Crystal structure and Temperature-Dependent Luminescence Characteristics of KMg4(PO4)3:Eu2+ phosphor for White Light-emitting diodes

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T11878 Solid-state spectroscopy and crystallography

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The KMg4(PO4)3:Eu(2+) phosphor was prepared by the conventional high temperature solid-state reaction. The crystal structure, luminescence and reflectance spectra, thermal stability, quantum efficiency and the application for N-UV LED were studied respectively. The phase formation and crystal structure of KMg4(PO4)3:Eu(2+) were confirmed from the powder X-ray diffraction and the Rietveld refinement. The concentration quenching of Eu(2+) in the KMg4(PO4)3 host was determined to be 1 mol% and the quenching mechanism was certified to be the dipole-dipole interaction. The energy transfer critical distance of as-prepared phosphor was calculated to be about 35.84 Å. Furthermore, the phosphor exhibited good thermal stability and the corresponding activation energy ΔE was reckoned to be 0.24 eV. Upon excitation at 365 nm, the internal quantum efficiency of the optimized KMg4(PO4)3:Eu(2+) was estimated to be 50.44%. The white N-UV LEDs was fabricated via KMg4(PO4)3:Eu(2+), green-emitting (Ba,Sr)2SiO4:Eu(2+), and red-emitting CaAlSiN3:Eu(2+) phosphors with a near-UV chip. The excellent color rendering index (Ra = 96) at a correlated color temperature (5227.08 K) with CIE coordinates of x = 0.34, y = 0.35 of the WLED device indicates that KMg4(PO4)3:Eu(2+) is a promising blue-emitting phosphor for white N-UV light emitting diodes (LEDs).

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SUBJECT AREAS:

MATERIALS FOR DEVICES

INORGANIC LEDS

Received
11 December 2014

Accepted
16 March 2015

Published

9    April 2015

Correspondence and

requests for materials

should be addressed to

[Y.G.L. (liuyang@cugb.](mailto:liuyang@cugb.edu.cn)

[edu.cn)](mailto:liuyang@cugb.edu.cn)

## Crystal structure and Temperature-Dependent Luminescence Characteristics of KMg4(PO4)3:Eu 21 phosphor for White Light-emitting diodes

Jian Chen, Yangai Liu, Lefu Mei, Haikun Liu, Minghao Fang & Zhaohui Huang

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing, 100083.

The KMg4(PO4)3:Eu 21 phosphor was prepared by the conventional high temperature solid-state reaction.
The crystal structure, luminescence and reflectance spectra, thermal stability, quantum efficiency and the
application for N-UV LED were studied respectively. The phase formation and crystal structure of
KMg4(PO4)3:Eu 21 were confirmed from the powder X-ray diffraction and the Rietveld refinement. The
concentration quenching of Eu 21 in the KMg4(PO4)3 host was determined to be 1 mol% and the quenching
mechanism was certified to be the dipole–dipole interaction. The energy transfer critical distance of
as-prepared phosphor was calculated to be about 35.84 A ˚ . Furthermore, the phosphor exhibited good
thermal stability and the corresponding activation energy DE was reckoned to be 0.24 eV. Upon excitation at
365 nm, the internal quantum efficiency of the optimized KMg4(PO4)3:Eu 21 was estimated to be 50.44%.
The white N-UV LEDs was fabricated via KMg4(PO4)3:Eu 21, green-emitting (Ba,Sr)2SiO4:Eu 21, and
red-emitting CaAlSiN3:Eu 21 phosphors with a near-UV chip. The excellent color rendering index (Ra 5 96)
at a correlated color temperature (5227.08 K) with CIE coordinates of x 5 0.34, y 5 0.35 of the WLED device
indicates that KMg4(PO4)3:Eu 21 is a promising blue-emitting phosphor for white N-UV light emitting
diodes (LEDs).

