Optimized photoluminescence properties of a novel red phosphor LiSrAlF6:Mn4+ synthesized at room-temperature

Article abstract of DOI:10.1016/j.jallcom.2018.09.172

A red phosphor with a broad excitation band in blue region and sharp emission peaks in red region is needed for compensating the red components of white light-emitting diodes (WLEDs) by converting blue light with a yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce). The red phosphor LiSrAlF₆:Mn⁴⁺ (LSAF:Mn) with morphology of flaky particles about 50 μm in diameter has been synthesized by an ion-exchange reaction route at room-temperature in air. The as-synthesized phosphors were characterized by x-ray powder diffraction (XRD), scanning electron microscope (SEM), and temperature-dependent photoluminescence (PL) spectra. The excitation spectrum of LSAF:Mn shows a broad-band excitation from 400 to 500 nm that is due to the spin-allowed transition of ⁴A_2g → ⁴T_2g of Mn⁴⁺, indicating a potential application for solid state lighting. The optimal PL intensity of the phosphor LSAF:Mn has been obtained by optimizing the synthetic parameters.

Full text of DOI:10.1016/j.jallcom.2018.09.172

Accepted Manuscript
4+
Optimized photoluminescence properties of a novel red phosphor LiSrAlF :Mn
6
synthesized at room-temperature
Mengmeng Zhu, Yuexiao Pan, Mingmei Wu, Hongzhou Lian, Jun Lin
PII:
S0925-8388(18)33412-1
10.1016/j.jallcom.2018.09.172
JALCOM 47591
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 26 July 2018
Revised Date: 11 September 2018
Accepted Date: 15 September 2018
Please cite this article as: M. Zhu, Y. Pan, M. Wu, H. Lian, J. Lin, Optimized photoluminescence
4+
properties of a novel red phosphor LiSrAlF :Mn synthesized at room-temperature, Journal of Alloys
6
and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.09.172.
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ACCEPTED MANUSCRIPT
Optimized photoluminescence properties of a novel red phosphor
LiSrAlF6:Mn4+ synthesized at room-temperature
Mengmeng Zhu,a Yuexiao Pan,a* Mingmei Wu,c Hongzhou Lian,b Jun Linb*
aNanomaterials and Chemistry Key Laboratory, Faculty of Chemistry and Materials
Engineering, Wenzhou University, Zhejiang Province, Wenzhou 325027, P. R. China.
Tel. & Fax: (+86) 577-8837-3017. E-mail: yxpan8@gmail.com;
bState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
Fax: +86-431-85698041; Tel: +86-431-85262031. E-mail: jlin@ciac.ac.cn;
cMOE Key Laboratory of Bioinorganic and Synthetic Chemistry/ School of
Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, P.
R. China. Tel: 86-20-84111823 (Office). E-mail: ceswmm@mail.sysu.edu.cn
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Abstract
A red phosphor with a broad excitation band in blue and sharp emission peaks in
red is needed for compensating the red components of white light-emitting diodes
(WLEDs) by converting blue light with a yellow phosphor Y3Al5O12:Ce3+ (YAG:Ce).
The red phosphor LiSrAlF6:Mn4+ (LSAF:Mn) with morphology of flaky particles
about 50 µm in diameter has been synthesized by an ion-exchange reaction route at
room-temperature in air. The as-synthesized phosphors were characterized by x-ray
powder diffraction (XRD), scanning electron microscope (SEM), and
temperature-dependent photoluminescence (PL) spectra. The excitation spectrum of
LSAF:Mn shows a broad-band excitation from 400 to 500 nm that is due to the
4
spin-allowed transition of 4A2g → T2g of Mn4+, indicating a potential application for
solid state lighting. The optimal PL intensity of the phosphor LSAF:Mn has been
obtained by optimizing the synthetic parameters.
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1. Introduction
WLEDs have attracted much attention due to their high efficiency, long serving
time, and environmental friendly compared to conventional incandescent and
fluorescence lightings.1-3 The main strategy to obtain WLEDs is mixing the blue light
from InGaN LED chips and yellow light from phosphors YAG:Ce, which suffers from
the problems of low color rendering index (CRI < 80) and high correlated color
temperature (CCT > 5000 K) because of the insufficient red contribution in their
white PL spectra.4-9 Combination of a red phosphor to compensate the red component
is a feasible approach to obtain white light with controlled emission color properties
and a CRI higher than 90.4-9 In this regard, it is essential to develop red phosphors that
can be efficiently excited by the blue LED chips.
