Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ forWarm White Light-Emitting DiodesYe Jin,†,‡ Mu-Huai Fang,† Marek Grinberg,§ Sebastian Mahlik,§ Tadeusz Lesniewski,§ M.G. Brik,⊥,||,#

Guan-Yu Luo,○,△ Jauyn Grace Lin,△ and Ru-Shi Liu*,†,⬡

†Department of Chemistry, ○Department of Physics, and △Center for Condensed Matter Sciences, National Taiwan University,Taipei 10617, Taiwan‡School of Optoelectronic Information, Chongqing University of Technology, Chongqing 400054, China§Institute of Experimental Physics, University of Gdan sk, 80-952 Gdansk, Poland⊥College of Sciences, Chongqing University of Posts and Telecommunications, Chongqing 400065, China||Institute of Physics, University of Tartu, Tartu 50411, Estonia#Institute of Physics, Jan Dlugosz University, Czestochowa PL-42200, Poland⬡Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University ofTechnology, Taipei 106, Taiwan

*S Supporting Information

ABSTRACT: Red phosphors AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) havebeen widely studied due to the narrow red emission bands around 630 nm. The differentemission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL,taking KNaSiF6:Mn4+ as an example, was investigated here. The effects of pressure on crystalstructure and luminescence were determined experimentally and theoretically. The increaseof band gap, red shift of emission spectrum and blue shift of excitation spectrum wereobserved with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge)were found clearly distorted from 180° in MF6

2− octahedron with strong ZPL intensity. Thelarger distorted SiF6

2− octahedron, the stronger ZPL intensity. This research provides a newperspective to address the ZPL intensity problem of the hexafluorosilicate phosphors causedby crystal distortion and pressure-dependence of the luminescence. The efficacy of the devicefeaturing from Y3Al5O12:Ce

3+ (YAG) and KNaSiF6:Mn4+ phosphor was 118 lm/W with thecolor temperature of 3455 K. These results reveal that KNaSiF6:Mn4+ presents goodluminescent properties and could be a potential candidate material for application in back-lighting systems.

KEYWORDS: KNaSiF6:Mn4+, red phosphor, ZPL, high pressure, warm white LED

White light-emitting diodes (WLEDs) are used extensivelyin backlights, flashlights, and automobile headlamps as

replacements for conventional incandescent and fluorescentlamps because of their high energy efficiency, durability,reliability, and capacity for use in products with various sizesand more eco-friendly components.1,2 Phosphors have beenwidely investigated for use in phosphor-converted WLEDs. Themost widely used WLED combines the yellow phosphorY3Al5O12:Ce

3+ (YAG) with a blue InGaN LED chip. However,this LED presents a low color rendering index (CRI) (Ra < 80)and high correlated color temperature (CCT > 6000 K)because of the absence of red emission in its spectrum.3

Therefore, red phosphors, such as Sr2Si5N8:Eu2+ and (Sr,Ca)-

AlSiN3:Eu2+ or Mn4+-doped red phosphors, have been added to

the devices in order to improve their color-renderingproperties.4−15 The broad emission band of nitride phosphorsis often longer than 650 nm, and serious reabsorption rendersthem unsuitable for improving the efficiency of a device.

Therefore, Mn4+-doped red phosphors, especially AMF6:Mn4+

(A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge) with narrow bandemissions at around 630 nm, have recently attracted increasedresearch attention.5−15 Team of Adachi et al.7−12 synthesized aseries of fluorides, including KNaSiF6, using wet chemicaletching. The authors demonstrated the luminescence propertiesof the phosphor at normal pressure and showed that theKNaSiF6:Mn4+ system is an intermediate of homodialkalineK2SiF6:Mn4+ and Na2SiF6:Mn4+ narrow emission band redphosphors,11 whereas Zhu et al.13 successfully synthesizedK2TiF6:Mn4+ through a simple cation exchange reaction.The emission wavelength of the d−d transition of Mn4+ ions

is easily influenced by various environmental factors, contraryto the parity-forbidden f−f transitions of rare-earth ions. The

Received: February 15, 2016Accepted: April 22, 2016Published: April 22, 2016

Letter

www.acsami.org

© 2016 American Chemical Society 11194 DOI: 10.1021/acsami.6b01905ACS Appl. Mater. Interfaces 2016, 8, 11194−11203

AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge) havedifferent zero-phonon lines (ZPLs) because of their MF6

2−

octahedral structures.12 In this study, for example, whereasK2SiF6:Mn4+ shows no ZPL intensity,13,14 an intense ZPL isobserved for Na2SiF6:Mn4+.15 K2GeF6:Mn4+ exhibits twophases, one with an intense ZPL and another without ZPL.5