ince the first light emitting diode (LED) light source was invented by Nick Holonyak of General Electric, it
has drawn more and more attention to apply in solid-state lighting and create an enormous revolution on
the lighting industry. Recently, a great attention has been focused on white LEDs as solid-state lighting and
# S
as components of display devices because of their low energy consumption, high efficiency, long operational
lifetime (.100 000 h), environmental friendliness and high material stability 1–4 . There are several ways to
assemble the white LEDs. The most prevalent strategy is produced by pumping the blue InGaN chip with
yellow-emitting Y3Al5O12:Ce 31 (YAG) phosphor. Nevertheless, high correlated color temperature (CCT) and
low color rendering (CRI) index (Ra, 80) restrict it to provide sunlight-like illumination due to the deficiency in
red emission. In order to generate excellent CRI values and appropriate CCT white light for display or general
illumination light sources, the method of pumping blue, green, and red-emitting phosphors with near ultraviolet
(N-UV) LEDs has been investigated 5–7 . Thereinto, the BaMgAl10O17:Eu 21 (BAM), which is the most commonly
used commercial blue phosphor for N-UV LEDs as high efficiency, suffer from poor thermal stability 4,8,9 .
Accordingly, the development of excellent structure and thermal stability of blue phosphor host for N-UV
LEDs is highly desirable.

Eu 21 ion is the most frequently used blue-emitting activator in phosphor, which shows broad N-UV excitation
and visible emission in a specific host owing to the 4f–5d transitions. According to the impact of the strength of
the crystal field and covalent, Eu 21 can emit light from the ultraviolet to the infrared with broadband emitting
fluorescence in different matrixes, and the corresponding fluorescence lifetimes locate commonly in the range of
0.2–2.0 ms 10,11 . On basis of these characteristics, multifarious Eu 21 -activated phosphors have been widely studied
in LED lighting, 3D displays and scintillators in detection devices.

The phosphate KMg4(PO4)3 compound was first obtained from flux during crystallizing K2MgWO2(PO4)2
by Tomaszewski and co-workers 12 . Xiaofeng Lan et al. reported the luminescence properties of Eu 21 activated KMg4(PO4)3 by combustion-assisted synthesis method in 2012 13 . However, to our best knowledge,

the temperature-dependent luminescence characteristics as well as
the application of KMg4(PO4)3:Eu 21 pumped for n-UV LEDs have
not been investigated. In this paper, the KMg4(PO4)3:Eu 21 phosphor
was firstly prepared by the conventional high temperature solid-state
reaction method. The crystal structure, reflectance spectra, thermal
stability, quantum efficiency and applications in white NUV LED are
studied respectively. White LEDs was fabricated by combing an NUV LED chip (lmax 5 385 nm) with the KMg4(PO4)3:Eu 21, along
with green and red phosphors, and its optical properties have also
been investigated. The results demonstrate that the blue-emitting
KMg4(PO4)3:Eu 21 is a promising blue-emitting phosphor for white
N-UV LEDs.

Results
XRD Refinement and Crystal Structure. Figure 1 depicts the XRD
patterns of the series of as-synthesized KMg4(PO4)3:xEu 21 (x 5 0,
0.02, 0.06, and 0.1), and the standard pattern (JCPDS 15-4111) of
KMg4(PO4)3 is shown as a reference. It can be found from the
Figure 1 that all XRD patterns agree well with the standard pattern
and no other phase is observed, which demonstrates that the single
phase of KMg4(PO4)3:xEu 21 was obtained and the doping of Eu 21
ions did not cause any notable impurities or any structural variation.
Besides, the main diffraction peaks shift slightly to the higher angle
side with increasing Eu 21 concentrate, as shown in Figure 1(b). This
observation means that the lattice was distorted by substitution the
ions which are comparatively big radius in KMg4(PO4)3 host lattice
with Eu 21 ions 14 . Thus, it is reasonable to assume that Eu 21 (r 5
1.25 A ˚ for coordinate number (CN) 5 8 and r 5 1.17 A ˚ for CN 5 6)
ions substituted the position of K 1 sites (r 5 1.51 A ˚ for CN 5 8)
because both the Mg 21 (r 5 0.72 A ˚ for CN 5 6 and r 5 0.66 A ˚ when
CN 5 5) and P 51 (r 5 0.17 A ˚ for CN 5 4) sites are smaller than the
Eu 21 ions 15,16 . For further understanding the phase purity and the
occupancy of Eu 21 ions on K 1 sites in KMg4(PO4)3:Eu 21, the Rietveld
refinement of KMg4(PO4)3 and KMg4(PO4)3:0.06Eu 21 phosphors
were analyzed via the GSAS program as shown in Figure 2. The
KMg4(PO4)3 was served as an initial structural model. The results
of Rietveld refinement further demonstrate that neither the host nor
the doped 0.06 mol Eu 21 ions generated any impurity or secondary
phases in KMg4(PO4)3. The KMg4(PO4)3:xEu 21 belongs to an
orthorhombic structure with the space group Pnnm(58). For the
crystal of KMg4(PO4)3 host, the lattice parameters were fitted to be
a 5 16.3707(7) A ˚, b 5 9.5627(4) A ˚, c 5 6.1667(5) A ˚, cell volume (V)
5 965.361(23) A ˚ 3 and the weighted profile R-factor (Rwp), the
expected R factor (Rp) are 8.86% and 6.86%, respectively. As doped

