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Novel emission bands of Na2TiF6:Mn4+ phosphors induced by the cation exchange method
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Novel emission bands of Na2TiF6:Mn4+ phosphors induced by the cation exchange method Youmiao Liua, Tianman Wanga, Zanru Tana, Jianming Mengb, Wenjing Huangb, ⁎ ⁎⁎ ⁎⁎ Yingheng Huanga,b, , Sen Liaoa,b, , Huaxin Zhanga, a
School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi, 530004, China Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, School of Resources, Environment and Materials, Guangxi University, Nanning, Guangxi, 530004, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoluminescence Fluorides Red emitting phosphor Mn4+ Visible quantum cutting
A series of Na2TiF6:xMn4+ samples were prepared by the cation exchange method. Visible QC behavior and the strongest ZPL peaks were observed for these samples. The characteristic of the visible QC behavior is that PL intensity at 620 nm is 1.61 times as high as PLE intensity at 476 nm. The mechanisms of these phenomena were discussed, and the color purity, crystal-field and nephelauxetic effect, the critical distance and the multipolar interaction for the samples were determined. The result shows that the Mn4+ ions are located at a strong crystal field and the concentration quenching of Mn4+ is from the quadrupole-quadrupole interaction. The slightly increase of nephelauxetic ratio causes the slightly blue shift of emission. The color-purity and chromaticity coordinates of the optimal sample suggests that the strongest ZPL makes its red emission with high color-purity. Thus, Na2TiF6:0.08Mn4+ is a potential red-emitting phosphor for blue light-based WLEDs.
1. Introduction Red light emitting phosphors excited by blue light have attracted considerable attention for their using in the study of LEDs [1–4]. For example, Mn4+ doped fluotitanate phosphors [4–9] may show strong broadband absorption in the blue region and strong narrowband emission in the red region. However, their major emission peaks are generally at about 634 nm (deep red), which is not very sensible to human eyes. Mn4+-doped fluoride phosphors are common materials used in LEDs, however, their light purities are much less than those of well-studied Eu3+-doped phosphors [10]. Interestingly, some Na2TiF6:Mn4+ phosphors [11–14] have been reported to exhibit strong zero phonon lines (ZPL) at 617～620 nm, which are very closed to the emission of Eu3+ (5D0 → 7F2 transition) in Eu3+-doped phosphors. The narrowband emission of Na2TiF6:Mn4+ phosphors indicate that they have so good light purity as Eu3+-doped phosphors. The Na+ ions in these phosphors play a key role in their intensities of ZPL. For example, when the Na+ ions are replaced by K+ ions, K2TiF6:Mn4+ phosphors usually show much weaker ZPL [15–18] than Na2TiF6:Mn4+ phosphors. It is an important information that the synthesis methods may affect the intensities of their ZPL. For instance, the ZPL of the Na2TiF6:Mn4+ prepared with chemical etching method is ⁎
the strongest peak in its emission bands [11], while the ZPL of the Na2TiF6:Mn4+ prepared with co-precipitation method is the second strongest peak in its emission bands [12,13]. Furthermore, it meets our expectations, when the ZPL is the strongest peak, the main peak of the emission spectrum is effectively blue shifted from 634 to 620 nm. As a result, it is an effective method to improve the light purity of the Na2TiF6:Mn4+ phosphors by enhancing the intensities of ZPL. It has been reported that visible quantum cutting (QC) via downconversion can enhance the photoluminescence properties of the phosphors [19–33]. For example, the visible QC behavior in BaGdF5:Tb3+ [33] greatly enhances the emission intensity of Tb3+, which is 1.77 times stronger than that of the excitation peak. So far, most phosphors with visible QC behavior contain couples of rare earth ions [19–33], such as Gd3+–Eu3+ and Gd3+–Tb3+. In addition, the visible QC behavior has been found in some phosphors with Gd3+-Cu2+ couples, where the energy can be transferred from Gd3+ to Cu2+ via cross-relaxation. As we all know, there is visible QC behavior in GdCu0.2–0.6 glasses [34]. Among the Gd3+-based phosphors, the energy transfer between two different metal ions is an important contributor to the visible QC behavior. Na2SnF6:Mn4+ [35] without Gd3+ ion was found to exhibit similar visible QC behavior as above, and its Photoluminescence (PL) intensity
Correspondence to: School of Resources, Environment and Materials, Guangxi University, Nanning, Guangxi, 530004, China. Correspondence to: School of Chemistry and Chemical Engineering, Guangxi University, China. E-mail addresses: [email protected] (Y. Huang), [email protected], [email protected] (S. Liao), [email protected] (H. Zhang).
⁎⁎
https://doi.org/10.1016/j.ceramint.2018.12.104 Received 17 October 2018; Received in revised form 12 December 2018; Accepted 14 December 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Y., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.104
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is stronger than that of photoluminescence excitation spectra (PLE). The article aims to suggest that other Mn4+-doped fluorides may also show the similar visible QC behavior. From our previous work [15,36], it has been concluded that several kinds of Mn4+ doped fluotitanate phosphors were synthesized with a cation exchange method. As a part of our continuing studies focused on the synthesis of phosphors, we cover that some new Na2TiF6:xMn4+ phosphors synthesized by the cation exchange method. Interestingly, all obtained samples not only show the strongest ZPL emission bands, but also exhibit visible QC behavior under blue light excitation, which make them have potential application in warm WLEDs. 2. Experimental All chemicals were reagent-grade pure and purchased from the Sinopharm Chemical Reagent Co. Ltd., China. XRD was performed by using a Rigaku D/max 2500 V diffractometer at a scanning rate of 5°/ min from 5° to 70° for 2θ at room temperature, which was equipped with a graphite monochromator by utilizing monochromatic CuKα radiation (λ = 0.154178 nm). The morphologies of the samples were characterized by Hitachi S-3400 scanning electron microscopy (SEM) with an attached energy-dispersive X-ray spectrometer (EDS). Samples were mounted on an aluminum slice coated with Au. Photoluminescence spectra and photoluminescence excitation spectra (PLE and PL spectra) were recorded at room temperature by a Shimadzu RF-5301 spectrophotometer equipped with a xenon lamp as the excitation source. The luminescence decay curves were obtained from an Edinburgh FLS980 fluorescence spectrophotometer. Mn4+ source K2MnF6 was synthesized according to the method described in the literature [4,36,37]. For Na2TiF6:xMn4+ phosphors, a typical synthesis of Na2TiF6:xMn4+ (molar ratios of Mn4+/Na2TiF6, x = 0.08) is that, 0.988 g (4.0 mmol) K2MnF6, and 15 ml HF (40%) solution was added into a 50 ml plastic beaker and completely dissolved to obtain a yellow transparent solution by stirring. Then 10.39 g (50.0 mmol) Na2TiF6 (molar ratios of K2MnF6/ Na2TiF6, x = 0.08) was added into the above yellow transparent solution. After stirred for 2 h, the reaction mixture was kept at room temperature (about 25 °C) for 12 h, then the precipitates were collected, washed with acetone several times and dried at 98 °C for 2 h to get the sample, Na2TiF6: 0.08Mn4+. For comparison, six Na2TiF6:Mn4+ red phosphor samples with different molar ratios (x) of K2MnF6/Na2TiF6 were synthesized (x = 0.05～0.10).
