = 0.74 Å). Therefore, if the two Zn2+ ions in BZW:Eu3+ are replaced with a Eu3+ ion and a Li+ ion, the change in the crystal structure and lattice parameters of the molecule will be very small [32,33]. Furthermore, Li+ ions can easily occupy the sites of defects due to their small radius. [34].

The Commission internationale de l'éclairage (CIE) chromaticity diagram for the BZW:0.1Eu3+, 0.1Li+ phosphor calculated from the PL spectra at an excitation wavelength of 316 nm, is given in Fig. 7. The inset in Fig. 7 shows the digital image of the BZW:0.1Eu3+, 0.1Li+ phosphors under an excitation wavelength from a 254 nm UV lamp. There is an intense red emission. The CIE chromaticity coordinates (x, y) of this phosphor are (0.633, 0.366), both in the red region, and they hardly change as the doping Eu3+ concentration increases, as Table 1 shows. In fact, they are closer to the National Television Standard Committee (NTSC) standard coordinates for a red phosphor (0.670, 0.330) than the chromaticity coordinates of Y2O2S:Eu3+ (0.622, 0.351) are. These results demonstrate that red emitting BZW:Eu3+, Li+ phosphors show promise as a red component in near UV-based WLEDs.

4. Conclusions In sum, a series of Ba2Zn0.9-xWO6:xEu3+, 0.1Li+(x = 0.08, 0.09, 0.1, 0.11 and 0.12) red emitting phosphors was prepared using a high-temperature solid-state reaction method. The phosphor obtained has a cubic structure with

space group. Its

excitation and emission spectra show that the CT bands of O 2-→Eu3+ and O2-→W6+ complex (250-350 nm) are the most intense, and the 7F0-5L6, 7F0-5D1 and 7F0-5D2 transitions of Eu3+ ions in Ba2ZnWO6:xEu3+, 0.1Li+ phosphors can all be observed, showing efficient absorption in the UV region. In addition, there is a strong red emission (598 nm, 5D0-7F1 transition) that is due to the location of the Eu3+ ions at a symmetric position in the host. The optimal Eu3+ doping concentration is about 10 mol%. The critical transfer distance is 13.75 Å, and the mechanism for concentration quenching of Eu3+ emissions in the Ba2ZnWO6 host is apparently multipole-multipole interaction. The optimal doping concentration of the charge compensation Li+ ion is also about 10 mol% (y = 0.1). Finally, the CIE chromaticity coordinates of the Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.08, 0.09, 0.10, 0.11 and 0.12) phosphors are all in the red region, and they hardly change as the doping Eu3+ concentration increases. The very good CIE chromaticity coordinates (0.633, 0.366) of this kind of phosphor

suggest that it is an excellent candidate for use as the red emitting phosphor in UVbased WLEDs.

Acknowledgements This work was supported by the National Science and Technology Support Program of China (2014BAB15B02) and the National Key Basic Research Development Program of China (973 Program) (2012CB722707).

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Table 1. The CIE coordinates of Ba2 Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.08, 0.09, 0.10, 0.11 and 0.12) phosphors under 316 nm excitation. Ba2Zn1-2xWO6:xEu3+, 0.1Li+

λex = 316 nm X

Y

X = 0.08

0.6325

0.3670

X = 0.09

0.6327

0.3669

X = 0.10

0.6329

0.3667

X = 0.11

0.6330

0.3665

X = 0.12

0.6335

0.3661

Figure Captions Fig. 1. (a) The XRD patterns of Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.00, 0.05, 0.10, 0.15 and 0.20). (b) Partial enlarged detail of the diffraction peak. Fig. 2. The Raman spectra of Ba2ZnWO6 and Ba2 Zn0.9-xWO6:xEu3+, 0.1 Li+ (x = 0.05, 0.10, 0.15 and 0.20) Fig. 3. The SEM images of Ba2ZnWO6 (a) and Ba2Zn0.8WO6:0.1Eu3+, 0.1Li+ (b) phosphor. Fig. 4. (a) Photoluminescence excitation and (b) emission spectra of Ba2ZnWO6:0.1Eu3+, 0.1Li+at room temperature. Fig. 5. Emission (λex = 316 nm) spectra of Ba2Zn0.9-xWO6:xEu3+, 0.1Li+ (x = 0.08, 0.09, 0.10, 0.11, 0.12) samples doped with different contents of Eu3+ ions. The inset shows the influence of the Eu3+ ions on emission intensity. Fig. 6. Emission (λex = 316 nm) spectra of Ba2Zn0.9-xWO6:0.1Eu3+, yLi+(y = 0, 0.08, 0.09, 0.1, 0.11 and 0.12) samples doped with different contents of Li+ ions. Fig. 7. CIE chromaticity coordinates of the BZW:0.1Eu3+, 0.1Li+ phosphor excited at 316 nm, and the inset showed the luminescent image of BZW:0.1Eu3+, 0.1Li+ phosphor, which was excited by a 254 nm UV lamp.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Luminescent properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as novel orange emitting phosphor for WLED applications

Peng Chen, Dingming Yang*, Wenyuan Hu, Jing Zhang, Yadong Wu School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China *Corresponding author: Dingming Yang ([email protected] ;)

Graphical Abstract

Luminescent properties and structure of double perovskite Ba2ZnWO6:Eu3+, Li+ as novel orange emitting phosphor for WLED applications

Peng Chen, Dingming Yang*, Wenyuan Hu, Jing Zhang, Yadong Wu School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, China *Corresponding author: Dingming Yang ([email protected];)

Highlights Novel red-emitting Ba2ZnWO6:Eu3+, Li+ phosphors were investigated. The effect of charge compensation agent (Li+) in the phosphors was analyzed. The red emission intensity reaches a maximum with Ba2Zn0.8 WO6:0.1Eu3+, 0.1Li+. The CIE chromaticity coordinates of the phosphors was located in red region.