with Eu 21, the lattice parameters of KMg4(PO4)3:0.06Eu 21 became a
5 16.3563(4) A ˚, b 5 9.5570(2) A ˚, c 5 6.1663(1) A ˚ and V 5 963.904
(67) A ˚ 3 . The refinement data converged to Rwp 5 10.54% and Rp 5
7.97%, as summarized in Table 1. The volumetric constriction with
increasing Eu 21 doping concentration also indicates that the Eu 21

occupied the K 1 ions sites.

Figure 3 illustrates the crystal structure of KMg4(PO4)3 and the
coordination environment of the K 1 ions. The compound of
KMg4(PO4)3 is consist of PO4 tetrahedra, MgO6 octahedra and
MgO5 polyhedra which are linked by P–O–Mg bridges. The K 1 ions
are located in the tunnels along b-axis and surrounded by the threedimensional framework forming via interconnected polyhedra. As
presented in Figure 3c, the K 1 ions in KMg4(PO4)3 are eight-fold
coordinated by oxygen ions with 4g position and m site symmetry.

Reflectance and Photoluminescence properties of the KMg4(PO4)3:
xEu phosphor at RT. The reflectance spectra of KMg4(PO4)3 host and
KMg4(PO4)3:0.05Eu 21 are presented in Figure 4a. The KMg4(PO4)3
host shows an energy absorption band ranging from 200 to 300 nm,
and a high reflection ranging from 300 to 700 nm. The band gap of the
virgin KMg4(PO4)3 is calculated by using the following formula: 17

- ½FðR?Þhv� n ~C hv{Eg ð1Þ

in which hn means the energy per photon, C is a proportional constant
and Eg represents the value of the band gap, n 5 1/2 stands for an
indirect allowed transition, 2 means a direct allowed transition, 3/2
represents a direct forbidden transition, or 3 indicates an indirect
forbidden transion, R‘ 5 Rsample/Rstandard. The F(R‘) means the
Kubelka2Munk function which can be formulated to the following
equation:

where K, S, and R represent the absorption, scattering, and reflectance
parameter, respectively. As illustrated in Figure 4b, the band gap energy
of KMg4(PO4)3 host is estimated to be about 5.74 eV from the
extrapolation of the line for [F(R‘)hn] 2 5 0. As Eu 21 ions were
introduced into the host, strong broad absorption appeared in the
250–400 nm N-UV range, which is matched well with the excitation
spectrum.

The photoluminescence emission (PL, lex 5 300 and 365 nm) and
excitation (PLE, lem 5 450 nm) spectra of KMg4(PO4)3:0.06Eu 21 are
also depicted in Figure 4a. The PLE spectrum of KMg4(PO4)3:

K S ~ ð 1{ R R Þ 2

FðR?Þ~ K

ð2Þ
R R

Figure 1 | (a) XRD patterns of the as-prepared samples KMg4(PO4)3:xEu 21 (x 5 0, 0.02, 0.06, and 0.1) and the standard pattern (JCPDS 15-4111).
(b) The detailed XRD patterns ranging from 10u to 12u for the same samples

Figure 2 | Powder XRD pattern (3) of (a) KMg4(PO4)3 host and (b) KMg4(PO4)3:0.06Eu 21 samples with their corresponding Rietveld refinement
(solid line) and residuals (bottom).