In this work, KNaSiF6:Mn4+ was synthesized via a simplecoprecipitation method. The intermediate phase of KNa-SiF6:Mn4+ showed different structures and luminescent proper-ties, especially in terms of ZPL intensity, from K2SiF6: Mn4+

and Na2SiF6: Mn4+. The mechanism of the ZPL emergence, aswell as its intensity and position, has not been previouslyreported yet. This study aims to elucidate the mechanism of theZPL intensity in AMF6:Mn4+ (A = Na, K, Cs, Ba, Rb; M = Si,Ti, and Ge). We clearly found variations in the angles ∠FMnF= ∠FMF(M = Si, Ti, Ge) of the phosphors AMF6:Mn4+ (A =Na, K, Cs, Ba, Rb; M = Si, Ti, and Ge). Distortion of theMnF6

2− octahedron is the key factor influencing ZPL intensity.The properties of the resultant phosphors were examined byphotoluminescence (PL) under both ambient pressure andhigh pressure and are verified by the calculation method.Moreover, the LED obtained from mixing of the narrow-bandred emission phosphor KNaSiF6:Mn4+ with conventionalYAG:Ce3+ yellow phosphor can be excited by blue LED chip.This LED shows an enhanced CRI for lighting and backlightingapplications.The fluoride phosphor compounds, namely, KNaSiF6:0.06

Mn4+, K2SiF6:0.06 Mn4+, and Na2SiF6:0.06 Mn4+, wereprepared from high-purity NaF, KF, SiO2, HF, KMnO4, andH2O2 via two-step coprecipitation.14,15 K2MnF6 was preparedfirst A mixture consisting of 6.7 g of KF and 0.45 g of KMnO4was dissolved in a 30 mL solution of 48% HF. Then, 0.3 mL ofH2O2 was added to the solution drop by drop. The solutiongradually turned from deep purple to yellow, and a yellowprecipitate was obtained. The powder was washed several timesusing alcohol and acetone and then dried at 70 °C. Theprepared K2MnF6 powder (0.30 g) was dissolved in siliconfluoride solution (1.2 g of SiO2 in 35 mL of 48% HF) withstirring. Meanwhile, 1.26 g of NaF and 1.75 g of KF weredissolved in 15 mL of 48% HF. Then, the NaF and KF solutionwas added to the K2MnF6 solution with stirring for 5 min.Some yellow powder was obtained and then the powder waswashed with ethanol (99.9%) and dried at 70 °C. The sampleswere subsequently prepared for measurement. K2SiF6:0.06Mn4+ and Na2SiF6:Mn6+ were prepared using the same method.To prevent any injury from hydrogen fluoride, all theexperimental processes are conducted under good protectivecloth, gloves, and goggles.The samples were analyzed by X-ray diffraction (XRD) using

a D2 Phaser (Bruker) diffractometer operating with Cu Kαradiation (λ = 1.5418 Å) to identify their phase purity. Thesynchrotron XRD pattern of KNaSiF6:Mn4+ was also examinedfor refinement at the BL01C2 beamline of the NationalSynchrotron Radiation Research Center (Taiwan) at awavelength of 0.688808 Å. Total Pattern Analysis Solution(TOPAS) software was used to perform XRD Rietveldrefinement. PL spectra were measured using a FluoroMax-3spectrophotometer. Temperature-dependent luminescence(25−300 °C) was determined using a THMS-600 apparatuscombined with PL equipment. All solid-state nuclear magneticresonance (NMR) spectra were acquired on a 14.1 T wide-boreBruker Avance III spectrometer equipped with a 4 mm magic-angle-spinning (MAS) probe head. The Larmor frequency for

29Si was 119.2 MHz, the spinning rate for the 29Si MAS NMRspectra was 10 kHz, and the recycle delay was 60 s. PLexcitation (PLE) spectra were recorded using a systemconsisting of a 150 W Xe lamp, two monochromators SPM2,and two Hamamatsu R928 photomultipliers (the first forluminescence and the second for reference signal detection).Steady-state luminescence measurements were performed withan Andor SR-750-D1 spectrometer equipped with a CCDcamera (DU420A-OE). The excitation source was a He−Cdlaser at 442 nm. We used the system consisting of a PL 2143 A/SS laser and a PG 401/SH parametric optical generator toobtain luminescence decays. Emission signals were analyzedusing a Bruker Optics 2501S spectrograph and a HamamatsuStreak camera (model C4334−01). High hydrostatic pressurewas applied to a Merrill Bassett-type diamond anvil cell.Polydimethylsiloxane oil was used as the pressure-transmittingmedium, and pressure was measured in terms of shifts in the R1luminescence line of ruby.The composition, crystal structure, and phase purity of the

as-synthesized samples were investigated by powder XRD. Thesynchrotron XRD with synchrotron light source and TOPASfitting software were also used to obtain more detailedstructural information on KNaSiF6:Mn4+ phosphor. Figure 1adisplays the Rietveld structure refinement results of KNa-SiF6:0.06Mn4+. The crystallographic data of KNaSiF6 are usedas the initial structure model during refinement, and the actualstructure of KNaSiF6:0.06Mn4+ is constructed. The diffractionpeaks of KNaSiF6:Mn4+ could be indexed to an orthorhombicPnma structure (ICSD No. 07−1334). The lattice parameters,including a = 9.33208(21) Å, b = 5.50742(11) Å, c =9.80288(22) Å, α = β = c = 90°, and cell volume =503.825(19)Å3, are also accurately determined. The χ2 and R-Bragg valuesare 1.44 and 6.01, respectively, which indicates the validity ofthe refinement process.The bond angle of the central atom was examined to study

the relationship between the phosphor stucture and its PLspectrum. The refinement data show that there are twodifferenet bond angles. The angle of the axial bond pair isapproximately 179.29°, whereas the angle of two other bondpairs in the equatorial part is 174.11°. When Mn4+ ions areintroduced to the host, these ions substitute Si4+ sites to formthe MnF6

