Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+ Red-Emitting Phosphors with a

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RbSiF:Mn and RbCsSiF:Mn Red-Emitting Phosphors with a Faster Decay Rate Minseuk Kim, Woon Bae Park, Jin-Woong Lee, Jinhee Lee, Chang Hae Kim, Satendra Pal Singh, and Kee-Sun Sohn Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03542 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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Chemistry of Materials

Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+ Red-Emitting Phosphors with a Faster Decay Rate Minseuk Kim∥‡, Woon Bae Park∥‡, Jin-Woong Lee‡, Jinhee Lee‡, Chang Hae Kim†, Satendra Pal Singh‡*, and Kee-Sun Sohn‡* ‡ Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea † Korea Research Institute of Chemical Technology, Daejeon, 305-600, Republic of Korea ABSTRACT: A narrow-band red emission is a seminal issue for establishing the color-rendering index and color gamut of phosphor-converted light emitting diode (pc-LED) applications. In this regard, Mn4+-activated K2SiF6 phosphors (referred to as K216) have recently attracted a great deal of attention after their successful commercialization. As with K216 phosphors, Mn4+-activated K3SiF7 (referred to as K317) phosphors perform in a manner that is similar to that of K216 phosphors, and have been introduced as a narrow-band red phosphor. Despite the acceptable performances of both versions of phosphors, slower decay remains a shortcoming for applications to high-powered LEDs. We introduced K317 derivatives by replacing K with Rb and Cs. As a result, Mn4+-activated (Rb,Cs)3SiF7 phosphors were synthesized with different Rb:Cs ratios and exhibited faster decay times by comparison with both the K216 and K317 phosphors. The present study involved both structural and luminescent investigations along with density functional theory (DFT) calculations for novel (Rb,Cs)3SiF7:Mn4+ phosphors. This study reveals the possibility of a completely solid solution with single-phase formation within the entire range of Rb-Cs with retention of the tetragonal structure in the P4/mbm space group.

Introduction Phosphor converted white light emitting diodes (pcWELDs) are now rapidly replacing traditional incandescent bulbs and fluorescence tubes for general lighting because of their excellent efficiency, longer operational life and environment friendliness.1-3 The key factors determining importance of their application in the general lighting are luminous efficacy, color rendition and reliability. Most of the commercially available pc-WLEDs uses a Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor and a blue LED chip, which, however, suffer from poor color-rendering index (Ra < 75) and high correlated color temperatures (CCT > 4500 K) due to the lack of red component making unsuitable for high-quality general lighting.3-5 Thus a red lightemitting phosphor in combination with YAG:Ce3+ is highly desired to overcome these limitations. Eu2+-doped red light-emitting phosphors such as CaAlSiN3:Eu2+, Sr2Si5N8:Eu2+, and Sr[LiAl3N4]:Eu2+ have shown excellent performance for practical applications.6-9 However, strong photon re-absorption in the green or yellow spectral region due to 4f−5d transitions of the activator ion can cause color change and luminous reduction, which limit its applicability. Recently, Mn4+-doped fluoride phosphors have attracted considerable interest due to their practical merits. Brik and Srivastava have compiled a list of Mn4+activated phosphors.10 In the crystal structure of the Mn4+activated phosphors, Mn4+ is coordinated with six anions

forming an MnX6 octahedron. In particular, for Mn4+activated fluoride phosphors, the 2E-4A2 transition peaks with local vibronic modes of the MnF6 octahedron in Mn4+-activated phosphors produced narrow emission bands within the range of red-light emission, leading to a favorable color-rendering index and color gamut in phosphor-converted light emitting diode (pc-LED) applications.11-17 In this regard, Mn4+-activated K2SiF6 phosphors (referred to as K216) have recently been commercialized.18-26 Since there are two more K216 derivatives involving Rb and Cs instead of K at the 8c site of Fm-3m symmetry, the luminescent properties of Mn4+-activated Rb2SiF6 and Cs2SiF6 phosphors were investigated.27-30 Despite the relative advantages of a narrow-band emission for K216 phosphors, compared with the broad-band emission for Eu2+-activated nitride phosphors, a longer decay time limits the applicability of 216 phosphors. The decay behavior of K216 phosphors matters because a relatively long decay time in the millisecond range would raise problems when used in either high-power or acdriven LEDs. In this regard, we have recently discovered K3SiF7:Mn4+ (referred to as K317) as a possible alternative to the well-established commercially available K216, and we exclusively investigated the luminescent property and the decay behavior of the K317 phosphor.31.32 The decay rate of K317 phosphors was found to be faster than that of the K216 phosphor while the luminescent efficacy is simi-