0.06Eu 21 presents a broad hump ranging at 250–400 nm, which
originates from the 4f 7 –4f 6 5d transition of Eu 21 ions. It indicates
that the broad excitation spectrum of KMg4(PO4)3:Eu 21 matches
well with the emission of the commercial N-UV chip (365–
420 nm). The PL spectra of the KMg4(PO4)3:0.06Eu 21 phosphor
present a 52.2 nm full width at half-maximum (FWHM) broad blue
emission band extending from 400 to 525 nm peaking at 450 nm,
which is assigned to the 4f 6 5d–4f 7 transition of the Eu 21 ions.
Moreover, The PL of KMg4(PO4)3:0.06Eu 21 detected under
365 nm is similar to that under 300 nm in addition to the difference
of the relative intensity, which verifies that the Eu 21 ions occupy the
same lattice site (K 1 sites) in KMg4(PO4)3 host 18 . Above result is in
accord with the conclusion from Rietveld refinement. The CIE chromaticity coordinates of KMg4(PO4)3:0.01Eu 21 and commercial BAM
phosphors under 365 nm UV excitation are illustrated in Figure 5.
The color coordinates of KMg4(PO4)3:0.01Eu 21 and BAM are calculated to be (x 5 0.1507, y 5 0.0645) and (x 5 0.1471, y 5 0.0628),
respectively. The inset shows the digital photograph of
KMg4(PO4)3:0.01Eu 21 phosphor under a 365 nm UV lamp, which
indicates KMg4(PO4)3:Eu 21 phosphor can be used as a blue-emitting
phosphor for w-LEDs application.

As Figure 4a shows, the PL and PLE spectra of KMg4(PO4)3:
0.06Eu 21 overlap partially, which demonstrates the existence of
energy transfer between Eu 21 -Eu 21 19. As we know, two main
mechanisms can be explanatory for the resonant energy-transfer:

exchange interaction or electric multipolar interaction 20 . In order
to further investigate the process of energy transfer between activators or between sensitizer and activator, the Eu 21 concentrationdependent PL spectra of KMg4(PO4)3:xEu 21 (x 5 0.005, 0.01, 0.02,
0.04, 0.06, 0.08) phosphors under 365 nm light excitation are shown
in Figure 6. The optimal doping concentration of Eu 21 for the PL
intensity in KMg4(PO4)3:xEu 21 is 1% mol. When the doping content
of Eu 21 exceeded 0.01 mol%, the PL intensity began to decrease
because of the concentration quenching effect which is due to the
energy consumed via energy transfer from one activator to another 21 .
Thus, the critical distance (Rc) for energy transfer among Eu 21 is
necessary to obtain for further understanding the concentration
quenching interaction mechanism. The value of the critical distance
(Rc) can be reckoned via the following equation: 22

- �1

3V 3
Rc<2 ð3Þ
4pxcN

where V means the unit cell volume, xc represents the concentration
of activator ion where the quenching occurs and N is the number of
the K 1 ion in per unit cell. For the KMg4(PO4)3 host, xc 5 0.01, N 5
4, and V 5 965.361 A ˚ 3, hence, the value of Rc is calculated to about
35.84 A ˚ . Owing to the typical critical distance of the exchange interaction is about 5 A ˚ and the exchange interaction only fits the energy
transfer of forbidden transitions 5,23 . Therefore, the electric multipolar

Table 1 | Main parameters of processing and refinement of KMg4(PO4)3 host and KMg4(PO4)3:0.06Eu 21 samples

Compound KMg4(PO4)3 KMg4(PO4)3:0.06Eu

Sp.Gr. Pnnm Pnnm
a, A ˚ 16.3707(7) 16.3563(4)
b, A ˚ 9.5627(4) 9.5570(2)
c, A ˚ 6.1667(5) 6.1663(1)
a, u 90 90
b, u 90 90
c, u 90 90
V, A ˚ 3 965.361(23) 963.904(67)
2h-interval, u 5–90u 5–90u
Rwp, % 8.86 10.54
Rp, % 6.86 7.97
x 2 2.511 2.844

Figure 3 | Crystal structures of KMg4(PO4)3. The view of the KMg4(PO4)3 (a) along the [001] Direction, (b) perpendicular to the [010] direction, and the
(c) coordination geometry of anions around the K 1 ions.

interactions are dominant in the energy transfer process. The interaction type can be estimated via the following equation: 24