2− group, comparing with the ionic radii and valencestates (rSi

4+ = 0.040 nm and rMn4+ = 0.053 nm), and the unit cell

crystal structure and coordination environment of KNaSiF6 areshown in Figure 1b. A KNaSiF6 unit cell clearly contains fourformula units. Only one type of SiF6

2− octahedral group isobserved. The Si4+ ions are surrounded by six F− ions, with twodifferent angles: ∠F1SiF2 = 174.11° and ∠F3SiF4 = 179.29°(Figure 1c). The 29Si NMR spectrum of KNaSiF6:0.06Mn4+ isshown in Figure 1d. The spectrum of KNaSiF6 contains onlyone resonance at −188 ppm because this phosphor presentsonly one Si six-coordinate site.To investigate the structure well, we performed two sets of

density functional theory (DFT)-based calculations. Thesecalculations are based on generalized gradient approximation(GGA) with the Perdew−Burke−Ernzerhof (PBE) functional16and local density approximation (LDA) with the Ceperley−Alder−Perdew−Zunger (CA-PZ) functional.17,18 The calcu-lations are performed with the CASTEP module19 of theMaterials Studio package, and a summary of the experimentaland calculated structural data for KNaSiF6 is presented in Table1. The electron configurations are considered as follows: Na,2s22p63s1; K, 3s23p64s1; Si, 3s23p2; and F, 2s22p5. The cutoff

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energy is chosen as 370 eV. The k-point mesh is set to 5 × 8 ×5, corresponding to a separation between k-points of 0.02 1/Å.The convergence criteria are as follows: energy, 1 × 10−5 eV/atom; maximal force, 0.03 eV/Å; maximal stress, 0.05 GPa; andmaximal displacement, 0.001 Å.Comparative results from Table 1 show that the experimental

and theoretical structural data (including the lattice constantsand fractional atomic positions) present good agreementamong all data sets. The largest differences betweenexperimental and optimized lattice constants were +4% forconstant c (LDA calculations) and −3% for a constant (GGAcalculations). An overall agreement with the experimental datais beneficial for the GGA calculations.After optimization of the KNaSiF6 structure, the electronic

and optical properties of the phosphor were calculated atambient and elevated hydrostatic pressure. Figure 2 shows the

calculated band structure at ambient pressure. The band gap isdirect, and the band gap values are 7.417 eV (LDA) and 6.98eV (GGA). The real band gap is approximately 8−9 eV becauseDFT-calculated band gaps are always underestimated.The valence band of KNaSiF6 is remarkably flat, which

reveals the very low mobility of the holes. By contrast, thedispersion of electronic states forming the conduction band ishighly pronounced. Well-resolved sub-bands separated bynarrow gaps are found in the valence band. The nature ofthese gaps, as well as the origin and composition of allcalculated bands, can be understood with the help of thedensity of states (DOS) diagrams (Figure 3).

Figure 1. (a) Rietveld refinement results of the powder X-raydiffraction profile of KNaSiF6:0.06Mn4+ phosphor (Rwp = 10.48%, Rp =8.18% and χ2 = 1.44); (b) crystal structure of KNaSiF6:Mn4+; (c)crystal structure of the SiF6

2− octahedron; (d) 29Si NMR spectrum ofKNaSiF6:0.06Mn4+.

Table 1. Experimental and Calculated Structural Data of KNaSiF6

exp. LDA GGA

lattice constants (Å)a 9.3246 9.102674 (+2.4%) 9.606752 (−3.0%)b 5.4992 5.293121 (+3.7%) 5.583111 (−1.5%)c 9.7892 9.388901 (+4.1%) 9.840516 (−0.5%)

fractional atomic positionsx y z x y z x y z

K 0.5136 0.2500 0.1798 0.5154 0.2500 0.1747 0.5117 0.2500 0.1753Na 0.3718 0.2500 0.5596 0.3789 0.2500 0.5501 0.3736 0.2500 0.5506Si 0.2290 0.2500 0.9206 0.2300 0.2500 0.9226 0.2288 0.2500 0.9214F1 0.3274 0.4660 0.9941 0.3316 0.4757 0.9998 0.3288 0.4684 0.9952F2 0.1254 0.0367 0.8548 0.1221 0.0262 0.8525 0.1241 0.0322 0.8542F3 0.1244 0.2500 0.0601 0.1191 0.2500 0.0679 0.1230 0.2500 0.0642F4 0.3313 0.2500 0.7843 0.7806 0.2500 0.7806 0.3323 0.2500 0.7813

Figure 2. Calculated band structure (GGA-calculated, solid line; LDA-calculated, dashed line) of KNaSiF6.