To develop a novel efficient red-emitting phosphors being excitable by blue LED,
Mn4+ with outer 3d3 electron configuration exhibiting narrow-band emission lines in
red region and a broad-band excitation in blue region are considered to be efficient
candidate because the Mn4+ doped red phosphors can be effectively excited by blue
light and emit intense red luminescence.10-21 Generally, Mn4+ ions located at the
centers of octahedral sites in solid state inorganic compounds act as luminescence
emitters. Mn4+ ions in most hosts of red phosphors have a group of sharp emitting
peaks in red region between 600 and 650 nm. The fine structure of the emission
spectrum and the efficiency of the Mn4+ doped phosphors are strongly dependent on
the microenvironment Mn4+ experienced that including the crystallography structure,
coordination environments, and remote neighbouring-cations.10-14 For example, a red
phosphor Rb2SiF6:Mn4+ obtained by replacement of K+ with Rb+ shows better thermal
stability than K2SiF6:Mn4+.10 We have reported the formation mechanism and the
luminescence properties of Mn4+-substituted phosphors containing [GaF6]3- and
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[AlF6]3- octahedrons in Li3Na3Ga2F12 (LNGF) and Li3Na3Al2F12 (LNAF),
respectively.11,12 It is observed that the PL spectra of LNGF:Mn are very similar to
those of LNAF:Mn but the quantum yield of LNGF:Mn is nearly 3 times of that of
LNAF:Mn. Moreover, the luminescence color purity and luminescence efficiency of
K2NaAlF6:Mn4+ are much better than those of K2LiAlF6:Mn4+ due to slight
nephelauxetic effect in K2NaAlF6.13,14 Thus, the performance of the Mn4+-substituted
phosphors is strongly subject to the surroundings of the luminescence center in host
lattice.
To our knowledge, the preparation and optical properties of Mn4+ doped alkaline
earth metal fluoride phosphors have not been reported so far except for BaTiF4:Mn4+
and BaSiF4:Mn4+.15-17 Additionally, there is no Mn4+ doped alkaline earth metal
fluoride phosphors containing Sr2+ or Ca2+ reported previously. In this work, a novel
red Mn4+ doped hexa-fluoroaluminate LSAF:Mn has been firstly synthesized by the
ion-exchange method at room-temperature and its formation mechanism has been
discussed. The phosphor LSAF:Mn shows red sharp-peaky emission, a broad
absorption in the blue light region, and excellent thermal stability, which is promising
in improvement of the performance of WLEDs based on InGaN chips and YAG:Ce
phosphors. This work will open the opportunity for development of novel Mn4+ doped
fluoride phosphors containing alkaline earth metal.
2. Experimental
2.1 Synthesis of red phosphors
Starting materials
The raw materials used in this work include KMnO4, KHF2, H2O2 (30 wt%), LiF,
SrCO3, AlF3, HF (40 wt%) and ethyl alcohol (AR). All of thesis starting reagents were
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used directly without any further purification. All the chemicals were purchased from
Aladdin Reagent Co., Ltd (Shanghai).
Synthesis of K2MnF6
According to the operating procedures reported previously, 230 mmol KHF2 and
5.7 mmol KMnO4 were dissolved in 30 mL HF (40 wt%) solution, then 0.6 ml H2O2
(30 wt%) was dropwise added.22 After 30 min magnetically stirring, the deep purple
solution gradually turned yellow and a yellow precipitate was obtained at the same
time. The resulting yellow solid product K2MnF6 was collected carefully by sucking
o
filtration, washed extensively with methanol several times, and dried at 60 C for 4
hours.
Synthesis of host lattice LiSrAlF6
Herein, 0.01 mol LiF, 0.01 mol SrCO3, and 0.01 mol AlF3 were weighed and
dissolved in 20 ml HF (10 wt%) solution under stirring. The white precipitate of
LiSrAlF6 was produced at room-temperature after stirring for 4 h. The resulting white
solid compounds were filtered, washed with ethyl alcohol, and dried at 60°C for 4 h.
Synthesis of red phosphor LiSrAlF6:Mn4+
Red phosphors LiSrAlF6:Mn4+ were prepared via an ion exchange and
co-precipitation combined synthesis with different molecular values of
K2MnF6/LiSrAlF6. In a typical synthesis, 0.2 mmol K2MnF6 was added into 20 ml 10
wt % HF solution until completely dissolved, and 10 mmol LiSrAlF6 was added into
the yellow transparent solution. After magnetically stirring for 3 hours, the
precipitates were collected, washed with ethyl alcohol for several times, and dried at
60 oC for 4 hours.