Fig. 1. XRD pattern of sample, Na2TiF6:0.08 Mn4+.
doped of K2MnF6 in Na2TiF6 matrix. Fig. 3c shows the energy level diagram of Mn4+ excited by 377 and 476 nm lights. When the 4T1 g and 4T2 g levels of Mn4+ are excited, the electrons of the two levels relax and finally reach the 2Eg level (Fig. 3c) by non-radiative relaxation (NR). Then, red emission band is produced when the electrons of the 2Eg level jump to the 4A2 g. But, actually, the PL intensity at 620 nm and the PLE intensity at 476 nm are different, where the former is 1.61 times as high as the latter. This PL phenomenon of the samples is similar to that of Na2SnF6:Mn4+ [35]. Visible QC behaviors have been confirmed in some Eu3+-doped phosphors, where the energy transfer from Gd3+ to Eu3+ via crossrelaxation. The PL phenomenon found in the samples is similar to the visible QC behavior of Eu3+-doped phosphors [22–24]. Thus, when K2MnF6 is doped into Na2TiF6, the following reactions occur:
x K2MnF6 + Na2TiF6 = Na2TiF6⋅x K2MnF6
(1)
x K2MnF6 + Na2TiF6 = K2TiF6⋅x Na2MnF6
(2)
The results of the above reactions show that when Na2TiF6 is used as a host, the sample will contain two types of Mn4+ centers, which are coordination molecular centers K2MnF6 and Na2MnF6. On the contrary, when K2TiF6 is used as host, there will be only one Mn4+ center (K2MnF6) in the sample. Obviously, this will make the crystal field distortion of Na2TiF6:Mn4+ more intense than that of K2TiF6:Mn4+. The ZPL is attributed to local crystal field distortion, thus we can deduce that Na2TiF6:Mn4+ is more likely to give ZPL than K2TiF6:Mn4+. This assumption is also supported by the results of the literature. For example, all Na2TiF6:Mn4+ phosphors [11–14] obtained with different methods show intense ZPL, while non of K2TiF6:Mn4+ phosphors prepared with various methods [15–17] has intense ZPL. Furthermore, Tang et al. [18] reported that ZPL photon energy of Na2SiF6:Mn4+ (2.0072 eV) is larger than that of K2SiF6:Mn4+ (1.9954 eV), and ZPL photon energy of K2TiF6:Mn4+ (1.9916 eV) is similar to that of K2SiF6:Mn4+ (1.9954 eV). Therefore, it is reasonable that ZPL photon energy of Na2TiF6:Mn4+ is similar to that of Na2SiF6:Mn4+, which is larger than that of K2TiF6:Mn4+. Fig. 3a–b show that, excited by 476 nm light, the energy transfer from Na2[MnF6] to K2[MnF6] centers occurs efficiently. This results in the visible QC behavior and the PL intensity at 620 nm is 1.61 times as high as the PLE intensity at 476 nm. Fig. 3d shows the cooperative energy transfer from Na2[MnF6] to K2[MnF6] centers. Excited by 476 nm light, the electrons in 4T2 g levels of Mn4+ relax and finally reach the 2Eg level by nonradiative relaxation (NR). Then, the energy in the 2Eg level (about 2.00 eV) of Na2[MnF6] is transferred to the 2Eg level (about 1.99 eV) of K2[MnF6] by resonance between two neighbor centers, leading to the
3. Results and discussion Fig. 1 shows the XRD patterns of sample, Na2TiF6:0.08Mn4+. In Fig. 1, all the diffraction peaks of the sample in the patterns could be indexed and are agreement with those of pure hexagonal Na2TiF6 (PDF #43–0522). Ion radii of Mn4+ (CN = 6) and Ti4+ (CN = 6) ions are 0.53 and 0.61 Å, respectively, which are pretty similar. So, the result of XRD also indicates that Mn4+ ions have successfully occupied octahedral sites of Ti4+. Fig. 2 shows the SEM image and the corresponding EDS spectra of the sample, Na2TiF6:xMn4+ (x = 0.08). In Fig. 2a, it is found that the sample is composed with needle-like crystals with lengths of about 20 µm. Fig. 2b shows that the sample is comprised with elements of Na, K, Ti, Mn and F. Fig. 3 shows the fluorescent properties of Na2TiF6:0.08Mn4+. A strong broad excitation band from the spin-allowed 4A2→4T2 transition of Mn4+ [11,12] is found at about 476 nm (Fig., 3a). The full-width at half-maximum of the peak (55 nm) is much broader than that of bluechip emission (about 20 nm) [16]. Six emission peaks centered at 608, 612, 620, 629, 633 and 645 nm are attributed to as-ν4, as-ν6, ZPL, s-ν6, s-ν4 and s-ν3 of the spin forbidden 2Eg→4A2 of Mn4+ (Fig. 3b) [11,12], where "s" and "as" are abbreviations of stokes and anti-stokes, respectively. In Fig. 3a-b, the strongest emissions are from ZPL, which is similar to that of Na2TiF6:xMn4+ reported by Xu et al. [11]. The ZPL emission is attributed to the local crystal field distortion caused by 2
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Fig. 2. SEM image (a) and the corresponding EDS spectrum (b) of Na2TiF6:0.08 Mn4+ sample.