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The conduction band is formed by the 3s and 4s states of Naand Ka, respectively. The valence band is formed by the 2pstates of F and 3p states of Si. The 2p states of F ions atnonequivalent crystallographic sites are separated from eachother and form the unusual structure of the valence band. The3p states of Si sharply peak at the bottom of the valence band atapproximately −4 eV, whereas the F 2p states are spread overthe whole valence band. The 3p states of K produce a sharppeak at approximately −9 eV, whereas the 2p states of Na, 2sstates of F are spread from around −18 to −22.5 eV.The calculated effective Mulliken charges are as follows

(LDA/GGA data are given): K + 1.04/+1.02, Na + 0.74/+0.76,Si + 2.10/+2.16, and F −0.65/-0.65. Nearly all of the ions(except for K) show charges that are very different from thoseexpected from their chemical formula. These findings indicate aconsiderable degree of covalency between the Si−F and Na−Fbonds; K ions are bound to their neighbors in the crystal latticepredominantly by ionic bonds.The effects of pressure on the physical properties of a solid

can be modeled by optimizing the structure of a consideredmaterial at elevated pressures. The pressure range was set from0 GPa (ambient pressure) to 10 GPa, with a step size of 2 GPa.Decreases in the volume of a crystal with pressure can be fittedto the Murnaghan equation of state, as follows20

= + ′− ′

⎜ ⎟⎛⎝

⎞⎠

VV

BPB

1B

0

1/

(1)

where V and V0 are the volumes at elevated pressure P andambient pressure, respectively, B is the bulk modulus, and B′ isits pressure derivative. Figure 4a shows the results of therelative volume of a unit cell change in the LDA/GGA runs.The fit of the calculated data (symbols) to the Murnaghanequation (solid line) yields the following values: for LDA, B =24.25 ± 0.58 GPa, B′ = 6.18 ± 0.25; and for GGA, B = 22.83 ±0.32 GPa, B′ = 4.13 ± 0.12. The lattice constants a, b, and c alsodecrease with pressure according to the linear (for a and c) andquadratic (for b) fits shown in Figure 4b. Furthermore, theband gap changes with pressure (Figure 4c). Figure 4d showsthe cross-section of the electron density difference. In thisfigure, the loss of electrons is described by a blue coloration,and electron enrichment is indicated by a red coloration. Thewhite regions correspond to a very small difference in electrondensity compared with that of a free atom. These characteristicsconfirm our conclusion obtained from the analysis of effective

Mulliken charges that the chemical bonds between Si and Fions are strongly covalent because of the overlap of red andblue colors, whereas Na and K ions are bound to the crystallattice by ionic bonds because of the scarce overlap of colors.PL and PLE spectra of the phosphors measured at room

temperature are shown in Figure 5a. The excitation spectrumcontains two broad bands around 355 and 460 nm (matchingwith the LED chips), corresponding with the spin-allowtransitions 4A2g →

4T1g and4A2g →

4T2g of Mn4+, respectively.The phosphor emits red emissions originating from the spin-forbidden d-d transition 2Eg →

4A2g with several narrow bandsat 610−650 nm under 460 nm excitation. The strongestemission is found at 629 nm. According to group theory, thereare six fundamental internal vibronic modes, namely, ν1, ν2, ν3,ν4, ν5, and ν6, of the octahedral group with Oh symmetry.13

Three peaks (longer than 620 nm) belong to Stokes ν6, ν4, andν3 peaks, and two other peaks (shorter than 620 nm) are anti-Stokes ν6 and ν4 peaks. The peak at approximately 620 nm isthe ZPL (Figure 5b), which is the electronic dipole forbiddenin the octahedral MnF6

2−. The MnF62− octahedron is distorted

because of ∠F1MnF2 = ∠F1SiF2 = 174.11° and ∠F3MnF4 =∠F3SiF4 = 179.29° (Figure 1c). Intense ZPL could be observedat 620 nm. The higher distortion of MnF6

2− octahedron givesrise to the stronger the ZPL intensity. The distortion of acoordination octahedron around Mn4+ ion removes the centerof inversion and then the parity selection rule is lifted (there areno odd/even states). The following examples and the PLspectra of KNaSiF6:Mn4+ under high pressure are in goodagreement with that fact or conclusion. ZPL intensities aredifferent among K2SiF6:Mn4+, KNaSiF6:Mn4+, andNa2SiF6:Mn4+ (at 616 nm) (shown in Figure S1) because oftheir distinct distortions. The crystal structures of K2SiF6 and

Figure 3. Calculated density of states diagrams of KNaSiF6.