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lar, which means that the K317 phosphor would outperform the K216 phosphor in some applications. Although there have been many investigations related to K216 phosphors and derivatives,11-26 there have been no further reports of the luminescence or decay behavior for K317 derivatives. In addition, a further improvement in decay time for K317 is required despite the superiority to K216. In this regard, Mn4+-activated Rb3SiF7 and Cs3SiF7 phosphors (referred to as R317 and C317) and their solid solution deserve detailed investigations, since Rb and Cs based fluorides are also known to constitute the structure in the P4/mbm space group similar to K317.33 In the present work, we employed a combinatorial materials search approach to systematically screen Mn4+-activated (K,Rb,Cs)3SiF7 ternary composition space in terms of luminescence and decay properties while maintaining the 317 structure. Based on a superiority in phase purity, luminescence, and more importantly decay rate, we focused on Mn4+-activated Rb-Cs binary solid solutions from the Mn4+-activated (K,Rb,Cs)3SiF7 ternary composition space, and examined these binary phosphors comprehensively in terms of XRD, photoluminescence, decay curve, diffuse reflectance, and density functional theory (DFT) calculations. Experimental Section The following analytic reagent-grade chemicals with a minimum assay of 99% or more were used during the synthesis: HF solution (Avantor 49 wt.%), KF (High Purity Chemicals 99%), RbF (Sigma Aldrich 99.8%), CsF (Sigma Aldrich 99.9%), H2SiF6 solution (Sigma Aldrich 33.5~35 wt.%), and K2MnF6 (Trikaiser 99.9%). A3SiF7 (A=K, Rb, Cs) was synthesized using a two-step synthesis approach wherein A2SiF6 was synthesized first followed by A3SiF7. For the synthesis of A2SiF6, a stoichiometric amount of AF (A=K, Rb, Cs) was dissolved in HF and mixed with H2SiF6 solution and stirred for 30 min at 25 °C. After a complete precipitation, the solution was filtered and the filtrate was washed several times with anhydrous ethanol, and dried at 110 °C. The synthesized A2SiF6 powder was mixed with a stoichiometric amount of AF using an agate mortar and pestle. The mixed raw materials were transferred to a specially designed sample container so called combi-chem container made of BN (80 × 40 × 20 mm), which involved 18 sample sites that were 8.5 mm in diameter and 16 mm in depth. The raw materials were then fired at an optimized temperature of 300 °C and 400 °C for 3 h under a mixed gas flow (H2/N2 5%, 500 cc/min) in a sealed-tube furnace. The total amount of raw materials at each sample site was about 0.3 g, which produced a sufficient amount of final phosphor powder available for use in various structural and optical characterizations. In order to examine the effects of AF and A2SiF6 (A = K, Rb, Cs) on the final phase formation, these compounds were synthesized with four different proportions of AF: 0.5, 1.0, 1.5, and 2.0

