Journal of Luminescence 131 (2011) 2652–2660

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Optical properties of Mn4þ-activated Na2SnF6 and Cs2SnF6 red phosphors

Yusuke Arai, Sadao Adachi n

Graduate School of Engineering, Gunma University, Kiryu-shi, Gunma 376-8515, Japan

a r t i c l e i n f o

Article history:

Received 18 April 2011

Received in revised form

14 June 2011

Accepted 15 June 2011Available online 7 July 2011

Keywords:

Red phosphor

Na2SnF6

Cs2SnF6

K2SiF6

K2TiF6

Hexafluorometallate

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.06.042

esponding author. Tel.: þ81 277 30 1710; fax

ail address: adachi@el.gunma-u.ac.jp (S. Adac

a b s t r a c t

Alkaline hexafluorostantanate red phosphors Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ are synthesized by

chemical reaction in HF/NaMnO4 (CsMnO4)/H2O2/H2O mixed solutions immersed with tin metal. X-ray

diffraction patterns suggest that the synthesized phosphors have a tetragonal symmetry with the space

group D4h14 (Na2SnF6:Mn4þ) and a trigonal symmetry with the space group D3d

3 (Cs2SnF6:Mn4þ).

Photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and the Raman scattering techni-

ques are used to investigate the optical properties of the phosphors. The Franck–Condon analysis of the

PLE data yields the Mn4þ-related optical transitions to occur at �2.39 and �2.38 eV (4A2g-4T2g) and at

�2.83 and �2.76 eV (4A2g-4T1g) for Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ , respectively. The crystal field

parameters (Dq) of the Mn4þ ions in the Na2SnF6 and Cs2SnF6 hosts are determined to be �1930 and

�1920 cm�1, respectively. Temperature-dependent PL measurements are performed from 20 to 440 K

in steps of 10 K, and the obtained results are interpreted by taking into account the Bose–Einstein

occupation factor. Comprehensive discussion is given on the phosphorescent properties of a family of

Mn4þ-activated alkaline hexafluoride salts.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Searching for new red phosphors or those containing a redemission component is a useful approach to the realizationof white-light-emitting diode (white LED) applications [1–4].Alkaline hexafluoride red phosphors, such as K2SiF6:Mn4þ andK2TiF6:Mn4þ , have recently been demonstrated to meet theefficacy and color-quality targets of future ‘‘warm-white’’ LEDdevices [5]. These phosphors exhibit efficient red emission underblue (�460 nm) or UV excitation (�360 nm) [6,7]. The emissionspectra of MnF2�

6 ions in such red phosphors show large numbersof sharp vibronic-progressed lines in a region of 1.9–2.1 eV due tothe 2Eg-

4A2g transition [6,7].Incomplete phosphorescent and vibronic data are available in the

literature on the MnF2�6 ions in a number of alkaline hexafluoride

complexes. Hexafluorostantanates, K2SnF6 �H2O, Na2SnF6, andCs2SnF6, belong to a family of alkaline hexafluorides. Although uniquephosphorescent properties of the K2SnF6 �H2O:Mn4þ hydrate phos-phor have been reported by the present authors [8], no detailedstudy has been performed on Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ untilnow. Only a report [9] was found in which photoluminescence (PL)spectrum of Cs2SnF6:Mn4þ measured at 80 K was reported. Theauthor of this study prepared Cs2SnF6:Mn4þ phosphor by fluorinatinga mixture of CsCl, MnCl2 �4H2O, and Cs2SnCl6 at 350 1C.

ll rights reserved.

: þ81 277 30 1707.

hi).

K2SnF6 �H2O is only the hydrate material among a family ofA2XF6 salts (A¼an alkaline ion, X¼a tetravalent ion). In our previouswork [8], we found that vacuum evacuation, immersion in metha-nol, or heating in air leads to partial dehydration of K2SnF6 �

H2O:Mn4þ . The dehydration-induced lattice deformation resultedin the splitting of the Mn4þ-related emission lines, together with anenhancement of the zero-phonon line (ZPL) emission intensity. Suchunique photoluminescent properties have not been observed pre-viously in any other phosphor or luminescent material.

This study is conducted to determine the optical properties ofMn4þ-activated Na2SnF6 and Cs2SnF6 using PL analysis and PLexcitation (PLE) spectroscopy. Many of their fundamental propertieshave not been thoroughly evaluated or are even unknown. Thephosphors used here were prepared by the reaction of tin metalwith HF/alkaline permanganate/H2O2/H2O solutions. To the best ofour knowledge, no such unique synthesis method has been usedpreviously for obtaining pure or Mn4þ-activated hexafluorostanta-nate. Raman scattering spectroscopy is also used to investigate theinternal vibronic properties of the SnF2�

6 and MnF2�6 ions in pure and

Mn4þ-activated Na2SnF6 and Cs2SnF6. It is interesting to investigatedifference in the phosphorescent properties between the variouskinds of Mn4þ-activated alkaline hexafluorides. Comprehensivediscussion is also given on this subject.

