Materials Research Bulletin 88 (2017) 166–173
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Color–tunable luminescence and energy transfer behaviors of Dy3+/Eu3+
co–doped SrLaMgTaO6 phosphors for solid state lighting applications
Yue Guoa , Byung Kee Moona , Byung Chun Choia , Jung Hyun Jeonga,* , Jung Hwan Kimb
a
b
Department of Physics, Pukyong National University, Busan 608-737, South Korea
Department of Physics, Dongeui University, Busan 614-714, South Korea
A R T I C L E I N F O
Article history:
Received 5 August 2016
Received in revised form 20 December 2016
Accepted 27 December 2016
Available online 27 December 2016
Keywords:
Rare-earth
Energy transfer
TaO6 group
Phosphors
A B S T R A C T
A series of color–tunable SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors were synthesized via solid–state
reaction. The X–ray diffraction, diffuse reflectance spectra, luminescent spectra, lifetimes and thermal
quenching were applied to characterize the obtained phosphors. The X–ray diffraction results revealed
that the obtained phosphors possessed pure monoclinic phase. The band gap of SrLaMgTaO6 was
calculated from Vienna ab initio simulation package and diffuse reflectance spectra. The energy transfers
from host to Dy3+ and Eu3+ ions were confirmed by the luminescence spectra. Furthermore, the
SrLa0.92MgTaO6: 0.07Dy3+, 0.01Eu3+ phosphor had excellent thermal stability, while the integrated PL
intensity still kept about 83.34% at 150 C of that measured at room temperature (30 C). In addition, the
warm and cold white light emission could be generated with different Eu3+ contents. All the results of
SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors show great potential as a single–phase white–emitting
phosphor for solid state lighting applications.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Nowadays, phosphor–converted white light–emitting diodes
(w–LEDs) have been drawing worldwide attention as a new
lighting source for the next generation due to their extraordinary
advantages such as luminous efficiency, low energy consumption,
good stability properties in chemical and physical, and environmental benefits [1–4]. As we know, the combination of blue InGaN
chip (450–470 nm) and Y3Al5O12: Ce3+ (YAG: Ce3+) yellow–
emitting phosphor is the most mature method to generate
w–LED. However, this strategy suffers from a poor color rendering
index due to lack of red–light component. This problem may be
solved by using a UV/near–UV LED chip coated with tri–color
phosphors (red CaAlSiN3: Eu2+, green (Ba, Sr)2SiO4: Eu2+, and blue
Ca2PO4Cl: Eu2+). Unfortunately, in this tri–color phosphors system,
the complex coating and different thermal quenching behaviors of
each component are still problems hard to tackle. Moreover, the
luminous efficiency is relatively low since the blue light is strongly
reabsorbed by the green and red phosphors [5,6]. In these regards,
a single–phased phosphor with white light–emitting for UV/near–
UV pumped w–LEDs can be a good choice as it reveals definite
advantages, such as excellent color rendering index, high color
* Corresponding author.
E-mail address: jhjeong@pknu.ac.kr (J.H. Jeong).
http://dx.doi.org/10.1016/j.materresbull.2016.12.035
0025-5408/© 2016 Elsevier Ltd. All rights reserved.
quality, better reproducibility, improved chromatic stability, a
simplified fabrication process, and a competitive price [7,8].
The perovskite–type structural compounds, acting as one of the
promising types of inorganic functional materials, have a stable
crystal structure, as well as special physical and chemical
properties. Recently, the structure of double perovskite AA'BB'O6
(A = Ca, Sr, Ba; A' = La, Pr, Nd, Gd; B = Mn, Mg, Zn; B' = W, Mo, Ta, Ti,
Sb) doping with various ions have attracted much attention due to
the versatility of double–perovskites [9–12]. The special lattice
arrangement of double–perovskite is the combination of layered
ordering of the A (A') sites and long–range rock–salt ordering of the
B (B') sites. For SrLaMgTaO6 compound, the A (A') sites are
randomly assigned by Sr2+ and La3+ ions, while the B (B') sites have
long–range rock salt arrangement by Mg2+ and Ta5+ ions. The
special lattice arrangement and versatility of the double–
perovskites derive from both the inherent stability and the ability
to accommodate unmatched A–O, B–O–B' bond lengths in the
AA'BB'O6 structure. So SrLaMgTaO6 can be a notable host lattice for
luminescence and laser material. In our previous work, we
observed that the Eu3+ ions occupied A' sites in SrLaMgTaO6
phosphors can exhibit bright red emission and can have promising
application in w–LEDs [13].