higher PL intensity than PLE intensity (visible QC behavior). This is also found in reported Na2SnF6:Mn4+ phosphors [35]. When NTF:0.08Mn4+ is compared to Na2TiF6:xMn4+ prepared with co-precipitation method [12,13], their emissions also have some differences: (i) for the former, PL intensity is much stronger than PLE intensity. But, for the latter, PL intensity is basically the same as PLE intensity. (ii) The ZPL of the former is the strongest peak, while that of the latter is a second strongest peak. But their stokes peak intensities are all stronger than those of the anti-Stokes peak. The Na2TiF6:xMn4+ prepared with chemical etching method show some different emission properties from the reported NTF:0.08Mn4+ that also has pure hexagonal Na2TiF6 phase (PDF #43–0522) and shows the strongest ZPL peak [11]. For example, the PL intensity of the former is much stronger than its PLE intensity, however, the PL and PLE intensities of the latter are similar. In addition, differing from the later, the peak intensities of Stokes peaks (on the right side of ZPL) of the former are stronger than those of corresponding anti-Stokes peaks (on the left side of ZPL). So, different synthetic methods may result in different emission modes of Mn4+ in Na2TiF6 host. The visible QC behavior in these Na2TiF6:xMn4+ samples were further investigated. In the cation exchange method the distributions of Mn4+ in samples are inhomogeneous, and there is concentration difference, due to fact that the diffusion and exchange of Mn4+ in solid Na2TiF6 hosts carry out from outside to inside, where diffusion process is dominant. However, for the chemical etching method, diffusion and exchange process of Mn4+ in the solid Na2TiF6 hosts is not dominant. Therefore, the distributions of Mn4+ in samples are uniform. As mentioned above, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers, as well as the concentration difference of Mn4+ in the samples, resulting in their visible QC behavior. This is also supported by reported Na2SnF6:Mn4+ synthesized from Na2SnF6 and K2[MnF6] by the cation exchange method [35]. Fig. 3e shows the decay curve of Na2TiF6:0.08Mn4+ sample. It can be fitted by a single-exponential function with a constant term, and the lifetime is 5.83 ms, which is similar to that of K2TiF6:Mn4+ samples [4]. The chromaticity coordinates (x = 0.6723, y = 0.3275) of the sample Na2TiF6:0.08Mn4+ are shown in Fig. 3f, which are in agreement with the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). The results suggest that sample Na2TiF6:0.08Mn4+ emits high color-purity red light at 620 nm. Moreover, the color purity of the sample can be calculated with the Eq. (3) [38,39]:
(x − x i )2 + (y − yi )2
Cp =
(x d − x i )2 + (yd − yi )2
× 100% (3)
Here, (x, y) are the color coordinates of the sample (x = 0.6723, y = 0.3275), (xi, yi) are the CIE of an equal-energy illuminant with a value of (0.33, 0.33), and (xd, yd) are the chromaticity coordinates corresponding to the dominant wavelength of the light source (The chromaticity coordinates of excitation are calculated to be: (i) xd = 0.1063, yd = 0.945 for excitation of 476 nm). The color purity of the sample is 105.4% (Table 1), indicating that the appearance of the strongest ZPL helps to improve the color purity of the sample. The experienced local crystal-field strength Dq for the Mn4+ of the sample is obtained by Eq. (4) [38–40].
Dq =
E (4 A2g − 4T 2g ) (4)
10
The mean energy gaps of the A2 g → T2 g and A2 g → T1 g transitions derived from excitation spectrum measured at 298 K (Fig. 3a) are 26,525 cm−1 (377 nm) and 21,008 cm−1 (476 nm) for sample Na2TiF6:0.08Mn4+, respectively. The calculated results of Dq are in Table 1. On the basis of the energy difference between the transitions 4A2 g 4 → T2 g and 4A2 g → 4T1 g of Mn4+, the parameter x and the Racah parameter B are evaluated by Eqs. (5)–(7) [38–40]. 4
4
4
4
E (4 A2g − 4T1g ) − E (4 A2g − 4T 2g )
x=
Dq
Dq 15(x − 8) = 2 B x − 10x B=
(5)
(6)
(x 2 − 10x )*Dq 15(x − 18)
(7)
The Racah parameter C is determined using Eqs. (8)–(10) [38–41]. 2
E ( E g → 4 A2g )/ B = 3.05C / B + 7.9 − 1.8B / Dq C=
B (E (2E g → 2 A2g )/ B + 1.8B / Dq − 7.9) 3.05 2
β1 =
(8)
2
⎛B⎞ +⎛C⎞ ⎝ B0 ⎠ ⎝ C0 ⎠ ⎜
⎟
(9)
⎜
⎟
(10)
The nephelauxetic ratio β1 can describe the degree of nephelauxetic effect, and it can be calculated by the Eq. (10). Where B, C, B0 and C0 3
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Fig. 3. Fluorescent properties of Na2TiF6:0.08 Mn4+: (a) PLE and PL, (b) An expanded PL, (c) energy level diagram, (d) cooperative energy transfers from Na2[MnF6] to K2[MnF6] excited by 476 nm, (e) Emission decay curve, (f) CIE chromaticity diagram.
are Racah parameter. For free ions of Mn4+, B0 and C0 are 1160 cm−1 and = 4303 cm−1, respectively. Based on the Tanabe–Sugano energy level diagram of Mn4+ (Fig. 4a, b), the energy gaps between the excited states 4T1 g (4T1) and 4 T2 g (4T2) increase with the increasing of crystal field. So, the excitation
spectrum of Mn4+ is strongly affected by the crystal field. However, the energy levels of 2Eg (2E) are independent of the crystal field. (Fig. 4a), therefore, the emission energy of Mn4+ is mainly determined by the nephelauxetic effect. The emitting 2Eg energy level of Mn4+ ion is a linear function of β1, indicating that the larger β1 will give higher 2Eg 4
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I = k [1 + β (x )θ/3]−1 x
Table 1 Calculation results of equations in the paper. −1
Cp /%
Dq/cm
105 4
2100 84
−1
x
B/cm
2 626
504 7
C/cm
−1
4052 6
Dq/B
β1
Rc/ Å
θ
4 163
1 037
14 41
12 81
Here, I is the emission intensity of Mn , x is the doping concentration of Mn4+, and x ≥ xc. The k and β are constants for same excitation condition, θ = 10, 8, and 6 correspond to the quadrupolequadrupole, dipole-quadrupole, and dipole-dipole interaction, respectively. Fig. 4c shows that the dependence of Log (I/x) on Log (x) for Na2TiF6: xMn4+ samples can be fitted by a straight line, and the slope of the line is − 4.2715 (− θ/3). The θ calculated by the slope is 12.81 (Table 1), which is close to 10. Therefore, the concentration quenching of Mn4+ in the Na2TiF6:xMn4+ samples may be from the quadrupolequadrupole interaction. The results also suggest that the energy transfer from Na2[MnF6] centers to K2[MnF6] centers is attributed to the quadrupole-quadrupole interaction mechanism.