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mol. The Mn4+ content was fixed at 1 mol.% during synthesis. Each fired sample was then ground and subjected to powder X-ray diffraction (XRD) and photoluminescence (PL) analysis. The powder XRD measurements were carried out using a Rigaku Miniflex 600 powder diffractometer operating at 600 W (X-ray tube) with Cu Kα radiation and fitted with a 1D silicon strip detector for fast, high-resolution scanning. The emission spectra were monitored either at 360 or 460 nm excitation in a pseudo high-throughput manner using an in-house-fabricated, continuous-wave (CW) PL system equipped with a xenon lamp. The time-resolved emission spectra were measured using an in-house photoluminescence system involving a picosecond Nd:YAG (Continuum, Santa Clara, CA) laser with an excitation wavelength of 355 nm and a chargecoupled device sensor with a time resolution of 10 ns. We fixed both the delay time and the gate time at 0.1 ms. DFT Calculations. A generalized gradient approximation (GGA) parameterized by Perdew, Burke, and Ernzerhof (PBE)34-37 or a hybrid approach based on HeydScuseria-Ernzerhof (HSE) functionals38,39 were employed as an exchange correlation potential in the Vienna ab initio simulation package (VASP5.4).34,37,40 In addition, to reasonably deal with the localized 3d electrons of Mn, the DFT+U method41-43 was adopted with an on-site interaction and exchange parameter (Ueff = U − J) of 3.9 eV.44 We adopted a plane wave basis set with projectoraugmented wave (PAW) potentials.45,46 A 5 x 5 x 5 and 3 x 3 x 4 k-mesh grids based on the Monkhorst–Pack47 scheme were adopted for A2SiF6:Mn4+ and A3SiF7:Mn4+, respectively. The cut-off energy was 500 eV and the selfconsistency field tolerance threshold was 10-5 eV/atom. The positions of all atoms and lattice parameters were fully relaxed until the atomic forces converged to 0.02 eV Å−1. Spin polarizations were considered in the structural optimization. Results and Discussion (K,Rb,Cs)3SiF7:Mn4+ternary combinatorial library. We constructed a three-dimensional prismatic combinatorial library including four ternary combinatorial libraries inside with each ternary library corresponding to a different alkali fluoride (AF)/216 molar ratio. Since the 317 phosphors were synthesized by incorporating AF (A=K, Rb, Cs) to the respective 216 phosphors during synthesis, the composition in the ternary library represents the 216 ternary composition. An additional composition parameter designating an alkali element in AF should be taken into account. In addition, we adopted two different synthesis temperatures. As a result, six prismatic combinatorial libraries were required to represent the search space of all the compositional and processing parameters. It should also be noted that every composition in the combinatorial library is not the final composition but is, instead, a processing composition.


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Figure 1 Six prismatic combinatorial libraries comprising of pie graphs for a total of 504 sample sites obtained after the (a) structural and phase fraction analysis using XRD patterns and (b) measurement of the luminescent intensity, a darker color signifies a higher luminescent intensity. The AF/216 ratio increases from the left to the right side (0.5, 1, 1.5, and 2), as shown by the arrows from left to right. In Figure S1 in the supporting information, both the 3-D-type structural and luminescent combinatorial libraries are unfolded and a more detailed 2-D representation is available.

Figure 1 (a) shows six prismatic combinatorial libraries comprising of pie graphs for a total of 504 sample sites (consisting of one or more phases) obtained after the structural and phase fraction analysis using X-ray diffraction (XRD) patterns. The constituent phases present in the libraries include 216, 317, AHF2, and AF. Table 1

shows the constituent phase information involving the space group and the maximum peak index that was used for the construction of the pie graphs. The principles for the construction of a pie graph are described in our previous reports.48-50 Since the target structure was 317, the amber section in each pie graph in the library should be



maximized. However, the residual 216 phase was also observed as well as some other impurity phases such as AHF2 and AF. Single-phase 317 phosphors such as K317, R317, and C317 were successfully synthesized under different conditions, and these are marked with green arrows. The higher AF/216 ratio (2) took effect for K317 and C317, but a single phase of R317 was obtained with an AF/216 ratio value of 1. Although we achieved almost 100% pure, single-phase R317 and C317, however, K317 include minor impurity phases. By considering the previous finding that the optimum synthesis additive for K317 was AHF2 rather than AF,31 the appearance of the impurity phases in the K317 sample could be acceptable since we did not focus on the synthesis of K317, which was already well established, but focused on R317 and C317 phosphors in the present investigation. AF proved to be a more suitable agent for the synthesis of both R317 and C317. Single-phases of K317, R317, and C317 were obtained both at 300 and 400 ℃, but we focused only on the 300 ℃ library due to the higher luminescence intensity at 300 ℃ than that at 400 ℃, as shown in Figure 1(b). Six prismatic combinatorial libraries are represented in terms of luminescent intensity. It is well known that the Mn4+ ion oxidizes and precipitates as KMnF3 at temperatures higher than 300 °C,51 which should be the reason for the lower luminescence intensity at 400 °C despite the better crystallinity. By considering information from both the structural and luminescent combinatorial libraries, we pinpointed the most promising region in the combinatorial library, as highlighted by a pink rectangle in Figure 1(a). The binary solid solution of the Rb-Cs fluoride with a 317 structure was of particular concern in the present investigation. Readers should note that the rightmost entry in the pink rectangle region in Figure 1(a), i.e., the C317 side, does not actually stand for the pure C317 but includes a certain amount of Rb since we used RbF as a synthesis agent for 317 phosphors when constructing the combinatorial library. In order to reconstruct a more promising Rb-Cs binary solid solution region in the combinatorial library, even that involving pure C317, we separately prepared four Rb3SiF7-Cs3SiF7 binary solid solution samples (solid solutions and two pure R317 and C317 samples). Figure 2 (a) shows the XRD patterns for Rb-Cs binary compositions (marked as 1, 3, and 6) extracted from the combinatorial library in Figure 1(a). The XRD peaks for C317 were shifted to a lower angle compared with those of R317. This was because the ionic size of Cs, and in turn the lattice volume of C317, was larger. When scrutinizing the highest main peak for samples 1, 3 and 6, we noticed a peak shift slightly towards the lower 2θ values with an increase in the Cs content. Figure 2 (b) shows the XRD patterns of newly prepared Rb3SiF7-Cs3SiF7 binary solid solutions. Better crystallinity and fewer impurity phases with a similar peak shift and an increase in the content of Cs were also observed in the case of newly prepared Rb3SiF7Cs3SiF7 binary solid solutions. We turned to a full-pattern Rietveld refinement using Fullprof52 to further explain the crystal structures of the R317 (Rb:Cs=3:0) and RC317