In the last decades, many investigations were devoted to grow
the single–phase white light– emitting phosphor based on the
efficient energy transfer from the sensitizer to activator, such as
Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
Ca9Gd(PO4)7:Eu2+/Mn2+ [14] and Ca5(PO4)3F:Tb3+/Eu3+ [15]. The
emission properties of trivalent dysprosium (Dy3+) ions consist of
two characteristic emission bands in the blue (480 nm)
and yellow (580 nm) regions originating from 4F9/2 ! 6H15/2
and 4F9/2 ! 6H13/2 transitions, respectively. Therefore, by tuning
the yellow–to–blue (Y/B) intensity ratio, the trivalent dysprosium
(Dy3+) ions have significant potential to obtaining white light.
However, one drawback is that the molar absorption coefficients
aroused by spin–forbidden f–f transitions is quite low, so the direct
excitation of Dy3+ ions is not sufficient to achieve efficient
luminescence characteristics [16]. An alternative approach is by
exciting an appropriate host as a sensitizer which can effectively
transfer energy to activator ion. For instance, in the Dy3+–doped
Y6WO12 phosphor, WO6 group plays the role of sensitizer which
absorbs the light near 400 nm, and then transfer it to Dy3+ ions [17].
For SrLaMgTaO6, the host can absorb the UV light and transfer the
energy to Dy3+ and Eu3+ ions. Furthermore, the TaO6 group can
absorb the near UV light and then give a broad blue emission band
from 420 nm to 600 nm, which is helpful to generate the white
light. Another weakness is that the white light emission obtained
from Dy3+ ions has a poor color rendering due to lack of the red
light component. It could be solved by combining Dy3+ and Eu3+
ions in SrLaMgTaO6 system. As we known, the Eu3+ ion is one of the
most popular activators for red emission in various host lattices.
Herein, we suggest that the SrLa0.93-yMgTaO6: 0.07Dy3+, yEu3+
phosphors with warm white light emission can be a notable
luminescence material. To the best of our knowledge, the study on
single–phase white light–emitting phosphor based on the energy
transfer from host to Dy3+ and Eu3+ ions in SrLaMgTaO6 is sparse.
Following this reasoning, a growing interest has been aroused to
study the Dy3+ and Eu3+ ions co–doped SrLaMgTaO6 double–
perovskite phosphor.
2. Experimental
2.1. Sample preparation
The un–doped, Eu3+, Dy3+ single–doped and Dy3+/Eu3+ co–
doped SrLaMgTaO6 phosphor were prepared by a conventional
high temperature solid–state reaction in air. In the synthesis,
dysprosium oxide (Dy2O3, 99.99%), europium oxide (Eu2O3,
99.99%), strontium carbonate (SrCO3, 99%), lanthanum oxide
(La2O3, 99.99%), magnesium carbonate basic ((MgCO3)4Mg
(OH)25H2O, 99%), tantalum oxide (Ta2O5, 99.5%) were used as
starting ingredients. All the samples were prepared according to
the formula SrLa(1-x-y)MgTaO6: xDy3+, yEu3+ (x = 0, y = 0; x = 0.07,
y = 0; x = 0, y = 0.01; x = 0.07, y = 0.005; x = 0.07, y = 0.01; x = 0.07,
y = 0.03; x = 0.07, y = 0.05; x = 0.07, y = 0.07; x = 0.07, y = 0.10). The
mixture based precursors were grinded thoroughly for 30 min in
an agate mortar and pre-heated at 650 C for 2 h in a muffle furnace
to decompose the carbonates, then the harvest mixtures were
reground and re–heated at 1400 C for 12 h. Finally, these solid
solutions were ground into powder for characterization. The Dy3+,
Eu3+ co–doped SrLaMgTaO6 phosphor was supplied by the Display
and Lighting Phosphor Bank at Pukyong National University.
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recorded by a Photon Technology International (PTI, USA) fluoro–meter equipped with a 60 W xenon lamp as the excitation source.
The fluorescence lifetime curves were measured with a phosphorimeter attached to the fluorescence spectrophotometer with a
25 W xenon flash lamp. The temperature–dependent luminescence properties were measured on a fluorescence spectrophotometer (SCINCO FS–2) with a heating apparatus (NOVA ST540).