energy level. As shown in Table 1, the Racah parameters B and C of the sample are smaller than B0 and C0, indicating that the chemistry bonds in the sample are not complete ionic bonds. Meanwhile, for the d3 electron configuration of Mn4+, when the Dq/B value is larger than 2.2, the Mn4+ is in the octahedral crystal field corresponding to a stronge crystal field [38–40]. The Dq/B value of the sample is 4.163, indicating that the Mn4+ ions are located at a site of a strong crystal field in the host lattice of the sample. In addition, the nephelauxetic effect parameter β1 of the sample is 1.307, which is slightly larger than that of K2NaAlF6 reported by zhu et al. [41], leading to a small blue shift of the emission. The s-ν6 peaks of the sample and reported K2NaAlF6 are 629 nm and 630 nm, respectively [41]. Fig. 5 shows PLE (Fig. 5a) and PL (Fig. 5b) spectra of Na2TiF6:xMn4+ with different concentrations of Mn4+. First, the PLE and PL intensities of the samples increase with the increase of molar ratios of Mn4+, and reach maximum values at x = 0.08; then they reduce with the continue increase of molar ratios of Mn4+ more than 0.08. To further explore the concentration quenching mechanism in Na2TiF6:xMn4+, the critical distance Rc between Mn4+ ions in Na2TiF6:xMn4+ is calculated based on the Blasse equation (Eq. (11)) [42].
4. Conclusions A number of Na2TiF6:xMn4+ samples were prepared by the cation exchange method at room temperature. The samples were characterized by means of XRD, SEM and PL&PLE, etc. Visible QC behavior and the strongest ZPL emission were found in these samples. The characteristic of the visible QC behavior is that PL intensity at 620 nm is 1.61 times as high as PLE intensity at 476 nm. The mechanisms of these phenomena were proposed as follows: (1) there are two coordination molecular centers K2MnF6 and Na2MnF6 in the samples. And the cation exchange method for the synthesis of samples results in the concentration difference of Mn4+ in crystal particles. (2) Intense local crystal field distortions were caused by the two coordination molecular centers K2MnF6 and Na2MnF6, and the concentration difference of Mn4+ in the samples lead to their strongest ZPL emissions. (3) When excited at 467 nm, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers, as well as the concentration difference of Mn4+ in the samples, result in their visible QC behavior. Besides, for these samples, the color purity, crystal-field and nephelauxetic effect, the critical distance and the multipolar interaction were determined. The results reflect the Mn4+ ions are located in a strong crystal field (Dq/B = 4.163) and the concentration quenching of Mn4+ in the samples is from the quadrupole-quadrupole interaction. Meanwhile, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers is attributed to the quadrupole-quadrupole interaction mechanism. The slightly increase of nephelauxetic ratio (β1 =1.037) causes the slightly blue shift of emission, and the optimal sample is Na2TiF6:0.08Mn4+. The colorpurity and chromaticity coordinates of the optimal sample are as follows, Cp = 105.4%, x = 0.6723, and y = 0.3275, indicating that the strongest ZPL makes its red emission with high color-purity. So,
1/3
3V ⎤ Rc ≈ 2 ⎡ ⎢ ⎣ 4πx c N ⎥ ⎦
(12) 4+
(11)
Where V is the unit cell volume of sample, xc is the critical concentration of Mn4+, and N is the number of sites that activators can substitute per unit. When Rc ≤ 5 Å, the quenching process is belong to exchange interaction mechanism, while when Rc > 5 Å, it is attributed to the multipolar interaction mechanism. For the Na2TiF6:0.08Mn4+ sample, the values of V, xc and N are 375.94 Å3, 8.0% and 6, respectively. Thus, the Rc is calculated to be 15.86 Å (Table 1), which is much larger than 5 Å. This result suggests that the energy transfer among Mn4+ ions in Na2TiF6:0.08Mn4+ is not belong to exchange interaction mechanism, but non-radiative energy transfer mechanism that may be dominantly attributed by multipolar interactions. The related type of multipolar interaction among Mn4+ in the Na2TiF6: xMn4+ can be further determined by Eq. (12) [38–41].
Fig. 4. Tanabe-Sugano energy level diagram for 5d3 electron configuration of Mn4+ in the center of octahedron: (a) Curve diagram, (b) Line diagram. 5
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Fig. 5. The photoluminescent properties of Na2TiF6:xMn4+: (a) PLE spectra, (b) PL spectra, (c) the related type line of multipolar interaction.
Na2TiF6:0.08Mn4+ is a potential red-emitting phosphor for blue lightbased WLEDs.
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Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 21561003 and No. 21661006), the Scientific Research Foundation of Guangxi University (Grant No. XDZ140116), the Open Foundation of Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University (Grant No. GXYSOF1804), and the Students Experimental Skills and Innovation Ability Training Fund Project of Guangxi University (No. 201710593234 and No. 201710593183). Conflict of interest statement The authors declare that they have no conflict of interest. References [1] H. Jia, Y.N. Duan, J.S. Gao, X.L. Yuan, H.Q. Wang, G.G. Li, Enhanced red emission for BaSiF6: Mn4+ hexagonal nanorod phosphor via using spherical silica, Mater. Lett. 223 (2018) 163–165. [2] M.Z. Rong, X.Y. Zhou, R.M. Xiong, N. Wang, Q. Wang, Z.L. Wang, Luminescent properties and application of Rb2GeF6: Mn4+ red phosphor, Mater. Lett. 207 (2017) 206–208.