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(Rb:Cs=2:1) phosphors that showed better luminescence. The initial structural model for the Rietveld refinement, a tetragonal structure in the P4/mbm was adopted from Hofmann et al.33 In the Rietveld refinement, a pseudovoigt function and a fifth-order polynomial were used to define the profile shape and the background, respectively. All the structural, profile and atomic displacement parameters were varied during the refinement. Because of the low concentration of Mn4+ (1 mol.%), it was not incorporated in the structural refinement, but due to the comparable ionic radii it is believed to occupy the Si4+ site in the structure. A very good fit between the observed and calculated pattern with an almost flat bakground was obatined for R317 and RC317, respectively, as shown in Figure 2 (c) and (d). This figure shows that both R317 and RC317 crystallized into an identical structure in the P4/mbm symmetry similar to that of K317. The structural parameters obtained from the Rietveld refinement are shown in Tables S1 and S2 in the supporting information. Table 1 Pie graph representation for the constituent phases with their chemical formula, space group, range of 2θ angle for the strongest peak in the XRD pattern, and the respective peak index. Pie

Chemical

Graph

Formula

Space Group

Angle 2θ For A=K 18.7~18.9

A2SiF６

Fm-3m

(A= K, Rb,Cs)

For A=Rb 29.7~29.9 For A=Cs 28.2~28.4

A3SiF7 (A= K, Rb,Cs)

P4/mbm

26.1~29.9 For A= K 34.4~34.6

AHF2 (A= K, Rb,Cs)

I4/mcm

For A=Rb 32.8~33.0 For A=Cs 29.9~30.0 For A=K 33.4~33.6

AF (A= K, Rb,Cs)


Fm-3m

For A=Rb 38.7~39.1 For A=Cs 37.4~37.7

Peak Index (1 1 1)

(2 2 0)

(2 2 0)

(2 0 1)

(1 1 2)

(1 1 2)

(1 1 0)

(2 0 0)

(1 1 0)

(1 1 0)

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The Rietveld refinement results clearly indicated that the incorporation of Cs ions with an ionic radius larger than that of Rb ions gave rise to volume expansion. A systematic variation of the volume with increases in the content of Cs is shown in Figure S2 in the supporting information. The crystal structure of A3SiF7 possesses two Wyckoff sites, 2a and 4h, for the A-site cations. Cations at the 2a site were coordinated with 6 F ions and those at the 4h site were coordinated with 10 F ions. Both the 2a and 4h Wyckoff sites were occupied by Rb ions in Rb3SiF7:Mn4+, while the 2a site was occupied by Cs ions

and the 4h site was occupied by Rb ions in the case of Rb2CsSiF7:Mn4+. The bond distances obtained from the Rietveld refinement ranged from 2.8977(4) to 3.011(6) Å and from 2.9825(3) to 3.105(14) Å at the 2a sites; from 2.770(5) to 2.990(5) Å and from 2.910(10) to 3.083(10) Å at the 4a sites for Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+, respectively. The incorporation of Cs ions in the lattice increased the bond distances and led to a significant reduction (from 0.061 Å to 0.0534 Å) in the difference between the average bond distance between the 2a and 4a sites. No significant difference in the average Si-F bond