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2.2 Characterization
XRD patterns of the products were collected on a Bruker (Karlsruhe, Germany) D8
Advance x-ray powder diffraction (XRD) with graphite monochromatized Cu Kα
radiation (λ= 0.15418 nm) at the scan rate of 10o.min-1 in the two degrees range from
10 o~80 o. The morphology and elemental composition of the as-prepared products
were studied by a Nova NanoSEM 200 scanning electron microscope (FE-SEM, FEI
Inc.) with an attached energy-dispersive x-ray spectrum (EDS) , transmission electron
microscopy (TEM), and selected area electron diffraction (SAED), elemental
compositions analysis obtained using an energy dispersive spectrometer (EDS) with a
JEOL 2100F high-resolution transmission electron microscope using an accelerating
voltage of 200 kV. The thermal stability the red phosphors are investigated by
thermogravimetrics (TG) analysis and different scanning calorimeter (DSC; Netzsch
STA 449 C, at a heating rate of 10 K.min-1). UV-vis diffuse reflectance spectra were
measured by using a Shimadzu UV-3600 spectrometer. Photoluminescence (PL)
spectra were recorded on FluoroMax-4 spectrofluorometer (Horiba Jobin Yvon Inc.)
with a 150 W xenon lamp (as the excitation source) with a heating attachment.
3. Results and discussions
3.1. Phase purity, morphology and composition analysis
The phase purities of the products prepared at different temperatures and with
various molecular ratios of K2MnF6/LiSrAlF6 by keeping other reaction conditions
consistent, have been characterized using XRD as shown in Fig. 1. All of the strong
and sharp diffraction peaks of the samples LSAF:Mn obtained at temperatures from
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room-temperature to 200 oC and with K2MnF6/LiSrAlF6 ranging from 0.5 to 16 mol%
can be indexed as a pure hexagonal LSAF phase. All the locations and relative
intensities of the diffraction peaks coincide well with the values in standards card
(ICDD No.48-1640) of LSAF. No other impurity phase can be observed, which
indicates that the pure hexagonal phase LSAF can be readily prepared using this
method. The formation of the LSAF precipitate can be expressed as following:
LiF + SrCO3 + AlF3 + 2HF = LiSrAlF6 + H2O + CO2
(1)
The red phosphor LSAF:Mn has been obtained through ion exchange and the
partial anions [AlF6]3- were replaced by [MnF6]2- when mixing K2MnF6 and LSAF in
HF solution based on the following reactions:
LiSrAlF6 + [MnF6]2- = LiSrAlF6:Mn4+
(2)
In order to further study the crystal structure information of the obtained phosphors,
the Rietveld refinement of LSAF:Mn with the single crystal structure date of LSAF
(ICSD No. 68905) as the initial model was carried out using the General Structure
Analysis System (GSAS) program, and the result is shown in Fig. 2a. The black line
represents the difference between experimental and calculated, and the blue sticks
mark the Bragg reflection position. It shows that they match very well with that
obtained from the experimental curve, indicating that the doped Mn4+ ions does not
cause any impurity in the LSAF host structure.
The final refinement structural parameters and the the atom coordinates are
summarized in Table 1. It demonstrate that LSAF:Mn belong to hexagonal structure
with a group of P-31c(163). The unit-cell parameters are a = b = 5.071 Å, c = 10.189
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Å, α = β = 90°, γ = 120° and V = 226.91 Å3, N = 2. The final results of refinement are
converged to Rwp = 5.24% and Rp = 11.14%, indicating good refinement quality and
the formation of a single-phase of LSAF. According to the Rietveld refinement result,
the crystal structure is drawn using Diamond software and shown in Fig. 2b. In the
crystal lattice and the unit cell of LSAF, each Al3+ is surrounded by 6 F- ions to form a
regular [AlF6]3- octahedral structure. In HF acid solution, the partly exchange between
ions [MnF6]2- and [AlF6]3- occurs during stirring, and the Mn4+ ions were stabilized at
the lattice sites of Al3+ ions.
The corresponding histograms of size distribution for the LSAF:Mn is depicted in
Fig. 3a. The average diameter for the particles is about 1281+192 nm. The LSAF:Mn
was taken to investigate the elemental composition by energy dispersive spectrometry
(EDS) as given in Fig. 3b. The EDS spectrum indicates that the composing element F,
Sr, Al, and Mn atoms were detected, but the element Li is too light to be detected. The
results indicate the flaky particles of the phosphor are confirmed to be pure LSAF:Mn.
The scanning electron microscopy (SEM) images of LSAF:Mn shown in Fig. 3b
(inset) exhibit that the samples consist of flaky particles about 1-3 µm in diameter.
To further confirm the morphology of the as-synthesize samples, the TEM,
HRTEM, and elemental mapping were carried out and shown in Fig. 4a-c. In HRTEM,
the lattice fringes are found to be about 0.1499 nm, which is consistent with the
d-spacing of the (118) plane of LSAF crystal. The mapping images clearly show that
the elements present in the synthesized sample are Sr, Al, F, and Mn, which further
provides proof that the obtained samples are pure LSAF phases, and Mn4+ ions have
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been successfully doped into the host lattice.