1x ~K h 1zb ð x Þh3i{1

ð4Þ

activator concentration (x); K and b are constants under the same
excitation condition of host lattice; and h is a function of electric
multipolar character. h 5 6, 8, 10 for dipole–dipole (d–d), dipole–
quadrupole (d–q), quadrupole–quadrupole (q–q) interactions,
respectively. In order to estimate the h value, the dependence of
lg(I/x) on lg(x) is illustrated in the inset of Figure 6. A relatively linear
relation can be observed and the slope of the straight line is fitting to

in which x is the concentration of activation, which is not less
than the critical concentration, I/x is the emission intensity (I) per

Figure 4 | (a) Excitation and emission spectra of KMg4(PO4)3:0.06Eu 21 (lem 5 450 nm for excitation and lex 5300 and 365 nm for emission); Diffuse
reflection spectra of KMg4(PO4):xEu 21 (x 5 0 and 0.06); (b) Absorption spectra of KMg4(PO4)3:0.06Eu 21 matrix calculated by the Kubelka2Munk
equation All spectra were taken at RT

Figure 5 | CIE coordinates of the KMg4(PO4)3:0.01Eu 21 phosphor and
the commercial BAM:Eu 21 . The inset shows a digital photograph of the
blue-emitting KMg4(PO4)3:0.01Eu 21 sample.

21.4911 which equals to 2h/3. Hence, the value of h is 4.4733, which
is close to 6, demonstrating that the interaction type in KMg4(PO4)3:
Eu 21 is dipole-dipole interactions.

To further explore the energy transfer process, the room temperature luminescence decay curves of Eu 21 ions in KMg4(PO4)3:xEu 21

(x 5 0.005, 0.01, 0.02, 0.04, 0.06, 0.08; lex 5 365 nm, lem 5 450 nm)
were measured, as shown in Figure 7. The decay curves can be fitted
with an approximate single-exponential decay model as:

I~I0zAexpð{t=tÞ ð5Þ

where I0 represents the initial emission intensity when t is 0 and t
means the lifetime. The average lifetimes of Eu 21 ions of
KMg4(PO4)3:xEu 21 (x 5 0.005, 0.01, 0.02, 0.04, 0.06, 0.08) were
estimated to be 1.246, 1.262, 1.245, 1.176, 1.163 and 1.161 ms,
respectively. All measured decay times are reasonable for the 5d–4f
allowed transition of Eu 21 in solids (,1 ms). Besides, the decay time
increases to maximum, when the concentration of Eu 21 increases to
0.01, and then the decay time reduces acutely, indicating an efficient
energy transfer between Eu 21 ions and causing concentration
quenching 25,26 .

Influence of Temperature on emission intensity and FWHM. The
thermal quenching of luminescence is one of important
technological parameters to be considered for phosphor materials
applied in high power LEDs, because it significantly affects the light
output and service life. The temperature-dependent PL spectra of the
KMg4(PO4)3:0.01Eu 21 exited by 365 nm N-UV light is depicted in
Figure 8. With increasing temperature (30uC–300uC), the emission
intensity decreases from 100% to 74.26% of that at 30uC, and the
FWHM of the emission band increases from 50.34 to 60.42 nm. The
inset of Figure 8 illustrates the comparison of the thermal
luminescence quenching of KMg4(PO4)3:0.01Eu 21 with that of
commercial BAM:Eu 21 and the FWHM of KMg4(PO4)3:0.01Eu 21
emission as a function of the temperature. As shown in Figure 8, it
can be found only 9% decay at 150uC for KMg4(PO4)3:0.01Eu 21,
which indicates that the thermal stability of KMg4(PO4)3:Eu 21 is
superior to that of commercial BAM:Eu 21 below 200uC and this
phosphor could be used as a promising phosphor for high-power
LED application. To better understand the thermal quenching
process, the configurational coordinate diagram can be used to

Figure 6 | PL spectra for KMg4(PO4)3:xEu 21 (x 5 0.005–0.08) phosphors. The inset shows the fitting line of lg(I/x) versus lg(x) in KMg4(PO4)3:xEu 21 (x
5 0 005 0 08) phosphors beyond the quenching concentration

by using the Arrhenius equation: 28

��

I0 ~ ln A{ DE

ln I0

DE ð6Þ

in which I0 and I are the luminescence intensity of KMg4(PO4)3:Eu 21

at room temperature and a given temperature, respectively; A is a
constant; k is the Boltzmann constant (8.617 3 10 25 eV K 21 ). From
above equation, the DE is calculated to be about 0.24 eV (Figure 9).