Figure 4. (a) Calculated dependence of the relative volume V/V0change (filled/open symbols, LDA/GGA) of KNaSiF6 and fitting tothe Murnaghan equation (solid lines); (b) calculated dependence ofthe lattice constants (filled/open symbols, LDA/GGA) of KNaSiF6and fitting as a function of pressure P (solid lines); (c) calculateddependence of the band gap (filled/open symbols, LDA/GGA) ofKNaSiF6 and fitting as a function of pressure (solid lines); (d) cross-section of the electron density difference in KNaSiF6 in the planeperpendicular to the b axis.

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Na2SiF6 are shown in Figures S2 and S3. Only one type ofMnF6

2− octahedron without distortion (∠F1MnF1 = ∠F1SiF1 =180°) is found in K2SiF6. Thus, no ZPL or very weak is foundin the PL spectrum of this phosphor.8,9,13,14 By contrast twotypes of MnF6

2− octahedrons are found for Na2SiF6. Oneoctahedron is without distortion (∠F1MnF1 = ∠F1SiF1 = 180°),but the other is seriously distorted (∠F2MnF3 = ∠F2SiF3 =171.48°). Thus, a strong ZPL is observed in KNaSiF6:Mn4+

phosphor. The position of the ZPL depends on theperturbation of the cation ion. The full-width at half-maximumof the PLE provides more evidence of distortion. The orbitaloverlaps or hybridizes because of the distortion. Moreover, thePLE spectrum is broadened. As the angles of K2SiF6, KNaSiF6and Na2SiF6 are 180, 174.11, and 171.48°, respectively (justconsidering only larger distortions), the full width at half-maximum of the PLE should follow the order: K2SiF6 <KNaSiF6 < Na2SiF6. These characteristics are consistent withthe following experimental results: 54, 69, and 75 nm forK2SiF6, KNaSiF6 and Na2SiF6, respectively (Figure S1). Theroom-temperature PL spectra of KNaSiF6:Mn4+ measured atdifferent hydrostatic pressures are presented in Figure 6a. ThePL spectra consist of six characteristic lines related to the 2Eg→4A2g transition (the ZPL and phonon repetition). Allluminescence lines have red-shift as the pressure increases.The high-pressure PL spectra in Figure 6a show that the ZPLintensity increases with pressure (Figure S4). Increasingpressure results in linear (for a and c) and quadratic (for b)decreases in lattice constants a, b, and c (shown in Figure 4b),which indicates that distortion is more serious with increasing

pressure. The ZPL intensity also increases with pressure. Allexperimental results, including the high-pressure PL, andcalculation results, support our supposition of the ZPL intensityof Mn4+ in fluoride, which has not been previously reported.The room temperature PLE spectra of KNaSiF6:Mn4+

measured under different pressures are shown in Figure 6b.The excitation spectra contain two broad bands at 355 and 460nm, corresponding to the spin-allowed transitions 4A2g →

4T1gand 4A2g →

4T2g, respectively. Both excitation bands have blueshift as the pressure increases. The energy of the maxima shiftsnearly linearly with increasing pressure at a rate ofapproximately 90 cm−1/GPa (Figure 7). As will be shown inthe next paragraph the PLE blue shift is related to increase ofthe crystal field strength and the PL red shift could be ascribedto hybridization of the orbital or an energy level split due to thedecrease in lattice parameters a, b, c and cell volume (V) withincreasing pressure, as shown by the calculation results inFigure 4. Also, although the emission transition from 2Eg to4A2g in the T-S diagram does not change with the pressure, the

Figure 5. (a) Photoluminescence (PL) and PLE spectra ofKNaSiF6:Mn4+; (b) PL spectra of KNaSiF6:Mn4+ phosphor.

Figure 6. (a) Emission spectra of KNaSiF6:Mn4+ under differentpressures upon 442 nm excitation; (b); excitation spectra ofKNaSiF6:Mn4+ under different pressures observed at maximumluminescence. Dashed curves present the spectra measured afterdecompression, to show that the pressure effect is reversible.

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Reach parameters B and C decrease. Therefore, the emissionenergy will decrease and show the red-shift spectrum. Figure 7shows the energy of the excitation bands, as well as position ofthe ZPL of the 2Eg →

4A2g transition in KNaSiF6:Mn4+ underpressure. The dependence of the PL energy on pressure is notcompletely linear. Two ranges of up to 6 GPa and above 7 GPacould be distinguished from the PL spectra.Mn4+ contains three electrons in its 3d shell and replaces Si4+

ions in octahedral coordination with six F− ions. The electronicconfiguration of Mn4+ (3d3) is similar to that of Cr3+, which isone of the best known and most extensively investigatedtransition metal ions. Thus, many results obtained for the Cr3+

system can be used to describe Mn4+ because of thisequivalence; in particular, the model that describes the pressuredependence of the luminescence of the octahedrally coordi-nated Cr3+ on pressure could be related to Mn4+.21 The energylevels of Mn4+ on the crystal field strength are well-illustratedby the Tanabe-Sugano energy diagram (Figure 8a),22 where the

energetic structure of the 3d3 system is described quantitativelyby the Racach integrals B and C and by the crystal field strength10Dq.23,24 The Tanabe-Sugano diagram presents the depend-ence of the energies of localized states on the quantity of Dq/B.The configurational coordinate diagram in Figure 8b presentsthe ground and the first excited states of Mn4+. Theconfiguration coordinate Q in this diagram represents the

positions of F− with respect to Mn4+. Because the ground 4A2gstate and the doublet state 2Eg (as well as the 2T1g and