2. Experimental

The starting material used in this study was metallic tin platelets(b-Sn) with 99.9% purity. The Sn platelets were degreased using

Fig. 1. SEM images of (a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ synthesized by

wet chemical etching.

rb. u

nits

)

Before purification

Na2SnF6:Mn4+

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–2660 2653

organic solvents in an ultrasonic bath and rinsed in deionized water.They were then chemically etched in a mixture of HF/NaF(CsF)/H2O2. This etching procedure produced white Na2SnF6 and Cs2SnF6

powders. The Mn4þ-activated Na2SnF6 or Cs2SnF6 was synthesizedby immersing pure Na2SnF6 or Cs2SnF6 powder in a 100 cc HF (50%)/6 g NaMnO4 �H2O/100 cc H2O mixture or in a 100 cc HF (50%)/1.6 gCsMnO4/100 cc H2O mixture for about 20 min at room-temperature,respectively. It should be noted that the present synthesis producedsome manganese oxides as the fate of surplus Mn in our synthesizedNa2SnF6:Mn4þ powder. Because of this, Na2SnF6:Mn4þ powder waspurified by immersing in a solution of HCl:acetone¼1:10 for about5 min. The synthesized phosphor powders were finally rinsed inmethanol and then dried in air at 60 1C for 2 h.

The structural properties of our synthesized phosphors wereinvestigated by scanning electron microscopy (SEM) at an accel-erating voltage of 15 kV (JEOL, JSM-6330F). The crystallinity of thephosphors was examined by XRD analysis using an RAD-IIC X-raydiffractometer (Rigaku) with Cu Ka radiation. EPMA measure-ments were performed with a SHIMADZU EPMA-1610 at aprobe current of 100 nA and an accelerating voltage of 15 kV.The chemical compositions, such as Na, Cs, Sn, F, and Mn weredetected. Because of little Mn quantities, we could not determineits accurate values from the EPMA measurements. From ourprevious measurements on the X-ray photoelectron spectroscopyof K2SiF6:Mn4þ [10]; however, we estimate the Mn quantities inNa2SnF6:Mn4þ and Cs2SnF6:Mn4þ to be an order of 0.1 mol%.

PL and PLE spectra were measured using a fluorescencespectrometer (Hitachi, F-4500) by excitation at lex�470 nm (PL)or by monitoring at lem�630 nm (PLE) at room-temperature. Inaddition, temperature-dependent PL measurements were per-formed using a single monochromator equipped with a charge-coupled device (Princeton Instruments PIXIS 100) in a CryoMinicryostat (Iwatani Industrial Gases) and a stainless cryostat (Tech-nolo Kogyo) between 20 and 440 K in steps of 10 K. The 325 nmline of He–Cd laser (Kimmon, IK3302R-E) was used as theexcitation light source. The Raman spectrometer consisted of anArþ ion laser (Showa Optronics Corp., GLG 3110) and a charge-coupled device (Princeton Instruments, PIXIS 100) with an LP02-488 interference filter (Semrock, Inc.). The Raman spectra weremeasured at room-temperature.

After purification

XR

D in

tens

ity (a

20 30 40 50 60 70 80

Tetragonal (ASTM)

2� (deg)

Fig. 2. XRD patterns of Na2SnF6:Mn4þ (a) before and (b) after purification in HCl/

acetone mixture for about 5 min. (c) The ASTM card pattern of tetragonal Na2SnF6.

3. Results and discussion

3.1. Structural properties

Fig. 1a and b shows the SEM images of Na2SnF6:Mn4þ andCs2SnF6:Mn4þ synthesized in the HF/NaMnO4/Sn and HF/CsMnO4/Sn mixed solutions, respectively. Many particles with lengths of�100–300 mm (Na2SnF6:Mn4þ) and �10–20 mm (Cs2SnF6:Mn4þ)were prepared. The following chemical reaction can be proposed:

6HFþ2AMnO4þð1�xÞSn-A2ðSn1�xMnxÞF6þð2�xÞMnO2

þ3H2Oþ12ð2xþ1ÞO2, ð1Þ

where A2(Sn1�xMnx)F6 represents the Mn4þ-activated red phos-phor, i.e., A2SnF6:Mn4þ (A¼Na or Cs). Porous surface morphologyfor Na2SnF6:Mn4þ (Fig. 1a) was produced after chemical treatmentin the HCl:acetone¼1:10 solution (i.e., purification process).

Fig. 2a and b shows XRD traces measured in the y�2y scanmode of Na2SnF6:Mn4þ before and after purification in theHCl:acetone¼1:10 mixture, respectively. The XRD data of tetra-gonal Na2SnF6 powder obtained from the American Societyfor Testing and Materials (ASTM) card are also shown in Fig. 2c(#01-080-1155).