The temperature-dependent of emission FWHM is related to the
configuration coordinate model and the Boltzmann distribution, and
can be expressed by: 18,29

f f f f f f f f

ð7Þ

ð8Þ

FWHMðTÞ~W0|

f f i f f i f f i f f i f f i f f i f f i f f i f

coth hv

2kT

coth hv

pf f i f f i f f i f f
8ln2|hv|

W0~

pf f
S

Figure 7 | Decay lifetime tests of KMg4(PO4)3:xEu 21 (x 5 0.05–0.08)
detected at 450 nm for Eu 21 emission.

respond to this phenomenon. As the temperature increases, the
interaction of electron-phonon is intensive. Along with the
enhancing of phonon interaction, more electrons can be thermally
activated to the crossover between the 4f 6 5d excited state and 4f 7

ground state, whereupon release the energy by generating lattice
vibration 22 . The excellent thermal stability of KMg4(PO4)3:Eu 21

may relate to the indurative structure which is combined with PO4
tetrahedra, MgO6 octahedra and MgO5 polyhedra via P-O-Mg
bridges to form the three-dimensional framework. In this
indurative structure, the energy of the electrons in the excited state
is difficult to release via lattice vibration, which means smaller Stokes
shift in a configurational coordinate diagram model and higher
activation energy (DE) 27 . The activation energy can be estimated

where W0 is the FWHM at 0uC, hv represents the vibrational phonon
energy, S means the Huang2Rhys parameter, and k is the Boltzmann
constant. With the temperature increase, the excited electrons spread
to higher vibration levels and the radiative transitions from these
different levels cause the emission band broadening.

in which Ls represents the luminescence emission spectrum of the
sample; ER is the spectrum of the excitation light from the empty
integrated sphere (without the sample); ES means the excitation

Quantum efficiency and Electroluminescence properties of
White-Light LED Lamp. Quantum efficiency of phosphors is
another important technological parameter for practical application.
The internal quantum efficiency (QE) of KMg4(PO4)3:0.01Eu 21 were
measured and calculated by the following equations: 30

gQE~

LS
~~Ð~~ ~~Ð~~

ERz

~~Ð~~

ð9Þ

Figure 8 | Temperature-dependent PL spectra of KMg4(PO4)3:0.01Eu 21 . The inset shows a comparison of thermal quenching of KMg4(PO4)3:0.01Eu 21

with that of BAM:Eu 21 and dependence of the FWHM of KMg (PO ) :0 01Eu 21 emission on temperature

Figure 9 | The plot of ln (I0/I) vs. 1/T of the KMg4(PO4)3:0.01Eu 21

phosphors.

spectrum for exciting the sample. As given in Figure 10, the internal
QE of the KMg4(PO4)3:0.01Eu 21 phosphor is estimated to be about
50.44% under 365 nm excitation. As a comparison, the internal QE of
commercial BAM:Eu 21 phosphor is detected at the same condition
and calculated to about 88.99%. The QE of KMg4(PO4)3:0.01Eu 21 can
be further improved by optimization of the preparation conditions,
because the QE depends closely on the prepared conditions,
crystalline defects, particle size and morphology of the phosphor 31,32 .

To demonstrate the potential application of KMg4(PO4)3:Eu 21
phosphor, the electroluminescent spectrum of white LED lamp which
was fabricated via using N-UV LED chips (lmax 5 385 nm) combing

Figure 11 | EL spectrum of a white-emitting N-UV-LED chip (385 nm)
comprising of KMg4(PO4)3:Eu 21 (blue), (Ba,Sr)2SiO4:Eu 21 (green) and
CaAlSiN3:Eu 21 (red) phosphors driven by a 2 mA current.

with blue-emitting KMg4(PO4)3:0.01Eu 21 phosphor, green-emitting
(Ba,Sr)2SiO4:Eu 21 phosphor, and red-emitting CaAlSiN3:Eu 21 phosphor was measured as given in Figure 11 with forward bias current of
2 mA. The CIE color coordinates, correlated color temperature (CCT)
and color rendering index (Ra) of this fabricated WLED lamp are
determined to be (0.34, 0.35), 5227.08 and 96, respectively. The Ra
was decided from the full set of the first eight CRIs shown in Table 2.
The appropriate CCT value (5227.08) and high Ra value (96) demonstrate that the KMg4(PO4)3:Eu 21 can be a promising candidate for a
blue-emitting phosphor for application of WLEDs.