2T2gstates, which are not presented in Figure 8b) belong to theground electronic configuration t2

3, the transitions betweenthese states do not change the electrons special distribution.Therefore, the interactions between 3d3 electrons and fluoriteions are the same in these states. The related parabolas haveminima at the same value of Q:Q1. The excited quartet states4T2g and 4T1g belong to the excited electronic configurationt22e. Thus, transitions from the ground state to these states

change the spatial electrons distributions, which causesexpansion of the lattice around Mn4+ and the minima of therespective parabolas are shifted to position Q2. The latticerelaxation energy can be calculated as follows

ωℏ =−

Sk Q Q( )

22 1

2

(2)

where S and k are the Huang−Rhys factor and elastic constant,respectively, and ℏω is the energy of local vibration mode.Given that the PL spectrum consists of narrow bands related

to 2Eg →4A2g transition and their phonon repetitions, Mn4+ in

KNaSiF6 is the high-field system where the energy of the 4T2gstate is larger than that of the 2Eg state. According to the crystalfield model, the energy of 4T2g with respect to the ground state

i s e q u a l t o 1 0Dq , 2 3 − 2 5 w h e r e =D ZeR

354 5 a n d

∫= | |q R r r r( ) de2105 3d

2 4 .26 Thus,

∫= | |DqZe

RR r r r

6( ) d

2

5 3d2 4

(3)

where R in eq 3 is the average distance between Mn4+ and F−

ions, and R3d(r) is the radial part of the 3d wave function.According to eq 3, the value of 10Dq should increase whenpressure increases.The value of 10Dq (and Dq) from the excitation spectrum

can be determined by the following relation

ω= → − ℏDq E S10 ( A T )42g

42g (4)

where E(4A2g →4T2g) is energy of the lower excitation band

maximum and Sℏω is the energy of electron lattice coupling.Analysis of the excitation spectra reveals Sℏω = 2500 cm−1,which is independent of pressure. Under this assumption, eq 4allows calculation of the pressure dependence of Dq. Thefollowing relations allow calculation of B and C for differentpressures.

=Δ − Δ

Δ −B Dq

E Dq E DqE Dq

( / ) 10( / )10( / 8)

2

(5)

=Δ ′ − +

CE B B Dq9 9 /

3

2

(6)

where ΔE is the difference between the energy of the 4T1g and4T2g states and ΔE′ is the energy of the 2Eg state. All quantitiesdepend nearly linearly on pressure. The obtained values forambient pressure and pressure rates are listed in Table 2.Quantity of bulk modulus and their first derivative with

respect pressure obtained from ab initio calculations facilitateverification of eq 3 which is fundamental relation for crystalfield model.

Figure 7. Energies of the specific transitions of KNaSiF6: Mn4+ as afunction of pressure; solid lines represent the linear fittings (Table 2).

Figure 8. (a) Tanabe−Sugano diagram for Mn4+; (b) configurationalcoordinate diagram including the ground and first excited states;excitation and emission transitions are represented by arrows,respectively.

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Considering that pressure is isotropic and effects in thesimilarly in the microscopic and macroscopic scale, thefollowing relation should be valid

= =Dqp

DqR

Rp

DqB

dd

dd

dd

53 (7)

Our spectroscopic data yield the pressure rate for crystal fieldstrength as dDq/dp = 9.4 cm−1/GPa; the calculated value ofbulk modulus B = 24 GPa provides 5Dq/3B = 26 cm−1/GPa.The calculated result is over twice the experimental dDq/dp.Such a difference has been previously reported for materialsdoped with Cr3+ and is believed to be related to increases inlocal microscopic compressibility with respect to the bulkmaterial.27

The 2Eg →4A2g luminescence decay curves were measured at

different pressures under excitation at 442 nm to elucidate theluminescence behavior of KNaSiF6. The results are shown inFigure 9a. The decay of the emission intensity can be well-fittedby a single exponential equation for all considered pressures.The decay constant for Mn4+ emission is equal to 6.099 ms atambient pressure before compression and 6.416 ms at 1.3 kbarafter pressure-decompression. Figure 9b shows that the decaytime of the Mn4+ emission increases as pressure increases. Suchbehavior has been observed for Cr3+28,29 and for Mn4+ in othermaterials.30 The 2Eg →

4A2g transition is spin-forbidden, andluminescence can appear because of the admixture of the lowestquartet state 4T2g to the first excited state labeled 2Eg in Figure8, through spin−orbital interaction. The complete modelallowing the calculation of the probability of 2Eg → 4A2gtransition in high field materials considering the spin−orbitalinteraction and electron phonon coupling in the excited 2Eg,2T1g, and

4T2g electronic states with their vibronic structures, isdiscussed in detail in the literature.27 In a high-field systemwhere the 2Eg and