Figure 2 (a) XRD patterns for Rb-Cs binary solid solutions from the combinatorial library along with reference patterns, no.1, 3 and 6 corresponds to the compositions in the ternary diagram shown inside the rectangle in fig. 1(a), and (b) for the newly prepared binary solid solutions of R317 and C317 samples in different proportions. 4+ 4+ Rb2CsSiF7:Mn is designated as RC317 and RbCs2SiF7:Mn as CR317. A full-pattern Rietveld refinement fit was obtained after using a tetragonal structure in the P4/mbm space group on powder XRD data in the 2θ range from 10-90° for (c) Rb3SiF7 and (d) Rb2CsSiF7. The black dots, red line, and blue line represent the experimental, calculated, and difference profiles, respectively. The vertical tick marks above the difference profile denote the positions of the Bragg reflections.


25.0k

25.0k

(a)

R317 RC317 CR317 C317

20.0k 15.0k

Intensity (a.u.)

10.0k 5.0k

(b)

R317 RC317 CR317 C317

20.0k 15.0k 10.0k 5.0k 0.0

0.0 300

350

400

450

500

575

550

e0

K216 K317 R317 RC317

e-1

(d)

e-2 0

625

650

675

(c)

1.0 0.8 0.6

R317 RC317 CR317 C317

0.4 0.2 0.0 200

5

10

15

20

300

400

500

600

700

800

Wavelength (nm)

Wavelength (nm)

Wavelength (nm)

Normalized Intensity (a.u.)

600

Normalized Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 11 Normalized Intensity (a.u.)


e0

K216 K317 R317 RC317

e-1

(e)

e-2 0

5

Time (ms)

10

15

20

Time (ms)

Figure 3 (a) Photoexcitation spectra measured at an emission wavelength of 631 nm; (b) PL spectra at an excitation of 460 nm; (c) diffuse reflectance spectra for R317, RC317, CR317, and C317; and decay curves for (d) 1 mol.% and (e) 4+ 10 mol.% Mn concentrations detected at 631 nm with a Nd:YAG laser excitation at 355 nm for K216, K317, R317, and RC317.

distance was observed by the incorporation of Cs ions in the lattice, and only the range of the bond distance was changed from 1.714(6) to 1.723(5) Å in Rb3SiF7:Mn4+ to 1.715(11) to 1.720(14) Å in Rb2CsSiF7:Mn4+. Since the Mn4+ occupying the Si4+ site, which is the 2d site in the lattice, is responsible for the luminescence, the energy/wavelength corresponding to the excitation/emission spectra remained almost the same because of the identical average Si-F bond distances in Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+. However, the difference in the bond distances of Cs/Rb-F in the lattice might indirectly affect the luminescence behaviors. Table S3 in the supporting information depicts all the cation-anion bond distances obtained from the Rietveld refinement. Details of the luminescence behavior are discussed in the following section. Luminescent and decay behaviors of Rb3SiF7-Cs3SiF7 binary, solid-solution phosphors. The PL and PL excitation spectra for R317 (Rb:Cs=3:0), RC317 (Rb:Cs=2:1), CR317 (Rb:Cs=1:2), and C317 (Rb:Cs=0:3) were similar to those of the well known K317.33 The photoexcitation spectrum exhibited well-known 4A24T1 and 4A24T2 excitation bands at approximately 350 and 450 nm, respectively, as shown in Figure 3 (a). The emission spectrum in Figure 3 (b) shows that the emission intensity for RC317 was the highest and the further incorporation of Cs deteriorated the emission intensity. The photoexcitation spectra coincided with the diffuse reflectance spectra shown in Figure 3 (c), as shown by the positions of the two valleys assigned to 4A24T1 and 4A24T2 that were well matched with the corresponding excitation bands. It is obvious that a small amount of Cs incorporation improved the PL

intensity without shifting the emission/excitation peaks, as evidenced both in the combinatorial library and in the newly prepared binary phosphors. In this respect, the PL intensity for RC317 was the highest among the Rb-Cs binary solid solutions. The lattice size expansion induced by the slight incorporation of Cs might have been related to the PL intensity improvement, but excessive Cs incorporation was found to be detrimental to PL intensity. If the remarkable improvement in the PL intensity for RC317 was scrutinized with regard to the structure in more detail, the difference in the bond distances of Cs/Rb-F in the lattice might indirectly affect the PL intensity, as already discussed earlier. It should also be noted that despite the conspicuous lattice volume expansion, the consistent Si-F bond length gave rise to consistent emission/excitation peak wavelengths.