Figure 10 | Excitation line of BaSO4 and emission spectrum of the KMg4(PO4)3:0.01Eu 21 phosphor collected by an integrating sphere. The inset shows
the magnification of the emission spectrum

Table 2 | Full Set of 14 CRIs and the Ra of the Fabricated WLED

Ra R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14

96 96 98 98 94 96 94 96 92 77 95 89 96 96 99

Discussion
In conclusion, we report a systematic study on the preparation and
crystal structure analysis of blue-emitting KMg4(PO4)3:Eu 21 phosphor and investigate their reflectance spectra, thermal stability,
quantum efficiency and applications in N-UV LED. The phase composition and crystal structure of KMg4(PO4)3:Eu 21 were determined
via the powder X-ray diffraction patterns and Rietveld refinement
analysis. The optimal Eu 21 doping concentration in the KMg4(PO4)3
host is 1 mol%. The critical energy transfer distance of this phosphor
was calculated to be about 35.84 A ˚ and the concentration quenching
mechanism is proved to be the dipole–dipole interaction. The investigation results also reveal that the as-prepared phosphor shows good
thermal stability, and the internal quantum efficiency is 50.44%. The
white N-UV LEDs packaged by an N-UV chip with blue-emitting
KMg4(PO4)3:Eu 21, green and red-emitting phosphors generate white
light with high color rendering index (Ra 5 96) and an appropriate
correlated color temperature (5227.08 K). These results demonstrate
that KMg4(PO4)3:Eu 21 is a promising blue-emitting phosphor for NUV LEDs.

Methods
Materials and Synthesis. A variety of blue-emitting KMg4(PO4)3:xEu 21 phosphors
were prepared via a traditional high-temperature solid-state reaction. The constituent
raw materials KH2PO4 (A. R.), MgO (A. R.), NH4H2PO4 (A. R.) and Eu2O3 (A. R.)
were weighed in stoichiometric proportions and ground homogeneously in agate
mortar. Firstly, the mixtures were preheated at 600uC for 2 h in a muffle furnace in air
to release NH3, CO2, and H2O. Then, the precursor was reground and heated at
1000uC for 6 h in the thermal carbon reducing atmosphere (TCRA). Finally the
furnace cooled to room temperature and the mixtures were ground in an agate
mortar.

Materials Characterization. X-ray powder diffraction (XRD) patterns of the final
products were identified on a D8 Advance diffractometer (Bruker Corporation,
Germany) with Cu Ka radiation (l 5 0.15406 nm) radiation. High quality XRD data
for Rietveld refinement were collected by step scanning rate (8 s per step with a step
size of 0.02u) over a 2h range from 5u to 100u. The photoluminescent excitation/
emission (PLE/PL) spectra were detected by a Hitachi F-4600 fluorescence
spectrophotometer (Japan) equipped using a150 W Xe lamp as the excitation source.
The temperature-dependent luminescence properties were measured on the same
spectrophotometer which was assembled with a computer-controlled electric furnace
and a self-made heating attachment. The diffuse reflectance spectra were obtained by
a Varian Cary-5000 UV2vis2NIR spectrophotometer attached with an integral
sphere. The room-temperature luminescence decay curves were obtained from a
spectrofluorometer (Horiba, Jobin Yvon TBXPS) using a tunable pulse laser radiation
(nano-LED) as the excitation. Quantum efficiency was measured by a fluoromax-4
spectrofluorometer (Horiba, Jobin Yvon) with an integral sphere at room
temperature.

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Acknowledgments
This work was sponsored by National Natural Science Foundation of China (Grant No.
51472223), the Program for New Century Excellent Talents in University of Ministry of
Education of China (Grant No NCET 12 0951)

Author contributions

Y.G.L. and J.C. conceived the project. J.C. and H.K.L. designed and performed the
experiments. J.C., L.F.M. and M.H.F. analyzed the data. J.C. and Z.H.H. wrote the
manuscript. All the authors discussed the results and commented on the manuscript at all
stages.

Additional information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Chen, J. et al. Crystal structure and Temperature-Dependent

Luminescence Characteristics of KMg4(PO4)3:Eu 21 phosphor for White Light-emitting
diodes. Sci. Rep. 5, 9673; DOI:10.1038/srep09673 (2015).

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