The thermal stability of phosphor is a key parameter specially for LEDs with high
o
power because its working temperature could arrive at 150 C, which seriously affect
the performance of LEDs, such as the serving lifetime, color rending index,
chromaticity, and luminous efficiency. As shown in Fig. 5a, TG and DTA curves of
o
LSAF:Mn show that the weight has remained the constant until about 760 C. Such
high decomposition temperature suggests that the phosphor LSAF:Mn has high
thermal stability and meets the requirement in WLED devices. The diffuse reflection
spectra have been recorded for both undoped and Mn4+ doped LSAF as shown in Fig.
5b. According to the spectra, it can be seen that the two absorption bands peaking at
4
4
4
4
370 nm and 465 nm are assigned to the spin-allowed A2g → T1g and A2g → T2g
transitions of Mn4+, respectively. The strong absorption band between 200~250 nm is
originating from the absorption of hosts which found in both curves.
Fig. 6 illustrates the PL spectra of the red phosphor LSAF:Mn measured at 78 and
298 K, respectively. The room-temperature excitation spectrum monitored at 618 nm
shows two bands ranging from 300 nm to 500 nm. The bands centered at 365 and 467
4
2
4
2
nm are owing to A2g → T1g and A2g → T2g transitions, respectively.18 The
excitation spectrum demonstrates that LSAF:Mn phosphors could be well excited by
blue LED chips. It is worth noting that the spectral overlap between the excitation
spectrum of the red phosphor LSAF:Mn and the emission spectrum of the yellow
phosphor YAG:Ce is small, the photon reabsorption effect is smaller than nitride red
phosphors when they are co-coating on LED chips. Under excitation at 467 nm, the
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room-temperature emission spectrum of LSAF:Mn presents six emission peaks at 595,
605, 610, 618, 628, 632, and 646 nm, which belongs to anti-stokes vibronic
transitions v3(t1u), v4(t1u), and v6(t2u), zero phonon line (ZPL), and Stokes vibronic
transitions v6(t2u), v4(t1u), and v3(t1u), respectively.24 Compared to that measured at
298 K, the emission spectrum measured at 78 K represents blue-shift in all the Stokes
vibronic transitions of Mn4+ ions mainly due to a nephelauxetic effect aroused by
unit cell shrinkage.13 The decreased absorbed photons and less weak vibration
transition coupling associated with the vibration modes of [MnF6]2- octahedron at
lower temperauture also contribute the blue-shift in emitting transitions of Mn4+ ions.
The zero phonon line (ZPL) of Mn4+ ions is generally locating >620 nm, such as at
625 nm in K2LiAlF6 and 620 nm in Na3AlF6, which indicates red shift compared to
the ZPL at 618 nm in the emission spectrum of LSAF:Mn measured at 298 K.23,24 The
ZPL emission intensity depends on the local symmetry of sites that the Mn4+ ion
occupied. The distortion of the octahedron and the low symmetry will lead to the
stronger ZPL emission of Mn4+ ion.24-25 The location of ZPL of the Mn4+ might be
attributed to the crystal field strength and bond strength of Al-F bond in [AlF6]3-
octahedron. The electron deficiency of divalent Sr2+ ions is stronger than that of
monovalent alkaline metal ions such as K+ and Na+ in previous works, which leads to
lower electron cloud density and shorter Al-F bond length in LSAF than those in
K2LiAlF6 and Na3AlF6.23,24 Therefore, the wavelength of ZPL emission in LSAF is
shorter than that in alkaline metal fluoaluminates due to the stronger crystal field
strength aroused by Sr2+ ions in LSAF.
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According to the method reported previously, the ratio of Dq/B is 3.88 that is
higher than 2.2, indicating that Mn4+ ions experience a strong crystal field LSAF host
lattice.12,26 The Dq/B value in the LiNa2AlF6 host lattice is 3.80 according to the data
in previous work, which is smaller than that in LSAF.12 The result demonstrates the
crystal field in LSAF is stronger than that in LiNa2AlF6. The 2Eg level of Mn4+ ion is
almost independent of the crystal field strength according to the Tanabe-Sugano
2
4
energy level diagram. Whereas the emission peak energy Eg → A2g transition of
Mn4+ depends mainly on the nephelauxetic effect, which is mainly derived from the
27
covalent value of Mn4+-ligand bonding The as-prepared powder sample LSAF:Mn
is white under natural light and emits intense red luminescence under UV lamp as
shown in their photographs in Fig. 6b (inset), indicates that this red-emitting phosphor
can be used as a red component applied to LED technology. On the basis of the
emission spectrum of LSAF:Mn, the CIE chromaticity coordinate of LSAF:Mn are
calculated as x = 0.680 and y = 0.318.