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7
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Novel emission bands of Na2TiF6:Mn4+ phosphors induced by the cation exchange method Youmiao Liua, Tianman Wanga, Zanru Tana, Jianming Mengb, Wenjing Huangb, ⁎ ⁎⁎ ⁎⁎ Yingheng Huanga,b, , Sen Liaoa,b, , Huaxin Zhanga, a
School of Chemistry and Chemical Engineering, Guangxi University, Nanning, Guangxi, 530004, China Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, School of Resources, Environment and Materials, Guangxi University, Nanning, Guangxi, 530004, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Photoluminescence Fluorides Red emitting phosphor Mn4+ Visible quantum cutting
A series of Na2TiF6:xMn4+ samples were prepared by the cation exchange method. Visible QC behavior and the strongest ZPL peaks were observed for these samples. The characteristic of the visible QC behavior is that PL intensity at 620 nm is 1.61 times as high as PLE intensity at 476 nm. The mechanisms of these phenomena were discussed, and the color purity, crystal-field and nephelauxetic effect, the critical distance and the multipolar interaction for the samples were determined. The result shows that the Mn4+ ions are located at a strong crystal field and the concentration quenching of Mn4+ is from the quadrupole-quadrupole interaction. The slightly increase of nephelauxetic ratio causes the slightly blue shift of emission. The color-purity and chromaticity coordinates of the optimal sample suggests that the strongest ZPL makes its red emission with high color-purity. Thus, Na2TiF6:0.08Mn4+ is a potential red-emitting phosphor for blue light-based WLEDs.
1. Introduction Red light emitting phosphors excited by blue light have attracted considerable attention for their using in the study of LEDs [1–4]. For example, Mn4+ doped fluotitanate phosphors [4–9] may show strong broadband absorption in the blue region and strong narrowband emission in the red region. However, their major emission peaks are generally at about 634 nm (deep red), which is not very sensible to human eyes. Mn4+-doped fluoride phosphors are common materials used in LEDs, however, their light purities are much less than those of well-studied Eu3+-doped phosphors [10]. Interestingly, some Na2TiF6:Mn4+ phosphors [11–14] have been reported to exhibit strong zero phonon lines (ZPL) at 617～620 nm, which are very closed to the emission of Eu3+ (5D0 → 7F2 transition) in Eu3+-doped phosphors. The narrowband emission of Na2TiF6:Mn4+ phosphors indicate that they have so good light purity as Eu3+-doped phosphors. The Na+ ions in these phosphors play a key role in their intensities of ZPL. For example, when the Na+ ions are replaced by K+ ions, K2TiF6:Mn4+ phosphors usually show much weaker ZPL [15–18] than Na2TiF6:Mn4+ phosphors. It is an important information that the synthesis methods may affect the intensities of their ZPL. For instance, the ZPL of the Na2TiF6:Mn4+ prepared with chemical etching method is ⁎
the strongest peak in its emission bands [11], while the ZPL of the Na2TiF6:Mn4+ prepared with co-precipitation method is the second strongest peak in its emission bands [12,13]. Furthermore, it meets our expectations, when the ZPL is the strongest peak, the main peak of the emission spectrum is effectively blue shifted from 634 to 620 nm. As a result, it is an effective method to improve the light purity of the Na2TiF6:Mn4+ phosphors by enhancing the intensities of ZPL. It has been reported that visible quantum cutting (QC) via downconversion can enhance the photoluminescence properties of the phosphors [19–33]. For example, the visible QC behavior in BaGdF5:Tb3+ [33] greatly enhances the emission intensity of Tb3+, which is 1.77 times stronger than that of the excitation peak. So far, most phosphors with visible QC behavior contain couples of rare earth ions [19–33], such as Gd3+–Eu3+ and Gd3+–Tb3+. In addition, the visible QC behavior has been found in some phosphors with Gd3+-Cu2+ couples, where the energy can be transferred from Gd3+ to Cu2+ via cross-relaxation. As we all know, there is visible QC behavior in GdCu0.2–0.6 glasses [34]. Among the Gd3+-based phosphors, the energy transfer between two different metal ions is an important contributor to the visible QC behavior. Na2SnF6:Mn4+ [35] without Gd3+ ion was found to exhibit similar visible QC behavior as above, and its Photoluminescence (PL) intensity
Correspondence to: School of Resources, Environment and Materials, Guangxi University, Nanning, Guangxi, 530004, China. Correspondence to: School of Chemistry and Chemical Engineering, Guangxi University, China. E-mail addresses: [email protected] (Y. Huang), [email protected], [email protected] (S. Liao), [email protected] (H. Zhang).
⁎⁎
https://doi.org/10.1016/j.ceramint.2018.12.104 Received 17 October 2018; Received in revised form 12 December 2018; Accepted 14 December 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liu, Y., Ceramics International, https://doi.org/10.1016/j.ceramint.2018.12.104
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is stronger than that of photoluminescence excitation spectra (PLE). The article aims to suggest that other Mn4+-doped fluorides may also show the similar visible QC behavior. From our previous work [15,36], it has been concluded that several kinds of Mn4+ doped fluotitanate phosphors were synthesized with a cation exchange method. As a part of our continuing studies focused on the synthesis of phosphors, we cover that some new Na2TiF6:xMn4+ phosphors synthesized by the cation exchange method. Interestingly, all obtained samples not only show the strongest ZPL emission bands, but also exhibit visible QC behavior under blue light excitation, which make them have potential application in warm WLEDs. 2. Experimental All chemicals were reagent-grade pure and purchased from the Sinopharm Chemical Reagent Co. Ltd., China. XRD was performed by using a Rigaku D/max 2500 V diffractometer at a scanning rate of 5°/ min from 5° to 70° for 2θ at room temperature, which was equipped with a graphite monochromator by utilizing monochromatic CuKα radiation (λ = 0.154178 nm). The morphologies of the samples were characterized by Hitachi S-3400 scanning electron microscopy (SEM) with an attached energy-dispersive X-ray spectrometer (EDS). Samples were mounted on an aluminum slice coated with Au. Photoluminescence spectra and photoluminescence excitation spectra (PLE and PL spectra) were recorded at room temperature by a Shimadzu RF-5301 spectrophotometer equipped with a xenon lamp as the excitation source. The luminescence decay curves were obtained from an Edinburgh FLS980 fluorescence spectrophotometer. Mn4+ source K2MnF6 was synthesized according to the method described in the literature [4,36,37]. For Na2TiF6:xMn4+ phosphors, a typical synthesis of Na2TiF6:xMn4+ (molar ratios of Mn4+/Na2TiF6, x = 0.08) is that, 0.988 g (4.0 mmol) K2MnF6, and 15 ml HF (40%) solution was added into a 50 ml plastic beaker and completely dissolved to obtain a yellow transparent solution by stirring. Then 10.39 g (50.0 mmol) Na2TiF6 (molar ratios of K2MnF6/ Na2TiF6, x = 0.08) was added into the above yellow transparent solution. After stirred for 2 h, the reaction mixture was kept at room temperature (about 25 °C) for 12 h, then the precipitates were collected, washed with acetone several times and dried at 98 °C for 2 h to get the sample, Na2TiF6: 0.08Mn4+. For comparison, six Na2TiF6:Mn4+ red phosphor samples with different molar ratios (x) of K2MnF6/Na2TiF6 were synthesized (x = 0.05～0.10).