2.3. Details of calculation
The band structure calculation of SrLaMgTaO6 has performed
using the density functional theory (DFT) by Vienna ab initio
simulation package (VASP) [18,19]. Valence electrons were treated
explicitly and their interactions with ionic cores were described by
projector augmented wave (PAW) pseudopotentials [20,21]. In this
report, the generalized gradient approximation (GGA) [22] was
used for an exchange–correlation functional. All of the reported
results have been obtained with high plane–wave energy cut–off
700 eV. Thus, lattice constants of a = 5.640 Å, b = 5.642 Å, and
c = 7.972 Å were used with 20 atoms existed in the unit cell. The
Brillouin–zone integration was performed using a Gamma
centered k–point sampling with 11 11 9 k–mesh. The convergence criteria for energy and force were set to 0.1 meV and
0.01 eV Å1, respectively. It is worth noting that the unit cell has
been optimized to obtain the most stable lattice constant.
3. Results and discussion
3.1. Phase identification, crystal structure and energy gap
Fig. 1 presents the powder X–ray diffraction (XRD) patterns of
the un–doped, Eu3+, Dy3+ single–doped and Dy3+/Eu3+ co–doped
SrLaMgTaO6 phosphors. It is obvious that the XRD peaks of the un–
doped phosphor (Fig. 1(a)) were identified and indexed according
to the Atom Work Database at the National Institute for Materials
Science (NIMS). By comparing the sharp and the single diffraction
peak between them, the formation of phosphor is a single phase
compound, while no secondary phase was formed. The crystalline
solid of SrLaMgTaO6 phosphor has a monoclinic structure with P21/
n symmetry, which has the cell parameters of a = 5.6407 Å,
b = 5.6425 Å, c = 7.9720, V = 253.73 Å3, and z = 2 [23]. In addition,
the Eu3+, Dy3+ single–doped and Dy3+/Eu3+ co–doped SrLaMgTaO6
phosphors have similar patterns with un–doped SrLaMgTaO6
indicating the incorporation of small amounts of Dy3+ and Eu3+
ions successfully entered SrLaMgTaO6 lattice (see Fig. 1). Moreover,
2.2. Characterization
The phase purity and crystal structure of the synthesized
samples were collected on D8 Advance X–ray diffracto–meter
(Bruker, Cu Ka irradiation, l = 1.5406 Å) operating at 40 kV and
30 mA. The diffraction patterns with a fixed scanning step of 10 /
min were recorded over 2u ranging from 10 to 70 . UV–vis diffuse
reflectance spectra (UV–DRS) were measured on a V–670 UV–vis
spectrophotometer (JASCO Corp., Japan). The photoluminescence
(PL) and photoluminescence excitation (PLE) spectra were
Fig. 1. Powder XRD patterns of SrLaMgTaO6 with different rare earth ions. (a)
SrLaMgTaO6; (b) SrLaMgTaO6:0.05Eu3+; (c) SrLaMgTaO6:0.07Dy3+; (d-i) SrLaMgTaO6:0.07Dy3+, yEu3+ (y = 0.005, 0.01, 0.03, 0.05, 0.07, 0.10).
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Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
Fig. 2. Diffuse reflection spectra of SrLaMgTaO6 with different rare earth ions. (a) SrLaMgTaO6; (b) SrLa0.95MgTaO6:0.05Eu3+; (c) SrLa0.86MgTaO6:0.07Dy3+, 0.07Eu3+. The inset
puts the determination of the band gap of SrLaMgTaO6.
the dominant diffraction peak near 32 shifted to higher angle
because of the smaller Eu3+ (1.12 Å, CN = 12) and Dy3+ (1.05 Å,
CN = 12) ions substituting for the larger La3+ (1.17 Å, CN = 12) ions.
The enlarged XRD figure was added into Fig. 1. From the
consideration of charge and ionic radius, the Dy3+ and Eu3+ ions
were expected to substitute the La3+ sites in SrLaMgTaO6 crystal
structure.
The calculated band structure of SrLaMgTaO6 is given in
Fig. 2(a). All calculations were investigated by using the DFT. It can
be found that the compound SrLaMgTaO6 has a wide direct band
gap of approximately 4.1 eV from the maximum energy of the
valence band (VB) at point G to the lowest energy of conduction
band (CB) at point G. The calculated results confirm that the
compound SrLaMgTaO6 is a suitable host matrix.