The experimental XRD data in Fig. 2a indicate that thesynthesized phosphor without purification reveals a lot of diffrac-tion peaks. The purification in the HCl/acetone mixture forabout 5 min led to a greatly reduced peak number (Fig. 2b). Thepurified powder has a tetragonal structure with the space groupD14

4hðP42=mnmÞ (Fig. 2c) [11,12].Fig. 3a shows XRD trace of Cs2SnF6:Mn4þ , together with that

obtained from the ASTM card with a trigonal structure (space

Cs2SnF6:Mn4+

20 30 40 50 60 70 80

Trigonal (ASTM)

2� (deg)

XR

D in

tens

ity (a

rb. u

nits

)

Fig. 3. XRD patterns of Cs2SnF6:Mn4þ together with the ASTM card pattern of

trigonal Cs2SnF6.

Table 1Crystal structures and optical transition energies of 3d (Mn4þ) electrons in

Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ .

Property Alkaline hexafluorostantanate

Na2SnF6:Mn4þ Cs2SnF6:Mn4þ

Crystal structure Tetragonal Trigonal

Space group D144hðP42=mnmÞ D3

3dðP3m1Þ

Lattice constant a¼0.50532 nm a¼0.6322 nm

c¼1.0122 nm c¼0.5032 nm

ZPL energy (2Eg-4T2g) 2.005 eV 1.989 eV

ZPL energy (4A2g-4T2g) 2.39 eV (S¼4) 2.38 eV (S¼4)

Dq 1930 cm�1 1920 cm�1

ZPL energy (4A2g-4T1g) 2.83 eV (S¼9) 2.76 eV (S¼10)

1.85 1.90 1.95 2.00 2.05 2.1010-3

10-2

10-1

100ZPL

PL in

tens

ity (a

rb. u

nits

)Photon energy (eV)

Linear

PL in

tens

ity (a

rb. u

nits

)

Log

10-2

10-1

100

600620640660

ZPL

Wavelength (nm)

Linear

Stokes anti-Stokes

Log

�3

�4�6

�6

�6�6�4

�4

�3

�3

�3

Fig. 4. Room-temperature PL spectra of (a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ

plotted on a logarithmic and a linear scale. Vertical arrows show the ZPL energies

in the 2Eg-4A2g transition.

300 350 400 450 500 550 600 65010-4

10-3

10-2

10-1

100

Wavelength (nm)

PLE

(nor

mal

.)

ZPL 4A2g→4T2g ZPL PL (a

rb. u

nits

)2Eg→4A2g

10-4

10-3

10-2

10-1

100

2.02.53.03.54.0

ZPL4A2g→4T2g4A2g→-4T1g

4A2g→4T1g

PL

PLE

ZPL

Photon energy (eV)

2Eg→4A2g

Fig. 5. PLE and PL spectra of (a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ at 300 K.

Vertical bars represent the results calculated using Eq. (2). Vertical arrows show

the ZPL energies in the 4A2g-4T2g and 4A2g-

4T1g transitions (see also Table 1). To

clarify each ZPL position, the PLE spectra were plotted on a logarithmic scale.

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–26602654

group¼D33d�P3m1, #01-070-0141) in Fig. 3b [13]. No purification

was performed for Cs2SnF6:Mn4þ . The experimental XRD data inFig. 3a agree with the ASTM card pattern (Fig. 3b). The structuralparameters of Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ are listed inTable 1.

3.2. PL and PLE spectra

3.2.1. Room-temperature PL and PLE spectra

Fig. 4 shows room-temperature PL spectra of (a) Na2SnF6:Mn4þ

and (b) Cs2SnF6:Mn4þ measured by excitation with the 325 nm lineof a He–Cd laser. The PL spectra reveal main peaks at n3�1.93 eV(t1u stretching), n4�1.97 eV (t1u bending), n6�1.98 eV (t2u bending),n6�2.03, n4�2.05, and n3�2.08 eV in Na2SnF6:Mn4þ (Fig. 4a) andat n3�1.91, n4�1.95, n6�1.96, n6�2.01, n4�2.03, and n3�2.06 eVin Cs2SnF6:Mn4þ (Fig. 4b), where ni’s indicate the local vibronicmodes of the MnF2�

6 octahedron in A2SnF6. Essentially the sameemissions are observed in some Mn4þ-activated phosphors, such asCaAl12O19 [14], Gd3Ga5O12 [15], SrTiO3 [16], YAlO3 [17], YAl3(BO3)4

[18], and 3.5MgO �0.5MgF2 �GeO2 ([19], see also [20]). The long- andshort-wavelength emission lines on the zero-phonon line (ZPL)peaks in Fig. 4 are known as the Stokes and anti-Stokes lines,respectively. The ZPL energies are listed in Table 1. Many combina-tion lines of internal vibronic frequencies are also found in Fig. 4.