3.2 Optimized luminescence and concentration quenching mechanism
Fig. 7a shows the emission spectra for a series of LSAF:xMn (x = 0.5-16 mol%), in
which, x is the ratio of K2MnF6/LSAF. All of the spectra have identical features
except for their emission intensity. It is clearly that the luminescence intensity of
LSAF:Mn increases initially and reaches a maximum at x = 2 mol%, then decrease,
which is ascribe to concentration quenching. The luminescence decay curves of
LSAF:Mn with different concentrations of Mn4+ monitored at 618 nm and recorded
under excitation at 467 nm at room-temperature are shown in Fig. 7b. All the decay
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curves could be well fitted into singly exponential function as: 28-30
I = I exp −t /
τ
(
)
t
0
(3)
Where I0 and It are for the luminescence intensities at time t0 and tt, respectively; τ
denotes the decay time of luminescence from corresponding sample, which could be
easily calculated by fitting the decay curves. With the ratio of K2MnF6/LSAF
increasing from 0.5 to 16 mol%, the lifetime decreases from 4.11 to 3.35 ms.
Concentration quenching is usually caused by the occurrence of energy transfer
between the nearest Mn4+ ions. Since there is no overlap between the excitation and
emission spectra of Mn4+ ions, the energy transfer mechanism involved in the LSAF
crystal host is not radiation reabsorption. It may be due to exchange interaction or
electric multipolar interaction. Since there is only one kind of Mn4+ luminescent
center in the host lattice, the critical energy transfer distance (Rc) between Mn4+ ions
can be calculated by the following equation:13,14

1/3
3V
Rc ≈ 2




4π xc N
(4)
Where V is the volume of the unit cell, xc is the critical concentration of the activator
ion, and N is the number of formula units per unit cell. For LSAF host lattice, V =
228.7 Å3, N = 2, and the critical concentration of Mn4+ is found to be 2 mol%.
Therefore, Rc of Mn4+ is determined to be 22.19 Å. Non-radiative energy transfer
among Mn4+ ions usually occurs as a result of an exchange interaction, radiation
reabsorption, or a multipole-multipole interaction.
The exchange interaction responsible for the energy transfer for spin-allowed
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transition of Mn4+ ion is excluded because Rc of Mn4+ in LSAF:Mn is much larger
than 5 Å. In view of the little overlapping of emission and excitation spectrum of
LSAF:Mn phosphor, the reabsorption mechanism should not occur in this case.
Therefore, the multipole-multipole interaction is dominant for the concentration
quenching mechanism of Mn4+ emission in LSAF:Mn.
Generally, elevating reaction temperature is favorable to gain higher luminescence
efficiency for traditional rare earth ions doped inorganic phosphor. 31,32 We obtained a
o
series of LSAF:Mn samples at a reaction temperature ranging from 25 to 200 C. All
the samples are confirmed to be pure LSAF phase as shown in Fig. 1 and emit red
light as shown in emission spectra in Fig. 8. All the emission spectra have similar
features and their integrated luminescence intensities depend strongly on the reaction
temperature. With the reaction temperature increasing, the luminescence intensity
o
increases and reaches a maximum at 50 C, then decreases with reaction temperature
o
further increasing. The results indicate elevating reaction temperature to 50 C can
improve the luminescence intensity of LSAF:Mn slightly which might be due to the
improvement of crystallization. Whereas, further increasing reaction temperature to be
higher than 80 oC will lead to decomposing of K2MnF6 and decrease the concentration
of luminescence centers.33-36 Consequently, the doping and crystallization of Mn4+
doped fluorides can be actually completed at room-temperature and their formation
mechanism based on dynamically ion exchange is quite different from that for the
traditional rare earth ions doped phosphors. The typical internal quantum yield of the
sample LSAF:Mn is about 78% measured according to the method used in the
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previous works.12
3.3 Temperature-dependent luminescence and thermal quenching
The thermal quenching of luminescence is an important parameter to evaluate the
performance of phosphor materials. Fig. 9a depicts the dependence of luminescence
of LSAF:Mn on measurement temperature. With increasing temperature, the emission
intensity decreases gradually due to the temperature quenching. Note that the intensity
at 493 K still remains over 55% of that at room-temperature.
4. Conclusion
In conclusion, we explored a novel red fluoride phosphor LSAF:Mn including
strontium at room-temperature. The obtained phosphor LSAF:Mn shows broadened
excitation band around 470 nm, and enhanced red emissions due to the spin forbidden
transitions 2Eg → 4A2g of Mn4+. The chromaticity coordinates for phosphor LSAF:Mn
are located at x=0.68 and y=0.32, which are very close to standard red region (0.67,
0.33). The optimal concentration of K2MnF6 in the reaction system is 2.0 mol% of
o
LSAF and the optimal experimental temperature is 50 C to obtain the highest
o
luminescent intensity. The red phosphor LSAF:Mn keep undecomposed up to 700 C
which indicates excellent thermal stability and a suitable candidate for the fabrication
of blue InGaN based WLEDs.