Fig. 1. XRD pattern of sample, Na2TiF6:0.08 Mn4+.
doped of K2MnF6 in Na2TiF6 matrix. Fig. 3c shows the energy level diagram of Mn4+ excited by 377 and 476 nm lights. When the 4T1 g and 4T2 g levels of Mn4+ are excited, the electrons of the two levels relax and finally reach the 2Eg level (Fig. 3c) by non-radiative relaxation (NR). Then, red emission band is produced when the electrons of the 2Eg level jump to the 4A2 g. But, actually, the PL intensity at 620 nm and the PLE intensity at 476 nm are different, where the former is 1.61 times as high as the latter. This PL phenomenon of the samples is similar to that of Na2SnF6:Mn4+ [35]. Visible QC behaviors have been confirmed in some Eu3+-doped phosphors, where the energy transfer from Gd3+ to Eu3+ via crossrelaxation. The PL phenomenon found in the samples is similar to the visible QC behavior of Eu3+-doped phosphors [22–24]. Thus, when K2MnF6 is doped into Na2TiF6, the following reactions occur:
x K2MnF6 + Na2TiF6 = Na2TiF6⋅x K2MnF6
(1)
x K2MnF6 + Na2TiF6 = K2TiF6⋅x Na2MnF6
(2)
The results of the above reactions show that when Na2TiF6 is used as a host, the sample will contain two types of Mn4+ centers, which are coordination molecular centers K2MnF6 and Na2MnF6. On the contrary, when K2TiF6 is used as host, there will be only one Mn4+ center (K2MnF6) in the sample. Obviously, this will make the crystal field distortion of Na2TiF6:Mn4+ more intense than that of K2TiF6:Mn4+. The ZPL is attributed to local crystal field distortion, thus we can deduce that Na2TiF6:Mn4+ is more likely to give ZPL than K2TiF6:Mn4+. This assumption is also supported by the results of the literature. For example, all Na2TiF6:Mn4+ phosphors [11–14] obtained with different methods show intense ZPL, while non of K2TiF6:Mn4+ phosphors prepared with various methods [15–17] has intense ZPL. Furthermore, Tang et al. [18] reported that ZPL photon energy of Na2SiF6:Mn4+ (2.0072 eV) is larger than that of K2SiF6:Mn4+ (1.9954 eV), and ZPL photon energy of K2TiF6:Mn4+ (1.9916 eV) is similar to that of K2SiF6:Mn4+ (1.9954 eV). Therefore, it is reasonable that ZPL photon energy of Na2TiF6:Mn4+ is similar to that of Na2SiF6:Mn4+, which is larger than that of K2TiF6:Mn4+. Fig. 3a–b show that, excited by 476 nm light, the energy transfer from Na2[MnF6] to K2[MnF6] centers occurs efficiently. This results in the visible QC behavior and the PL intensity at 620 nm is 1.61 times as high as the PLE intensity at 476 nm. Fig. 3d shows the cooperative energy transfer from Na2[MnF6] to K2[MnF6] centers. Excited by 476 nm light, the electrons in 4T2 g levels of Mn4+ relax and finally reach the 2Eg level by nonradiative relaxation (NR). Then, the energy in the 2Eg level (about 2.00 eV) of Na2[MnF6] is transferred to the 2Eg level (about 1.99 eV) of K2[MnF6] by resonance between two neighbor centers, leading to the
3. Results and discussion Fig. 1 shows the XRD patterns of sample, Na2TiF6:0.08Mn4+. In Fig. 1, all the diffraction peaks of the sample in the patterns could be indexed and are agreement with those of pure hexagonal Na2TiF6 (PDF #43–0522). Ion radii of Mn4+ (CN = 6) and Ti4+ (CN = 6) ions are 0.53 and 0.61 Å, respectively, which are pretty similar. So, the result of XRD also indicates that Mn4+ ions have successfully occupied octahedral sites of Ti4+. Fig. 2 shows the SEM image and the corresponding EDS spectra of the sample, Na2TiF6:xMn4+ (x = 0.08). In Fig. 2a, it is found that the sample is composed with needle-like crystals with lengths of about 20 µm. Fig. 2b shows that the sample is comprised with elements of Na, K, Ti, Mn and F. Fig. 3 shows the fluorescent properties of Na2TiF6:0.08Mn4+. A strong broad excitation band from the spin-allowed 4A2→4T2 transition of Mn4+ [11,12] is found at about 476 nm (Fig., 3a). The full-width at half-maximum of the peak (55 nm) is much broader than that of bluechip emission (about 20 nm) [16]. Six emission peaks centered at 608, 612, 620, 629, 633 and 645 nm are attributed to as-ν4, as-ν6, ZPL, s-ν6, s-ν4 and s-ν3 of the spin forbidden 2Eg→4A2 of Mn4+ (Fig. 3b) [11,12], where "s" and "as" are abbreviations of stokes and anti-stokes, respectively. In Fig. 3a-b, the strongest emissions are from ZPL, which is similar to that of Na2TiF6:xMn4+ reported by Xu et al. [11]. The ZPL emission is attributed to the local crystal field distortion caused by 2
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Fig. 2. SEM image (a) and the corresponding EDS spectrum (b) of Na2TiF6:0.08 Mn4+ sample.