Fig. 5 shows room-temperature PLE and PL spectra of(a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ . Two broad excitationbands were observed with peaks at �470 and �370 nm. Theeffect of the crystal field on the energy level of the 3dn system canbe interpreted using the standard energy level diagram [21,22].

The red emissions observed in Figs. 4 and 5 are ascribed to the2Eg-

4A2g transition of the Mn4þ ion in the SnF2�6 octahedral site

of A2SnF6. The strong excitation band with a peak at �470 nm inFig. 5 can be assigned to the 4A2g-

4T2g transition. This excitation

600 610 620 630 640 650

1.921.962.002.04

Wavelength (nm)

300 K

200 K

100 K

20 K

ZPL

400 K

×2

×3×5

×10×30

×100

×2×2×2

×5×5×5×5

Photon energy (eV)

PL in

tens

ity (a

rb. u

nits

)

×10

�6�4

�3�4

�6

Fig. 6. Temperature-dependent PL spectra of Na2SnF6:Mn4þ between 20 and

440 K in increments of 20 K.

610 620 630 640 650

1.921.941.961.982.002.02

ZPL

Wavelength (nm)

300 K

200 K

100 K

20 K

×3×15×50

400 K

Photon energy (eV)

PL in

tens

ity (a

rb. u

nits

)

×10

�6�4

�6 �4 �3

Fig. 7. Temperature-dependent PL spectra of Cs2SnF6:Mn4þ between 20 and

440 K in increments of 20 K.

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–2660 2655

band usually consists of a number of components with a spacingof �55–65 meV and is understood as a vibronic progression ofthe fundamental frequency combined with an unsymmetricalvibration of the MnF2�

6 octahedron superimposed on the 4A2g-4T2g transition [6,7].

The intensity of the nth-order vibronic sideband Iexn can be

expressed using the ZPL intensity (Iex0 ) and Poisson distribution

function as [23,24]

Iexn ðnÞ ¼ Iex

0 expð�SÞSn

n!, ð2Þ

where S is the mean local vibronic number (Huang–Rhys factor).The vertical bars on the right-hand side of Fig. 5a and b showthe results calculated using Eq. (2) with S¼4 (4A2g-

4T2g). TheFranck–Condon analysis of Eq. (2) yielded the ZPL (n¼0) energiesto be �2.39 eV (Fig. 5a) and �2.38 eV (Fig. 5b) in the firstexcitation band (see Table 1). From these ZPL energies, we obtainthe crystal field parameters Dq to be �1930 and �1920 cm�1

for the Mn4þ ions in the Na2SnF6 and Cs2SnF6 hosts, respec-tively. The vibronic energies fit-determined here were �65 meV(Fig. 5a and b).

The vertical bars on the left-hand side of Fig. 5a and b show theresults calculated using Eq. (2) with S¼9 and 10 for the secondexcitation band (4A2g-

4T1g), respectively. Here we assumed thesame vibronic energies as those used in the first excitationband (4A2g-

4T2g). The corresponding ZPLs were determined tobe �2.83 eV (Fig. 5a) and �2.76 eV (Fig. 5b), as also listed inTable 1.

It should be noted that some of the ligand field-dependenttransitions have a vibronic oscillation structure that makes it verydifficult to exactly determine the ZPL energy. The Franck–Condonanalysis using Eq. (2) enabled the precision determination of theZPL energies in Na2SnF6 and Cs2SnF6. The present analysis alsodetermined vibronic quantum energy of �65 meV, which is thesame as that of the internal MnF2�

6 vibronic mode (n2, see Table 3later).

3.2.2. Temperature dependence of PL spectra

Fig. 6 shows temperature-dependent PL spectra of Na2

SnF6:Mn4þ between 20 and 440 K. The ZPL emission could notbe clearly observed at 300 K (Fig. 4), but it was observed as avery weak and sharp peak at low temperatures. This is becausethe electronic transitions 2Eg-

4A2g in the MnF62� octahedrons of

alkaline hexafluorides are electronic dipole forbidden. At 20 K,the red emission peaks n3, n4, and n6 are observed only on thelow-energy (Stokes) side of the ZPL emission. At 300 K, thesered emission peaks appear not only on the Stokes side butalso on the high-energy (anti-Stokes) side. Essentially the samespectral features were observed in Cs2SnF6:Mn4þ , as shown inFig. 7.

Figs. 8 and 9 show the PL spectra measured at 20 and 300 K inA2SnF6:Mn4þ with (a) A¼Na and (b) Cs, respectively. For com-parison, the PL spectra of cubic K2SiF6:Mn4þ at 20 and 300 K areshown in Fig. 10 [7]. All the vibronic-assisted PL peaks n3, n4, andn6 in Figs. 8 and 9 exhibited clear crystal field splittings inenergies of �3�6 meV. In K2SiF6:Mn4 (Fig. 10), the 2Eg state istwofold spin�orbit degenerate. We can, thus, expect no splittingsin the PL spectra of this phosphor. We have also observed nosplittings in the PL spectra of cubic Cs2XF6:Mn4þ with X¼Si andGe [25].