4T2g states are separated by a large energydistance, the quartet contribution to the 2Eg electronic manifoldcan be expressed using the perturbation approach. The lowestemitting state in this framework is the zero-phonon state of thefirst excited electronic manifold 2Eg. This state is represented bysuperposition of the spin doublet and spin quartet wavefunctions, as follows

∑

ψ χ ψ

χω

Φ = +

Δ + ℏ

−⎜ ⎟ ⎜ ⎟⎛⎝

⎞⎠

⎛⎝

⎞⎠q Q q Q q V

Q Sn

( , ) ,12

( ) ,32

( )

( )n

nn

0E E

0T s o

T 0

(8)

In eq 8, ψ ( )q,E12

and ψ ( )q,T32

are the electronic wave

functions describing the 2Eg and4T2g states, r is the electronic

coordinate, and χE0(Q), χT

n (Q) are the respective vibronic wavefunctions of the 2Eg and

4T2g electronic manifolds. Vs−o is thespin orbit coupling constant, which can be determined asfollows:

∫ ψ ψ= *− −⎜ ⎟ ⎜ ⎟⎛⎝

⎞⎠

⎛⎝

⎞⎠V q H q q,

12

,32

ds o E s o T (9)

where Hs−o is the spin−orbit coupling Hamiltonian and S0n isthe vibronic overlap integral, calculated as follows

∫ χ χ= =!

* −S Q Q QS

n( ) ( )d en

n Sn

0 E0

T/2

/2

(10)

2Eg → 4A2g luminescence lifetime can be calculated fromfollowing relation:

∑τ τ ω

= | |Δ + ℏ−V

Sn

1 1( )n

n

Ts o

2 02

2(11)

where 1/τT is the probability of4T2g →

4A2g radiative transition.When both Sℏω and ℏω are smaller than the separation energyΔ, the following relation can be obtained:

τ τ ω≈

| |Δ + ℏ

−VS

1 1( )

s o2

T2

(12)

Table 2. Parameters of the Crystal Field Model and TheirDependence on Pressure

value for ambientpressure (cm−1)

pressure shift(cm−1/GPa)

Dq 1925 ± 3 9.4 ± 0.2B 646 ± 20 −0.8 ± 0.3C 4065 ± 20 −7.1 ± 0.3E(2E) for pressure <7 GPa 16103 ± 3 −20 ± 0.7E(2E) for pressure >8 GPa 16000 ± 3 −12.5 ± 0.3Δ= E(4T2) − E(2E) forpressure <7 GPa

3250 ± 6 11.4 ± 0.27

Δ= E(4T2) − E(2E) forpressure >8 GPa

4092 ± 6 10.5 ± 0.27

Figure 9. (a) 2Eg →4A2g decay profiles of KNaSiF6:Mn4+; (b) decay

times of 2Eg → 4A2g emission in KNaSiF6:Mn4+ as a function ofpressure; the solid curve represents the calculated lifetime.

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The 2Eg →4A2g luminescence lifetime can be calculated using eq

12, and the results are presented by the solid curve in Figure 9b.

Fitting was achieved by assuming thatτ

| |−Vs o

T

2

is equal to (5.3 ±

0.5) × 109 cm−2 s−1 and is independent of pressure. Forpressures greater than 15 GPa the lifetime τ increase slower,then it is predicted by eq 12. This result can be explained by thenonhydroelasticity of pressure, which generates shear-strengthin the pressure transmitting medium and sample. The 4T2g →4A2g transition is parity forbidden and can be allowed due tointeractions with optical phonons and odd parity latticedistortions. The effects of shear-strength at high pressuregenerate odd parity distortion that diminishes the value of τTand this effect is responsible for additional diminishing of theMn4+ luminescence decay time at high pressure, not consideredin relation 12. It is consistent with the effect of the increase ofthe intensity of the ZPL with respect to phonon repetitionswith increasing pressure. Considering possible distortions onecan related this effect to odd parity static distortion of the MnF6system induced by pressure above 15 GPa.Figure 10 shows the temperature-dependent luminescent

spectrum of KNaSiF6:Mn4+ under 460 nm excitation. No peak

shifts while the intensity decreases as the temperatureincreasing. The integrated area, rather than the intensity, isconsidered in this study because of the narrow-band emissionsof Mn4+. Figure 10b shows the integrated area ofKNaSiF6:Mn4+ in the temperature range of 298−573 K. At

350 K, the relative intensity remains over 100% compared withthat at 298 K and then decrease to 79% at 400 K. This resultindicates excellent thermal stability for LEDs because the chiptemperature of LED is approximately 400 K. The KNa-SiF6:Mn4+ red-emitting phosphor is a prospective candidatematerial for LEDs.Quantum efficiency (QE), another important parameter for

practical applications, can be calculated using the followingexpression:

∫∫ ∫

η =−

L

E EQEs

R s (14)

where LS is the emission spectrum of the studied sample, ES isthe spectrum of the light used to excite the sample, and ER isthe spectrum of the excitation light without the sample in theintegrating sphere. The internal efficiency QE of KNa-SiF6:0.06Mn4+ at 460 nm excitation is 0.90, and its externalQE is 0.41. All of the results indicate that KNaSiF6:Mn4+

phosphor may be a candidate material for LED applications.Package experiments were performed to evaluate the

potential application ability for KNaSiF6:Mn4+ (Figure 11).Table 3 compares two different LED devices. The Ra and R9 of

Figure 10. (a) Temperature-dependent thermal luminescent spectrumof KNaSiF6:Mn4+ and (b) relative intensity as integrated emissionspectra.