higher PL intensity than PLE intensity (visible QC behavior). This is also found in reported Na2SnF6:Mn4+ phosphors [35]. When NTF:0.08Mn4+ is compared to Na2TiF6:xMn4+ prepared with co-precipitation method [12,13], their emissions also have some differences: (i) for the former, PL intensity is much stronger than PLE intensity. But, for the latter, PL intensity is basically the same as PLE intensity. (ii) The ZPL of the former is the strongest peak, while that of the latter is a second strongest peak. But their stokes peak intensities are all stronger than those of the anti-Stokes peak. The Na2TiF6:xMn4+ prepared with chemical etching method show some different emission properties from the reported NTF:0.08Mn4+ that also has pure hexagonal Na2TiF6 phase (PDF #43–0522) and shows the strongest ZPL peak [11]. For example, the PL intensity of the former is much stronger than its PLE intensity, however, the PL and PLE intensities of the latter are similar. In addition, differing from the later, the peak intensities of Stokes peaks (on the right side of ZPL) of the former are stronger than those of corresponding anti-Stokes peaks (on the left side of ZPL). So, different synthetic methods may result in different emission modes of Mn4+ in Na2TiF6 host. The visible QC behavior in these Na2TiF6:xMn4+ samples were further investigated. In the cation exchange method the distributions of Mn4+ in samples are inhomogeneous, and there is concentration difference, due to fact that the diffusion and exchange of Mn4+ in solid Na2TiF6 hosts carry out from outside to inside, where diffusion process is dominant. However, for the chemical etching method, diffusion and exchange process of Mn4+ in the solid Na2TiF6 hosts is not dominant. Therefore, the distributions of Mn4+ in samples are uniform. As mentioned above, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers, as well as the concentration difference of Mn4+ in the samples, resulting in their visible QC behavior. This is also supported by reported Na2SnF6:Mn4+ synthesized from Na2SnF6 and K2[MnF6] by the cation exchange method [35]. Fig. 3e shows the decay curve of Na2TiF6:0.08Mn4+ sample. It can be fitted by a single-exponential function with a constant term, and the lifetime is 5.83 ms, which is similar to that of K2TiF6:Mn4+ samples [4]. The chromaticity coordinates (x = 0.6723, y = 0.3275) of the sample Na2TiF6:0.08Mn4+ are shown in Fig. 3f, which are in agreement with the NTSC (National Television Standard Committee) standard values for red (x = 0.67, y = 0.33). The results suggest that sample Na2TiF6:0.08Mn4+ emits high color-purity red light at 620 nm. Moreover, the color purity of the sample can be calculated with the Eq. (3) [38,39]:
(x − x i )2 + (y − yi )2
Cp =
(x d − x i )2 + (yd − yi )2
× 100% (3)
Here, (x, y) are the color coordinates of the sample (x = 0.6723, y = 0.3275), (xi, yi) are the CIE of an equal-energy illuminant with a value of (0.33, 0.33), and (xd, yd) are the chromaticity coordinates corresponding to the dominant wavelength of the light source (The chromaticity coordinates of excitation are calculated to be: (i) xd = 0.1063, yd = 0.945 for excitation of 476 nm). The color purity of the sample is 105.4% (Table 1), indicating that the appearance of the strongest ZPL helps to improve the color purity of the sample. The experienced local crystal-field strength Dq for the Mn4+ of the sample is obtained by Eq. (4) [38–40].
Dq =
E (4 A2g − 4T 2g ) (4)
10
The mean energy gaps of the A2 g → T2 g and A2 g → T1 g transitions derived from excitation spectrum measured at 298 K (Fig. 3a) are 26,525 cm−1 (377 nm) and 21,008 cm−1 (476 nm) for sample Na2TiF6:0.08Mn4+, respectively. The calculated results of Dq are in Table 1. On the basis of the energy difference between the transitions 4A2 g 4 → T2 g and 4A2 g → 4T1 g of Mn4+, the parameter x and the Racah parameter B are evaluated by Eqs. (5)–(7) [38–40]. 4
4
4
4
E (4 A2g − 4T1g ) − E (4 A2g − 4T 2g )
x=
Dq
Dq 15(x − 8) = 2 B x − 10x B=
(5)
(6)
(x 2 − 10x )*Dq 15(x − 18)
(7)
The Racah parameter C is determined using Eqs. (8)–(10) [38–41]. 2
E ( E g → 4 A2g )/ B = 3.05C / B + 7.9 − 1.8B / Dq C=
B (E (2E g → 2 A2g )/ B + 1.8B / Dq − 7.9) 3.05 2
β1 =
(8)
2
⎛B⎞ +⎛C⎞ ⎝ B0 ⎠ ⎝ C0 ⎠ ⎜
⎟
(9)
⎜
⎟
(10)
The nephelauxetic ratio β1 can describe the degree of nephelauxetic effect, and it can be calculated by the Eq. (10). Where B, C, B0 and C0 3
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Fig. 3. Fluorescent properties of Na2TiF6:0.08 Mn4+: (a) PLE and PL, (b) An expanded PL, (c) energy level diagram, (d) cooperative energy transfers from Na2[MnF6] to K2[MnF6] excited by 476 nm, (e) Emission decay curve, (f) CIE chromaticity diagram.
are Racah parameter. For free ions of Mn4+, B0 and C0 are 1160 cm−1 and = 4303 cm−1, respectively. Based on the Tanabe–Sugano energy level diagram of Mn4+ (Fig. 4a, b), the energy gaps between the excited states 4T1 g (4T1) and 4 T2 g (4T2) increase with the increasing of crystal field. So, the excitation
spectrum of Mn4+ is strongly affected by the crystal field. However, the energy levels of 2Eg (2E) are independent of the crystal field. (Fig. 4a), therefore, the emission energy of Mn4+ is mainly determined by the nephelauxetic effect. The emitting 2Eg energy level of Mn4+ ion is a linear function of β1, indicating that the larger β1 will give higher 2Eg 4
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I = k [1 + β (x )θ/3]−1 x
Table 1 Calculation results of equations in the paper. −1
Cp /%
Dq/cm
105 4
2100 84
−1
x
B/cm
2 626
504 7
C/cm
−1
4052 6
Dq/B
β1
Rc/ Å
θ
4 163
1 037
14 41
12 81
Here, I is the emission intensity of Mn , x is the doping concentration of Mn4+, and x ≥ xc. The k and β are constants for same excitation condition, θ = 10, 8, and 6 correspond to the quadrupolequadrupole, dipole-quadrupole, and dipole-dipole interaction, respectively. Fig. 4c shows that the dependence of Log (I/x) on Log (x) for Na2TiF6: xMn4+ samples can be fitted by a straight line, and the slope of the line is − 4.2715 (− θ/3). The θ calculated by the slope is 12.81 (Table 1), which is close to 10. Therefore, the concentration quenching of Mn4+ in the Na2TiF6:xMn4+ samples may be from the quadrupolequadrupole interaction. The results also suggest that the energy transfer from Na2[MnF6] centers to K2[MnF6] centers is attributed to the quadrupole-quadrupole interaction mechanism.