In Fig. 11, we plot the n6-mode Stokes and anti-Stokes PL peakenergies versus temperature (T) in Na2SnF6:Mn4þ between 20and 300 K. For comparison, the excitonic-gap energy versus T

plots for GaSe are shown in Fig. 11 [26]. As understood fromFig. 11, this semiconductor has a direct band-gap energy, which isnearly the same as the red emission lines in Na2SnF6:Mn4þ . It can

be understood from Figs. 6 and 11 that not only the n3 mode,but also the n4 and n6 mode lines are relatively invariant withrespect to T. This fact clearly indicates that the red emissions are

-80 -60 -40 -20 0 20 40 60ΔE (meV)

300 K

20 K×5 ×5

PL in

tens

ity (a

rb. u

nits

) ~6 meV

~6 meV ~6 meV

~6 meV

ZPLanti-StokesStokes ↓

Na2SnF6:Mn4+

ν4ν6

�6

�6

�4

�4�3

�3

Fig. 8. PL spectra of Na2SnF6:Mn4þ at 20 and 300 K. The ZPL energies correspond

to 2.012 and 2.005 eV at 20 and 300 K, respectively.

-80 -60 -40 -20 0 20 40 60ΔE (meV)

300 K

20 K×5 ×5

PL in

tens

ity (a

rb. u

nits

) ~5.6 meV

~3.3 meV

~3.6 meV

ZPLanti-StokesStokes ↓

Cs2SnF6:Mn4+

�3�4 �6

�6

�4

�3 �4

�6

Fig. 9. PL spectra of Cs2SnF6:Mn4þ at 20 and 300 K. The ZPL energies correspond

to 1.990 and 1.989 eV at 20 and 300 K, respectively.

-80 -60 -40 -20 0 20 40 60ΔE (meV)

300 K

20 K×5

PL in

tens

ity (a

rb. u

nits

)

ZPLanti-StokesStokes ↓

K2SiF6:Mn4+

�6

�4

�3 �4

�6

�6�4

�3

Fig. 10. PL spectra of K2SiF6:Mn4þ at 20 and 300 K. The ZPL energies correspond

to 1.998 and 1.993 eV at 20 and 300 K, respectively.

0 50 100 150 200 250 3001.96

2.00

2.04

2.08

2.12

Na2SnF6:Mn4+

Temperature (K)

PL p

eak

ener

gy (e

V)

GaSe

�6 (anti-Stokes)

�6 (Stokes)

Fig. 11. Temperature-dependent n6-mode Stokes and anti-Stokes PL peak energies

in Na2SnF6:Mn4þ , together with the n¼1 excitonic peak energy in GaSe [26]. The

solid line shows the result calculated using Eq. (3).

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–26602656

due to the intra-d-shell transitions of the Mn4þ ion inNa2SnF6:Mn4þ .

The excitonic-gap energies of GaSe in Fig. 11 exhibit aremarkable dependence on T. The solid line represents the resultcalculated using the Varshni expression [27]

EgðTÞ ¼ Egð0Þ�aT2

Tþb, ð3Þ

where Eg(0)¼2.110 eV is the discrete-exciton (n¼1) energy atT¼0 K, a¼6.0�10�4 eV/K, and b¼200 K.

The linear temperature coefficient @ni/@T at T¼300 K in Fig. 11is ��2�10�5 eV/K (NaSnF6:Mn4þ). Differentiating Eq. (3) with

respect to T, we obtain

@EgðTÞ

@T¼�a 2T

Tþb�

T2

ðTþbÞ2

" #: ð4Þ

This equation gives the temperature coefficient @Eg/@T of�5�10�4 eV/K for GaSe at T¼300 K, which is nearly the sameas those for the tetragonally-bonded semiconductors (Si, GaAs,ZnSe, etc.) [27]. Thus, we can conclude that the temperaturevariations of the intra-d-shell transitions in many Mn4þ-dopedalkaline hexafluorides are about twenty times smaller than thoseof the band-gap (and excitonic-gap) energies in the tetragonally-bonded semiconductors.

Fig. 12 shows the integrated PL intensity (IPL) versus tempera-ture (T) plots for (a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ . The IPL

values were obtained by integrating the PL spectra in Figs. 6 and7. The experimental IPL data in Fig. 12 show a remarkabletemperature variation. The considerably large dip observed at

0 100 200 300 400 500

0

0.5

1.0

1.5

2.0

0

2

4

6

8

10

T (K)

I PL(T)

/I PL(

0)

Fig. 12. Integrated PL intensity IPL versus temperature T for (a) Na2SnF6:Mn4þ and

(b) Cs2SnF6:Mn4þ . The heavy and light solid lines indicate the results calculated

using Eqs. (7) and (8), respectively.