Table 2 Decay times obtain from single exponential fitting and practical 10% decay times for K216, K317, R317, RC317 Compounds

Decay Time (τ)

Decay Time (10%)

1 mol%

10 mol%

1 mol%

10 mol%

K216

8.45 ms

7.49 ms

21.22 ms

19.28 ms

K317

5.80 ms

4.95 ms

15.44 ms

14.52 ms

R317

5.38 ms

4.59 ms

13.89 ms

12.90 ms

RC317

5.29 ms

4.45 ms

13.50 ms

12.66 ms


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Figure 3 (d) and (e) shows the decay curves for K216, K317, R317, and RC317. Here, we synthesized the Mn4+doped phosphors with not only 1 mol.% but also 10 mol.%. All the 317 phosphors were shown to decay faster than the K216 phosphor. For a more systematic comparison, we reduced the Mn4+ concentration to 1 mol.%, although the conventional Mn4+ concentration corresponding to the promising luminescent efficacy was much higher (~ 10 mol.%). Of course, the decay rate for the 317 phosphors is consistently much faster than that of the K216 at higher Mn4+ concentrations, as shown in Figure 3 (e). The decay curves have been fitted with single exponential formula and the decay times obtained from the fitting results along with practical 10% decay times are shown in Table 2. Among those 317 phosphors, the decay times for the R317 and RC317 phosphors were similar but a bit faster than that of the K317 phosphor. The PL intensity, howev-

er, was reduced with faster decay. A reasonable compromise between PL intensity and the decay rate can be obtained by nominating both R317 and RC317 as promising representatives. Since the PL intensity depends not only on the intrinsic nature of the material, but also on extrinsic traits such as particle size, activator distribution, surface conditions, etc., we assumed there may be fairly high possibility that the PL intensity could be improved by modifying the synthesis and post-synthesis powder processing for R317 and RC317. In contrast to the PL intensity, however, the decay rate more strongly reflects a material’s intrinsic physical property, which suggests that no approach could substantially improve the decay behavior of K216. By considering the fact that the crystal structures for K317, R317, and C317 are identical in terms of a similar lattice size, but thoroughly different from that of the K216, the typical multi-polar or exchange energy transfer

Figure 4 Density of states (DOS) for K216, R216, C216, K317, R317, and C317 phosphors as well as for undoped hosts using the different exchange-correlation functionals GGA, GGA + U, and HSE.



behavior also would occur in different manner, and in turn would result in a faster decay rate. Theoretical comprehension based on a multipolar interaction scheme was reported in our previous work, wherein a mathematical model was introduced that elucidated the decay differences between K216 and K317.31 In addition to energy transfer, the intrinsic material nature also would be one of the reasons for the faster decay. In fact, the measured decay rate depends not only on the energy transfer that is closely related to the activator concentration (i.e., the inter-activator distance) and to the crystal structure that makes the energy-transfer routes possible, but it also depends on some other well-known physical properties such as refractive index, phonon energy, spectral overlap, and the radiative decay rate of isolated Mn4+.53,54 Table 3 DFT-calculated band-gap and crystal field splitting (10Dq) using various exchange correlation functional schemes such as GGA, GGA + U, and HSE06. The 10Dq values are given in the parenthesis. All the units are in eV.

Compounds

GGAundoped

K216

7.20

R216

7.05

C216

7.01

K317

5.80

R317

5.62

C317

5.67

GGA

GGA+U

HSE06

7.32

7.32

9.57

(2.14)

(2.18)

(3.44)

7.11

7.08

(2.23)

(1.78)

9.30 (3.41)

7.03

7.00

9.17

(2.20)

(2.08)

(3.35)

5.93

5.90

7.94

(2.30)

(2.00)

(3.33)

5.69

7.68

(2.22)

5.68 (2.09)

5.69

5.68

7.60

(2.19)

(2.02)

(3.31)

(3.37)