energy level. As shown in Table 1, the Racah parameters B and C of the sample are smaller than B0 and C0, indicating that the chemistry bonds in the sample are not complete ionic bonds. Meanwhile, for the d3 electron configuration of Mn4+, when the Dq/B value is larger than 2.2, the Mn4+ is in the octahedral crystal field corresponding to a stronge crystal field [38–40]. The Dq/B value of the sample is 4.163, indicating that the Mn4+ ions are located at a site of a strong crystal field in the host lattice of the sample. In addition, the nephelauxetic effect parameter β1 of the sample is 1.307, which is slightly larger than that of K2NaAlF6 reported by zhu et al. [41], leading to a small blue shift of the emission. The s-ν6 peaks of the sample and reported K2NaAlF6 are 629 nm and 630 nm, respectively [41]. Fig. 5 shows PLE (Fig. 5a) and PL (Fig. 5b) spectra of Na2TiF6:xMn4+ with different concentrations of Mn4+. First, the PLE and PL intensities of the samples increase with the increase of molar ratios of Mn4+, and reach maximum values at x = 0.08; then they reduce with the continue increase of molar ratios of Mn4+ more than 0.08. To further explore the concentration quenching mechanism in Na2TiF6:xMn4+, the critical distance Rc between Mn4+ ions in Na2TiF6:xMn4+ is calculated based on the Blasse equation (Eq. (11)) [42].
4. Conclusions A number of Na2TiF6:xMn4+ samples were prepared by the cation exchange method at room temperature. The samples were characterized by means of XRD, SEM and PL&PLE, etc. Visible QC behavior and the strongest ZPL emission were found in these samples. The characteristic of the visible QC behavior is that PL intensity at 620 nm is 1.61 times as high as PLE intensity at 476 nm. The mechanisms of these phenomena were proposed as follows: (1) there are two coordination molecular centers K2MnF6 and Na2MnF6 in the samples. And the cation exchange method for the synthesis of samples results in the concentration difference of Mn4+ in crystal particles. (2) Intense local crystal field distortions were caused by the two coordination molecular centers K2MnF6 and Na2MnF6, and the concentration difference of Mn4+ in the samples lead to their strongest ZPL emissions. (3) When excited at 467 nm, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers, as well as the concentration difference of Mn4+ in the samples, result in their visible QC behavior. Besides, for these samples, the color purity, crystal-field and nephelauxetic effect, the critical distance and the multipolar interaction were determined. The results reflect the Mn4+ ions are located in a strong crystal field (Dq/B = 4.163) and the concentration quenching of Mn4+ in the samples is from the quadrupole-quadrupole interaction. Meanwhile, the energy transfer from Na2[MnF6] centers to K2[MnF6] centers is attributed to the quadrupole-quadrupole interaction mechanism. The slightly increase of nephelauxetic ratio (β1 =1.037) causes the slightly blue shift of emission, and the optimal sample is Na2TiF6:0.08Mn4+. The colorpurity and chromaticity coordinates of the optimal sample are as follows, Cp = 105.4%, x = 0.6723, and y = 0.3275, indicating that the strongest ZPL makes its red emission with high color-purity. So,
1/3
3V ⎤ Rc ≈ 2 ⎡ ⎢ ⎣ 4πx c N ⎥ ⎦
(12) 4+
(11)
Where V is the unit cell volume of sample, xc is the critical concentration of Mn4+, and N is the number of sites that activators can substitute per unit. When Rc ≤ 5 Å, the quenching process is belong to exchange interaction mechanism, while when Rc > 5 Å, it is attributed to the multipolar interaction mechanism. For the Na2TiF6:0.08Mn4+ sample, the values of V, xc and N are 375.94 Å3, 8.0% and 6, respectively. Thus, the Rc is calculated to be 15.86 Å (Table 1), which is much larger than 5 Å. This result suggests that the energy transfer among Mn4+ ions in Na2TiF6:0.08Mn4+ is not belong to exchange interaction mechanism, but non-radiative energy transfer mechanism that may be dominantly attributed by multipolar interactions. The related type of multipolar interaction among Mn4+ in the Na2TiF6: xMn4+ can be further determined by Eq. (12) [38–41].
Fig. 4. Tanabe-Sugano energy level diagram for 5d3 electron configuration of Mn4+ in the center of octahedron: (a) Curve diagram, (b) Line diagram. 5
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Fig. 5. The photoluminescent properties of Na2TiF6:xMn4+: (a) PLE spectra, (b) PL spectra, (c) the related type line of multipolar interaction.
Na2TiF6:0.08Mn4+ is a potential red-emitting phosphor for blue lightbased WLEDs.
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Acknowledgements This research is supported by the National Natural Science Foundation of China (Grant No. 21561003 and No. 21661006), the Scientific Research Foundation of Guangxi University (Grant No. XDZ140116), the Open Foundation of Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University (Grant No. GXYSOF1804), and the Students Experimental Skills and Innovation Ability Training Fund Project of Guangxi University (No. 201710593234 and No. 201710593183). Conflict of interest statement The authors declare that they have no conflict of interest. References [1] H. Jia, Y.N. Duan, J.S. Gao, X.L. Yuan, H.Q. Wang, G.G. Li, Enhanced red emission for BaSiF6: Mn4+ hexagonal nanorod phosphor via using spherical silica, Mater. Lett. 223 (2018) 163–165. [2] M.Z. Rong, X.Y. Zhou, R.M. Xiong, N. Wang, Q. Wang, Z.L. Wang, Luminescent properties and application of Rb2GeF6: Mn4+ red phosphor, Mater. Lett. 207 (2017) 206–208.
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