1.90 1.95 2.00 2.05 2.10

PL in

tens

ity (a

rb. u

nits

)

Photon energy (eV)

ZPL

1 atm Vacuum

↓Stokes

A=K

Na

Cs

NH4�3

�3

�3�4

�6

�6�4

�4�6

�6�4

�3

�3

�4 �6

�6

�4

�3

anti-Stokes

Fig. 13. Room-temperature PL spectra of A2SnF6:Mn4þ with (a) A¼K (K2SnF6 �H2O:

Mn4þ), (b) Na, (c) Cs, and (d) NH4.

1.90 1.95 2.00 2.05 2.10

PL in

tens

ity (a

rb. u

nits

)

Photon energy (eV)

ZPL

1 atm Vacuum

↓Stokes

X=Si

Ge

Sn

Ti

A=K

anti-Stokes

�4�3

�6

�6�4

�3

Fig. 14. Room-temperature PL spectra of K2XF6:Mn4þ with (a) X¼Si, (b) Ge, (c) Sn

(K2SnF6 �H2O:Mn4þ), and (d) Ti.

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–2660 2657

T�300 K in Fig. 12a arose from the competition between thedecreased PL peak height and the increased spectral width inthe temperature range �250�370 K. In Fig. 12b, the IPL

value gradually increased with increasing T and then abruptlydecreased with a further increase in T (Z350 K). Thus, the IPL

versus T data in Fig. 12b can be explained using the two differentmechanisms below and above T�350 K.

The temperature dependence of the PL intensity can beexpressed as [28]

IPLðTÞ ¼ cX

j ¼ 3,4,6

/i9Mj9fS�� ��2 E0þ

12 81

2

� �4njþ

12 81

2

� �, ð5Þ

with

nj ¼1

expðhnj=kBTÞ�1, ð6Þ

where c is a proportionality constant, 9/i9Mj9fS9 is an opticaltransition operator, E0 is the ZPL energy, h is the Planck constant,kB is the Boltzmann constant, and nj is the local vibronicfrequency. The upper and lower signs in Eq. (5) correspond tothe Stokes and anti-Stokes processes, respectively, and nj is theBose–Einstein occupation number.

Using Eq. (3), IPL(T) can be written as

IPLðTÞ ¼ I0PLþ I1

PL 1þ2

expðhn=kBTÞ�1

� �, ð7Þ

where n is the effective vibronic frequency. The heavy solid linesin Fig. 12 show the results of fitting the experimental data to Eq.(7) with I0

PL ¼ 0:8 and I1PL ¼ 0:2 for Na2SnF6:Mn4þ and I0

PL ¼�2:2and I1

PL ¼ 3:2 for Cs2SnF6:Mn4þ . The effective vibronic frequencieshn determined here are 25 meV for both Na2SnF6:Mn4þ andCs2SnF6:Mn4þ .

The integrated PL intensities in Fig. 12 exhibited strongthermal quenching above �350�380 K and can be fitted withthe following equation:

IPLðTÞ ¼IPLð0Þ

1þexpð�Eq=kBTÞ: ð8Þ

The light solid lines in Fig. 12 show the results calculated usingEq. (8). The quenching energies determined here are Eq¼1.00 and0.98 eV for Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ , respectively.

3.3. Comprehensive discussion on PL spectra of Mn4þ-activated

alkaline hexafluorides

We compare in Fig. 13 the room-temperature PL spectra ofA2SnF6:Mn4þ with (a) A¼K (K2SnF6 �H2O:Mn4þ), (b) Na, (c) Cs,and (d) NH4. The A¼K and NH4 spectra have recently beenmeasured in our laboratory. The Mn4þ ion substituted for theSn site in the A2SnF6:Mn4þ host is positioned at the exact centerof an octahedral array of F� ions (MnF2�

6 ). In Fig. 13, cleardifference in the PL peak position of each vibronic mode (ni) canbe found between the different cation species (A¼K, Na, Cs, orNH4). Furthermore, the ZPLs in Fig. 13 are found to possess at thedifferent energies among those phosphors that are at 1.993 eV(K), 2.005 eV (Na), 1.989 eV (Cs), and 1.984 eV (NH4). This factindicates that the intra-d-shell electrons accept considerableperturbation from the cations in the host crystal. It should benoted that vacuum evacuation leads to partial dehydration ofK2SnF6 �H2O:Mn4þ , resulting in the splitting of the Mn4þ-related

1.90 1.95 2.00 2.05 2.10

PL in

tens

ity (a

rb. u

nits

)

Photon energy (eV)

ZPL↓Stokes

X=Si

Ge

Sn

Ti

A=Na�4�6 �3

�6�4

�3

anti-Stokes

Fig. 15. Room-temperature PL spectra of Na2XF6:Mn4þ with (a) X¼Si, (b) Ge,

(c) Sn, and (d) Ti.