HSE06undoped 9.55 9.13 9.15 7.80 7.54 7.52

DFT calculations for 216 and 317 phosphors. We examined the density of states (DOS) for 216 and 317 phosphors using DFT calculations. Figure 4 shows the DOS for K216, R216, C216, K317, R317, and C317 phosphors using the exchange correlation functional schemes GGA, GGA + U, and HSE. The DOS graphs for all the undoped 216 and 317 host compounds for the GGA and HSE methods are also included in Figure 4. Note that the on-site correction for localized d electrons for Mn ions, the GGA + U approach, showed no effect for undoped host compounds. The band-gap for GGA and GGA+U was similar, but increased significantly for HSE. Such a large band-gap value for the HSE method is generally well known.55 We should also note that the doping of Mn ions had no influence on the band gap. Table 3 shows the band-gap energy for all entries appearing in Figure 4. The ground states of t2g and eg caused by the splitting of Mn4+ ions in the octahedral site are prominent in the DOS

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plot, and the Mn-F hybridization is also conspicuous, for use in all the calculation schemes. All the t2g and eg states were located in the band gap as empty states for the 216 phosphors when the GGA method was applied. In all other cases, however, one of the t2g spin states is located below the highest occupied level, which falls below the top of the valance band. The crystal field splitting (10Dq) was estimated as an energy difference between the averages for the t2g and eg states over both the majority and minority spins. The 10Dq value is also listed in Table 3. All the 216 and 317 phosphors exhibited similar values of 10Dq when the same exchange-correlation functional was used. These calculated results indicated that the crystal field strength should be similar for all six phosphors since the octahedral local environments around the Mn4+ activator were all similar. Therefore, the calculated DFT results coincide with the experimental finding that the emission and excitation energies for all six phosphors are similar. The value of 10Dq was also more or less affected by the choice of exchange-correlation functional. The 10Dq for GGA and GGA+U was similar for all the 216 and 317 phosphors, whereas the HSE returned 10Dq values that were somewhat higher. In recent work, Du56 approximated the DFT-calculated E-4A2 emission energy with the spin flip of t2g states between a symmetric octahedron and the Jahn-Teller distorted octahedron that determines the level for the splitting of t2g (and eg). This approximation was successfully employed for predicting a consistent trend in the DFTcalculated 2E-4A2 emission energies for various fluorideand oxide-based host materials, although the exact value of the prediction was somewhat dubious. This approximation followed for K216, R216, and C216 when GGA was adopted as the model for exchange-correlation potential. However, one of the t2g spin states resides inside the valence band in all cases other than K216, R216, and C216 compounds, in which case the approximation can hardly be applicable. Furthermore, the experimentally measured 2 4 E- A2 emission and excitation energies for all the Mn4+ phosphors in the present investigation showed an almost complete coincidence, so that it was not necessary to establish a trend for the DFT-calculated 2E-4A2 emission energy. Therefore, we did not attempt to calculate the emission energy using the approximation. 2

Conclusions We discovered novel Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+ phosphors that crystallize into P4/mbm symmetry. These phosphors were pinpointed from the three-dimensional prismatic ternary combinatorial library. Incorporation of Cs led to improvement in the emission intensity, and thereby Rb2CsSiF7:Mn4+ was proven to be superior to the Rb3SiF7:Mn4+. The discovered phosphors exhibit a red light emission, similar to the well-known and commercially available K216 phosphors. However, these phosphors have a shorter decay time compared with that of K216, and even slightly shorter than our recently discovered K317 phosphors. The shorter decay time could be a crucial advantage when used in LED applications. All of the 216 and 317 phosphors have been examined in terms of


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DFT-calculated density of states. The discovered novel Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+ phosphors would enlarge the scope of red light-emitting phosphors for use in LED applications considering the fact that the K216 phosphors and variants are seen only as an option at their current stage of development.

ASSOCIATED CONTENT Supporting Information. Tables for the structural parameters obtained after the Rietveld refinement, 2-D representation of pie graphs, and variation of unit cell volume with Cs content in (Rb,Cs)3SiF7. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ∥M. K. and W. B. P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M3D1A1069705).

ABBREVIATIONS K216, K2SiF6; R216, Rb2SiF6; C216, Cs2SiF6; K317, K3SiF7; R317, Rb3SiF7; C317, Cs3SiF7; RC317, Rb2CsSiF7; CS317, Cs2RbSiF7; GGA, Generalized Gradient Approximation; PBE, Perdew, Burke, and Ernzerhof; VASP, Vienna ab initio simulation package; PAW, Projector Augmented Wave;

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Table of Content (TOC)

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Rb3SiF7:Mn4+ and Rb2CsSiF7:Mn4+ Red-Emitting Phosphors with a

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