Figure 11. (a) Chromaticity coordinates of the devices of theCommission Internationale de l’Eclairage 1931 color spaces; (b) light-emitting diode (LED) device with blue chip and YAG phosphor; (c)LED device with blue chip, YAG, and KNaSiF6:Mn4+; (d) electro-luminescent spectra of blue chips with YAG and KNaSiF6:Mn4+.

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the device combining a blue LED chip with YAG phosphor areonly 69 and 0, respectively, because of the absence of redphosphor. Moreover, the CCT is 5318 K; thus, the device emitscool white light. The CCT should be lower than 4000 K and Rashould be higher than 80 to obtain warm white light. Forcomparison, the device consistent with the blue chip, YAG, andKNaSiF6:Mn4+ shows warm white light with Ra = 90 and R9 =93. The CCT of this device is much lower than that of theprevious one. Normally, adding red phosphor into LED deviceswill decrease their luminescent efficacy because of Stokesshifting. However, the efficacy of the device we fabricated fromYAG and KNaSiF6:Mn4+ phosphor was 118 lm/W as the colortemperature is 3455 K. These results reveal that KNaSiF6:Mn4+

presents good luminescent properties and could be a potentialcandidate material for use in for back-lighting systems.In summary, a simple two-step coprecipitation method was

used to synthesize the hexafluorosilicate phosphor compounds,namely, Na2SiF6:Mn4+, K2SiF6:Mn4+, and KNaSiF6:Mn4+. Weclearly demonstrated the luminescence properties and ZPLintensity of the phosphors based on their PL spectra underambient pressure and high pressures. Distortion of the MnF6

2−

octahedron was demonstrated based on the crystal structure ofthe phosphors. Analytical results from the PL spectra underambient pressure and high pressure, XRD measurements, andcorresponding calculations of lattice parameters showed thatthe ZPL intensity of the phosphors behaves as follows: the ZPLintensity appears when the MnF6

2− octahedron is distorted andthe more distorted the angle ∠FMnF = ∠FMF(M = Si, Ti, Ge)is from 180°, the more intense the ZPL intensity. This studyprovides a new perspective of the ZPL intensity of AMF6:Mn4+

phosphors. The LED device was fabricated from YAG andKNaSiF6:Mn4+ phosphor and its efficacy was 118 lm/W withthe color temperature of 3455 K. These results reveal thatKNaSiF6:Mn4+ presents good luminescent properties and couldbe a candidate material for use in back-lighting systems.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b01905.

Crystal data and Rietveld refinement results of theKNaSiF6:0.06Mn4+ phosphor; PL and PLE spectra ofNa2SiF6:Mn4+, KNaSiF6:Mn4+ and Na2SiF6:Mn4+; Crystalstructures of K2SiF6:Mn4+ and the SiF6

2− octahedral;crystal structures of Na2SiF6:Mn4+, Si(1)F6

2− octahedral,and Si(2)F6

2− octahedral; dependence of IZPL/Iν6 ratioon pressure (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: rsliu@ntu.edu.tw.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors are grateful to the Ministry of Science andTechnology of Taiwan (Contracts MOST 104-2119-M-002-027-MY3 and MOST 104-2923-M-002-007-MY3) and Na-tional Center for Research and Development Poland Grant(SL-TW2/8/2015). Y.J. thanks the National Nature ScienceFoundation of China (No. 11104366), Chongqing ResearchProgram of Basic Research and Frontier Technology(cstc2014jcyjA50018), and the Scientific and TechnologicalResearch Program of Chongqing Municipal EducationCommission (KJ1500913). M.B. appreciates the support fromthe Programme for the Foreign Experts offered by ChongqingUniversity of Posts and Telecommunications, EuropeanRegional Development Fund (Center of Excellence “Meso-systems: Theory and Applications,” TK114), Ministry ofEducation and Research of Estonia, Project PUT430, theRecruitment Program of High-end Foreign Experts (No.GDW20145200225). M.B. also thanks the Institute of Physics,Polish Academy of Sciences, for the Visiting Professorshipposition. We thank Dr. G. A. Kumar (University of Texas atSan Antonio) for the Materials Studio package.

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Table 3. Photoelectric Parameters of the LEDs

chromaticitycoordinate

phosphor blendCCT(K) Ra R9 x y

efficacy (lmW−1)

YAG+KNaSiF6:Mn4+

3455 90 93 0.3977 0.3654 118

YAG 5318 69 0 0.3374 0.3614 164

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