1.90 1.95 2.00 2.05 2.10

PL in

tens

ity (a

rb. u

nits

)

Photon energy (eV)

ZPL↓Stokes

X=Si

Ge

Sn

Ti

×10

A=Cs

�3�4

�6

�6�4

ν6

anti-Stokes

Fig. 16. Room-temperature PL spectra of Cs2XF6:Mn4þ with (a) X¼Si, (b) Ge,

(c) Sn, and (d) Ti.

1.90 1.95 2.00 2.05 2.10

ZPL

×10X=Si

Ge

Sn

Ti

Stokes ↓

Photon energy (eV)

PL in

tens

ity (a

rb. u

nits

)

A=NH4

�3�4�6

�6�4

�3

anti-Stokes

Fig. 17. Room-temperature PL spectra of (NH4)2XF6:Mn4þ with (a) X¼Si, (b) Ge,

(c) Sn, and (d) Ti.

Table 2Summary of crystal classes, space groups, optical classifications, Mn4þ-related ZPL

emission intensities, piezoelectricities (PZ), and optical rotation (OR) activities in

A2XF6:Mn4þ (A¼K, Na, Cs, and NH4; X¼Si, Ge, Sn, Ti). ‘‘þ ’’ and ‘‘� ’’ indicate that

the intensities/phenomena are strong and weak (or forbidden in principle).

Alkaline hexafluorometallate

K2SiF6 K2GeF6 K2SnF6 �H2O Na2SiF6 Na2SnF6

Property Cs2SiF6 K2TiF6 Na2GeF6

Cs2GeF6 Cs2SnF6 Na2TiF6

(NH4)2SiF6 Cs2TiF6

(NH4)2GeF6

(NH4)2SnF6

(NH4)2TiF6

Crystal class Cubic Trigonal Orthorhombic Trigonal Tetragonal

Space group O5h ðFm3mÞ D3

3dðP3m1Þ D242h ðFdddÞ D2

3 ðP321Þ D144hðP42=mnmÞ

Optical

classification

Isotropic Uniaxial Biaxial Uniaxial Uniaxial

ZPL � � � þ �

PZ � � � þ �

OR � � � þ �

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–26602658

emission lines, together with an enhancement of the ZPL emissionintensity (Fig. 13a, see details in [8]).

We observed, however, no clear difference in the PL spectralfeature between the different X species in A2XF6:Mn4þ . InFigs. 14–17, we show the room-temperature PL spectra ofA2XF6:Mn4þ (X¼Si, Ge, Sn, and Ti) with A¼K (Fig. 14), Na (Fig. 15),Cs (Fig. 16), and NH4 (Fig. 17) [7,8,25,29–31]. We can see noremarkable difference in the PL peak position of each vibronic mode(ni) among the different X species. This is because the internalvibronic frequencies ni (i¼1�6) of the MnF2�

6 octahedron inA2XF6:Mn4þ can be principally determined from its moleculardynamics only. Furthermore, the ZPLs in Figs. 14–17 are observedat the same peak energies among the common-cation (A) phosphors.This fact indicates that the Mn4þ activators are nearly isolatedelectrically from the rest of the A2XF6 crystal (A¼K, Na, Cs, or NH4).

The PL spectra of K2SnF6 �H2O:Mn4þ (Figs. 13a and 14c),Na2SiF6:Mn4þ (Fig. 15a), Na2GeF6:Mn4þ (Fig. 15b), and Na2TiF6:Mn4þ (Fig. 15d) showed very strong ZPL emission even at

room-temperature. It is well known that crystals with the samesymmetry exhibit nearly the same physical behaviors [32]. Let ussummarize in Table 2 the crystal classes, space groups, opticalclassifications, ZPL emission intensities, piezoelectricities, andoptical rotation activities in the alkaline hexafluorometallates asreported in Figs. 14–17. It can be understood from Table 2 that ahost material with lower symmetry exhibits stronger ZPL emis-sion intensity. The observability of the ZPL is thought to besensitively dependent on the local symmetry of the environmentof the Mn4þ ion. Any piezoelectric or related effect, such as alongitudinal electric field induced by piezoelectrically activephonons and/or q-dependent Frohlich interaction, may activatethe ZPL emission in the orthorhombic D24

2h ðFdddÞ and trigonalD2

3 ðP321Þ symmetries.

3.4. Raman scattering spectra

The three ‘gerade’ mode frequencies n1 (a1g), n2 (eg), and n5

(t2g) can be determined by the Raman scattering, the ‘ungerade’mode t1u is an infrared-active mode (n3 and n4), and t2u is a silent

Table 3

Internal vibronic frequencies of SnF2�6 ions in Na2SnF6 and Cs2SnF6 and of MnF2�

6 ions

in Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ at room temperature. PS¼present study.