Acknowledgments
This research was jointly supported by the National Natural Science Foundation of
China (Grant No. 51572200, 51672266, 51628201, and 91433110), Zhejiang Province
(Grant No. Y16E020041), and the Public Industrial Technology Research Projects of
Wenzhou (Grant No. G20170005) and the Joint Project of National Natural Science
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Foundation of China (NSFC) and Yunnan Province (U1702254).
References
[1] Z.G. Xia, Z.H. Xu, M.Y. Chen, Q.L. Liu, Recent developments in the new
inorganic solid-state LED phosphors, Dalton Trans. 45 (2016) 11214-11232.
[2] G.G. Li, Y. Zhao, Y. Wei, Y. Tian, Z.W. Quan, J. Lin, Novel yellowish-green
light-emitting Ca10(PO4)6O:Ce3+ phosphor: structural refinement, preferential site
occupancy and color tuning Chem. Commun. 52 (2016) 3376-3379.
[3] Y.H. Kim, P. Arunkumar, B.Y. Kim, S. Unithrattil, E. Kim, S.H. Moon, J.Y. Hyun,
K.H. Kim, D. Lee, J.S. Lee, W.B. Im, A zero-thermal-quenching phosphor, Nat.
Mater. 16 (2017) 543.
[4] X. Ding, G. Zhu, W.Y. Geng, Q. Wang Y.H. Wang, Rare-earth-free high-efficiency
narrow-band red-emitting Mg3Ga2GeO8:Mn4+ phosphor excited by near-UV light
for white-light-emitting diodes, Inorg. Chem. 55 (2016) 154-162.
[5] X. Ding, Q. Wang, Y.H. Wang, Rare-earth-free red-emitting K2Ge4O9:Mn4+
phosphor excited by blue light for warm white LEDs, Phys. Chem. Chem. Phys.
18 (2016) 8088-8097.
[6] W.B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikhailovsky, J. Hu, B.F.
Chmelka, S.P. DenBaars, R. Seshadri, Efficient and color‐tunable oxyfluoride
solid solution phosphors for solid‐state white lighting, Adv. Mater. 23 (2011)
2300-2305.
[7] Z.G. Xia, Q.L. Liu, Progress in discovery and structural design of color
15

ACCEPTED MANUSCRIPT
conversion phosphors for LEDs, Prog. Mater. Sci. 84 (2016) 59-117.
[8] N.C. George, K.A. Denault, R. Seshadri, Phosphors for solid-state white lighting,
Annu. Rev. Mater. Res. 43 (2013) 481-501.
[9] H.M. Zhu, C.C. Lin, W.Q. Luo, S.T. Shu, Z.G. Liu, Y.S. Liu, J.T. Kong, E. Ma,
Y.G. Cao, R.S. Liu, X.Y. Chen, Highly efficient non-rare-earth red emitting
phosphor for warm white light-emitting diodes, Nat. Commun. 5 (2014) 4312.
[10] M.H. Fang, H.D. Nguyen, C.C. Lin, R.S. Liu, Preparation of a novel red
Rb2SiF6:Mn4+ phosphor with high thermal stability through a simple one-step
approach, J. Mater. Chem. C 3 (2015) 7277-7280.
[11] M.M. Zhu, Y.X. Pan, L.Q. Xi, H.Z. Lian, J. Lin, Design, preparation, and
optimized luminescence of a dodec-fluoride phosphor Li3Na3Al2F12:Mn4+ for
warm WLED applications, J. Mater. Chem. C 5 (2017) 10241-10250.
[12] M.M. Zhu, Y.X. Pan, Y.Q. Huang, H.Z. Lian, J. Lin, Designed synthesis,
morphology evolution and enhanced photoluminescence of a highly efficient red
dodec-fluoride phosphor, Li3Na3Ga2F12:Mn4+ for warm WLEDs, J. Mater. Chem.
C 6 (2018) 491-499.
[13] Y.W. Zhu, L.Y. Cao, M.G. Brik, X.J. Zhang, L. Huang, T.T. Xuan, J. Wang,
Facile synthesis, morphology and photoluminescence of a novel red fluoride
nanophosphor K2NaAlF6:Mn4+, J. Mater. Chem. C 5 (2017) 6420-6426.