SnF2�6 (cm�1) Compound Ref.

n1 n2 n3 n4 n5 n6

600 488 260 Na2SnF6 PS

592 477 559 300 252 Na2SnF6 [33]

594 480 560 255 Na2SnF6 [34]

590

582 468 256 Cs2SnF6 PS

572 460 555 239 247 Cs2SnF6 [35]

577 256

MnF2�6 (cm�1) Compound Ref.

n1 n2 n3 n4 n5 n6

522 609a 351a 238a Na2SnF6:Mn4þ PS

661a 399a 290a

564a 311a 312 215a Cs2SnF6:Mn4þ PS

609a 338a 243a

a T¼20 K.

Na2SnF6Na2SnF6:Mn4+

?×5?

?

200 300 400 500 600 700

Cs2SnF6Cs2SnF6:Mn4+

Wavenumber (cm-1)

×20

Ram

an in

tens

ity (a

rb. u

nits

)

�5 (Mn)

�5 (Sn)�2 (Sn)

�1 (Sn)

�2 (Sn)�5 (Sn)

�1 (Sn)

�2 (Mn)

Fig. 18. Stokes Raman lines in (a) Na2SnF6:Mn4þ and (b) Cs2SnF6:Mn4þ at 300 K.

The Raman spectra taken from pure Na2SnF6 and Cs2SnF6 samples are also shown.

Y. Arai, S. Adachi / Journal of Luminescence 131 (2011) 2652–2660 2659

mode (n6). Fig. 18 shows the room-temperature Raman spectraobtained from pure and Mn4þ-doped A2SnF6 with (a) A¼Na and(b) Cs. Here, pure Na2SnF6 and Cs2SnF6 were prepared in ourlaboratory by dissolving Sn in a mixture of HF/NaF (or CsF)/H2O2.

The Raman spectra in Fig. 18 reveal well-defined n1, n2, and n5

modes. The n1 mode was observed to be the most intense peak,whereas the n2 mode was observed to be a very weak peak. The n2

and n5 modes of the MnF2�6 octahedrons were also observed in

Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ , respectively. The unknownpeaks observed at 293 and 570 cm�1 in Fig. 18a may be arisingfrom any residue substance.

Table 3 summarizes the internal vibronic frequencies of theSnF2�

6 octahedrons in A2XF6 (A¼Na and Cs) determined from ourRaman scattering measurements. The literature values are alsolisted in Table 3 [33–35]. Because of its silent nature, we cannotfind in the literature any experimental data on the n6 modefrequency of the SnF2�

6 octahedrons in A2SnF6.No experimental data have been reported up to date on the

internal vibronic frequencies ni of the MnF2�6 ion in A2MnF6 or

A2SnF6:Mn4þ (A¼Na or Cs). The lower part of Table 3 lists a set ofinternal vibronic frequencies of the MnF2�

6 ions in A2SnF6:Mn4þ

determined from our PL and the Raman scattering measurements.It is understood from Table 3 that the vibronic quantum of�65 meV (�520 cm�1) used in the present PLE analysis corre-sponds to the n2 mode frequencies of the MnF2�

6 ions in theNa2SnF6 and Cs2SnF6 hosts.

4. Conclusions

Na2SnF6:Mn4þ and Cs2SnF6:Mn4þ were synthesized by thechemical etching of tin platelets in aqueous HF/NaMnO4- andCsMnO4-based solutions. These phosphors exhibited red lumines-cence in the 600–660 nm regions due to the 2Eg-

4A2g transitionunder blue (�470 nm) or UV excitation (�360 nm). The PLEdata analysis yielded the following ZPL energies and crystalfield parameters of the 3d3 (Mn4þ) electrons in Na2SnF6 andCs2SnF6: �2.39 and �2.38 eV (4A2g-

4T2g), �2.83 and �2.76 eV(4A2g-

4T1g), and �1930 and �1920 cm�1 (Dq), respectively.A set of ligand field-dependent vibronic frequencies (n1�n6) werealso determined from the Raman scattering measurements. Com-prehensive discussion was given on the phosphorescent proper-ties of Mn4þ-activated A2XF6 with A (an alkaline ion)¼K, Na, Cs,and NH4 and X (a tetravalent ion)¼Si, Ge, Sn, and Ti. From easeand cost of the production, it seems that the A2XF6:Mn4þ redphosphors with A¼K or Na and B¼Si, Ge, or Ti are the best onesfor warm or white LED applications.

Acknowledgments

This work was supported by a Grant-in-Aid for ScientificResearch (B) (23360133) from the Ministry of Education, Culture,Sports, Science and Technology, Japan.

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