[14] Y.W. Zhu, L. Huang, R. Zou, J.H. Zhang, J.B. Yu, M.M. Wu, J. Wang, Q. Su,
Hydrothermal synthesis, morphology and photoluminescent properties of an
Mn4+-doped novel red fluoride phosphor elpasolite K2LiAlF6, J. Mater. Chem. C
16

ACCEPTED MANUSCRIPT
4 (2016) 5690-5695.
[15] D. Sekiguchi, S. Adachi, Synthesis and optical properties of BaTiF6: Mn4+ red
phosphor, ECS J. Solid State Sci. Tech. 3 (2014) R60-R64.
[16] X.L. Gao, Y. Song, G.X. Liu, X.T. Dong, J. Wang, W.S. Yu, Narrow-band red
emitting phosphor BaTiF6:Mn4+: preparation, characterization and application for
warm white LED devices, Dalton Trans. 45 (2016) 17886-17895.
[17] X.Y. Jiang, Y.X. Pan, S.M. Huang, X. Chen, J.G. Wang, G.K. Liu, Hydrothermal
synthesis and photoluminescence properties of red phosphor BaSiF6:Mn4+ for
LED applications, J. Mater. Chem. C 2 (2014) 2301-2306.
[18] H.Z. Lian, Q.M. Huang, Y.Q. Chen, K. Li, S.S. Liang, M.M. Shang, M.M. Liu J.
Lin, Resonance Emission Enhancement (REE) for Narrow Band Red-Emitting
A2GeF6:Mn4+ (A = Na, K, Rb, Cs) Phosphors Synthesized via a Precipitation–
Cation Exchange Route, Inorg. Chem. 56 (2017) 11900-11910.
[19] L.Q. Xi, Y.X. Pan, M.M. Zhu, H.Z. Lian, J. Lin, Abnormal site occupancy and
high performance in warm WLEDs of a novel red phosphor, NaHF2:Mn4+,
synthesized at room temperature, Dalton Trans. 46 (2017) 13835-13844.
[20] Q. Zhou, Y.Y. Zhou, Y. Liu, Z.L. Wang, G. Chen, J.H. Peng, J. Yan, M.M. Wu, A
new and efficient red phosphor for solid-state lighting:Cs2TiF6:Mn4+, J. Mater.
Chem. C 3 (2015) 9615-9619.
[21] L.L. Wei, C.C. Lin, M.H. Fang, M.G. Brik, S.F. Hu, H. Jiao, R.S. Liu, A
low-temperature co-precipitation approach to synthesize fluoride phosphors
K2MF6:Mn4+ (M = Ge, Si) for white LED applications, J. Mater. Chem. C 3
17

ACCEPTED MANUSCRIPT
(2015) 1655-1660.
[22] T.F. Antokhina, S.B. Ivanov, N.N. Savchenko, L.V. Teplukhina, Synthesis and
physicochemical study of hexafluorocomplexes of tin with mixed alkali metal
cations, Bull. Acad. Sci., Divi.Chem. Sci. 36 (1987) 1785-1789.
[23] Y.W. Zhu, Y. Liu, L. Huang, T.T. Xuan, J. Wang, Optimized photoluminescence
of red phosphor K2LiAlF6:Mn4+ synthesized by a cation-exchange method, Sci.
China Technol. Sc. 60 (2017) 1458-1464.
[24] E.H. Song, J.Q. Wang, S. Ye, X.F. Jiang, M.Y. Peng, Q.Y. Zhang,
Room-temperature synthesis and warm-white LED applications of Mn4+ ion
doped fluoroaluminate red phosphor Na3AlF6:Mn4+, J. Mater. Chem. C 4 (2016)
2480-2487.
[25] L.L. Wei, C.C. Lin, Y.Y. Wang, M.H. Fang, H. Jiao, R.S. Liu, Photoluminescent
evolution induced by structural transformation through thermal treating in the red
narrow-band phosphor K2GeF6:Mn4+, ACS Appl. Mater. Inter. 7 (2015)
10656-10659.
[26] Y.H. Jin, Y.H. Hu, H.Y. Wu, H. Duan, L. Chen, Y.R. Fu, G.F. Ju, Z.F. Mu, M. He,
A deep red phosphor Li2MgTiO4:Mn4+ exhibiting abnormal emission: potential
application as color converter for warm w-LEDs, Chem. Eng. J. 288 (2016)
596-607.
[27] M. G. Brik, A. M. Srivastava, On the optical properties of the Mn4+ ion in solids,
ECS J. Solid State Sci. Technol. 2 (2013) R148.
18

ACCEPTED MANUSCRIPT
[28] K. Li, D.Q. Chen, R. Zhang, Y.L. Yu, Y.S. Wang, Eu2+:SrMg1-xMnxP2O7 (x = 0–1)
phosphors with tunable yellow–red emissions, J. Alloy. Compd., 555 (2013)
45-50.
[29] X.L. Tan, M. Fang, L.Q. Tan, H.N. Liu, X.S. Ye, T. Hayat, X.K. Wang, Core–
shell hierarchical C@Na2Ti3O7·9H2O nanostructures for the efficient removal of
radionuclides, Environ. Sci. Nano, 5 (2018) 1140-1149.
[30] M.G. Kong, M. Fang, X.L. Tan, M. Liu, G.L. Shang, G.T. Fei, L.D. Zhang, The
investigation on the mechanism of the increased decay time in red SrS:Eu2+,Dy3+
phosphor, Mater. Chem. Phys., 207 (2018) 161-166.[31] X.Y. Bao, S.P. Zhou, J.G.
Wang, L.J. Zhang, S.M. Huang, Y.X. Pan, Color tunable phosphor
CaMoO4:Eu3+,Li+ via energy transfer of MoO42−–Eu3+ dependent on morphology
and doping concentration, Mater. Res. Bull. 48 (2013) 1034-1039.
[32] Y.X. Pan, M.M. Wu, Q. Su, Comparative investigation on synthesis and
photoluminescence of YAG:Ce phosphor, Mater. Sci. Engin. B 106 (2004)
251-256.
[33] L. Huang, Y.W. Zhu, X.J. Zhang, R. Zou, F.J. Pan, J. Wang, M.M. Wu, HF-free
hydrothermal route for synthesis of highly efficient narrow-rand red emitting
phosphor K2Si1–xF6:xMn4+ for warm white wight-emitting diodes, Chem. Mater.
28 (2016) 1495-1502.
[34] L.F. Lv, Z. Chen, G.K. Liu, S.M. Huang, Y.X. Pan, Optimized
photoluminescence of red phosphor K2TiF6:Mn4+ synthesized at room
temperature and its formation mechanism, J. Mater. Chem. C, 3 (2015)
1935-1941.
19

ACCEPTED MANUSCRIPT
[35] L.Q. Xi, Y.X. Pan, M.M. Zhu, H.Z. Lian, J. Lin, Room-temperature synthesis and
optimized photoluminescence of a novel red phosphor NaKSnF6:Mn4+ for
application in warm WLEDs, J. Mater. Chem. C 5 (2017) 9255-9263.
[36] L.Q. Xi, Y.X. Pan, X. Chen, S.M. Huang, M.M. Wu, Optimized
photoluminescence of red phosphor Na2SnF6:Mn4+ as red phosphor in the
application in “warm” white LEDs, J. Am. Ceram. Soc. 100 (2017) 2005-2015.
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Figure captions:
Fig. 1 XRD patterns of samples prepared (a) at different temperatures with molecular
ratioof K2MnF6/LiSrAlF6 = 2 mol% and (b) with various molecular ratios of
K2MnF6/LiSrAlF6.
Fig. 2 X-ray diffraction patterns of LiSrAlF6:Mn4+ samples (a) obtained after
full-pattern Rietveld refinement and (b) the structure projection of LiSrAlF6:Mn4+
crystal structure plotted by software Diamond 3.1.
Fig. 3 (a) Size distribution and (b) EDS spectrum of LiSrAlF6:Mn4+ particles (inset:
SEM images of LiSrAlF6:Mn4+ flaky particles.
Fig. 4 (a) TEM, (b) HRTEM images, and (c) the element mapping of Sr, Al, F, and
Mn in selected area of LiSrAlF6:Mn4+ sample.
Fig. 5 (a) The thermogravimetrics (TG) analysis and different scanning calorimeter
(DSC) of red phosphor LiSrAlF6:Mn4+ and (b) the diffuse reflectance spectra of
undoped LiSrAlF6 and red phosphors LiSrAlF6:Mn4+.
Fig. 6 (a) Excitation and (b) emission spectra of LiSrAlF6:Mn4+ with molecular ratio
of K2MnF6/LiSrAlF6 = 2 mol% measured at 78 and 298 K, respectively. (Inset:
photographs of the phosphor LiSrAlF6:Mn4+ taken under visible light and UV light,
respectively.)
Fig. 7 (a) Emission spectra (inset: dependence of integrated emission intensity on the
concentrations of K2MnF6.) and (b) decay curves for red emission of phosphor
LiSrAlF6:Mn4+ prepared with various concentrations of K2MnF6.
Fig. 8 Emission spectra and the corresponding integrated luminescence intensity of
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LiSrAlF6:Mn4+ with molecular ratio of K2MnF6/LiSrAlF6 = 2 mol% dependent on
reaction temperature.
Fig. 9 (a) The emission spectra and (b) the luminescence intensity of LiSrAlF6:Mn4+
with molecular ratio of K2MnF6/LiSrAlF6 = 2 mol% dependent on measurement
temperature.
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Figures:
Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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