Fig. 2(b) shows the UV–vis DRS of the un–doped, Eu3+ single–
doped and Dy3+/Eu3+ co–doped SrLaMgTaO6 phosphors. These
samples show two absorption bands at 200–280 nm and 280–
350 nm, referred to herein as M and N, for easy distinguish. For the
un–doped sample (line (a)), the M band is attributed to host
absorption, namely electrons excited from the valence band to the
conduction band. The N band is originated from Ta5+–O2 charge
transfer band (CTB). For Eu3+ single–doped and Dy3+/Eu3+ co–
doped samples (line (b and c)), the intensities of M bands are
consistent with the un–doped sample, while the intensities of N
Fig. 3. The PLE and PL spectra of pure SrLaMgTaO6 monitored at 460 nm and excited
at both 250 and 378 nm at room temperature.
bands are much stronger than that of the un–doped sample due to
the CTB of Eu3+–O2. The band gap of SrLaMgTaO6 host can be
calculated by using the following equation [24,25]:
n
½F ðR1 Þhv ¼ C hv Eg
ð1Þ
Rsample
where, F ðR1 Þ is the Kubelka-Munk equation (R1 ¼
); hv
R
standard
is the energy per photon; C is a proportional constant; Eg
represents the value of the band gap; and n = 1/2 or 2 is for
indirect allowed or direct allowed electronic transitions, respectively. F ðR1 Þ can be obtained from the following function [26]:
F ð R1 Þ ¼
ð1 RÞ2 k
¼
s
2R
ð2Þ
where, R is the reflectance parameter; k is the molar absorption
coefficient of the sample; and s is the scattering coefficient. As
mentioned before, the compound SrLaMgTaO6 was calculated to be
a direct band gap material by using DFT, so we set the n value to 2.
2
According to the plot of ½FðR1 Þhv versus hv in the inset of Fig. 2(b),
the band gap Eg of SrLaMgTaO6 host is estimated to be about
2
4.62 eV when ½FðR1 Þhv = 0. The results show that the theoretical
and experimental values are close to each other.
3.2. Photoluminescence properties of un–doped, Dy3+ and Eu3+ single–
doped SrLaMgTaO6
Fig. 3 shows the PLE and PL spectra of pure SrLaMgTaO6
monitored at 460 nm and excited at both 250 nm and 378 nm at
room temperature. The PLE spectrum monitored at 460 nm
produced two absorption bands at 200–280 nm and 300–
400 nm, which are consistent with the results of the UV–vis
DRS. Under the excitation of 250 nm and 378 nm, the pure
SrLaMgTaO6 exhibits a broad blue emission band from 420 to
600 nm originating from the TaO6 group [27,28].
Fig. 4 shows a significant spectral overlap in the range of 400–
500 nm among the PLE spectra of Dy3+/Eu3+ single–doped
SrLaMgTaO6 and the PL spectra of un–doped/Dy3+ doped
SrLaMgTaO6 phosphor. According to this phenomenon, there
may be two kinds of energy transfer. One is from host to Dy3+
and Eu3+ ion, another is from Dy3+ to Eu3+ ion [29]. Recently, a few
investigations have shown that the energy transfer takes place
between Dy3+ and Eu3+ in various host materials, such as borates
[30], phosphate [31] and tungstate [32]. So it is possible to obtain
Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
Fig. 4. The spectral overlap happened between the both PLE spectra of Dy3+ and Eu3
+
single–doped SrLaMgTaO6 and the both PL spectra of un–doped and Dy3+ doped
SrLaMgTaO6 phosphor.
the white light from Dy3+ and Eu3+ co–doped SrLaMgTaO6
phosphor by varying the Dy3+/Eu3+ concentration ratio or by
adjusting the excitation light source.
3.3 Energy transfer and luminescence mechanism in
SrLa0.93-yMgTaO6:Dy3+, yEu3+ To study the energy transfer
between TaO6 group and Dy3+, as well as Eu3+ ions, we fixed the
Dy3+ concentration at 0.07 mol and changed the Eu3+ concentration
from 0 to 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10 mol in SrLaMgTaO6
host. The PLE spectra of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors monitored at 577 nm of the Dy3+ ions and at 615 nm of the
Eu3+ ions are shown in Fig. 5(a, b), respectively. As Eu3+
concentration increases, the integrated intensity of Dy3+ ions
monitored at 577 nm decreases while the integrated intensity of
Eu3+ ions monitored at 615 nm increases. The PL spectra of SrLa0.933+
3+
phosphors under the excitation of
yMgTaO6:0.07Dy , yEu
250 nm and 354 nm are displayed in Fig. 6(a, b). According to
169
Figs. 3 and 4, the 250 nm excitation source only can be absorbed by
the host. The 354 nm excitation source is the strongest characteristic absorption peak of Dy3+, but it is hardly absorbed by Eu3+ ions.
Therefore, the 250 nm and 354 nm excitation sources can be well
used to study the energy transfer in SrLa0.93-yMgTaO6:0.07Dy3+,
yEu3+ phosphors.
The PL spectra of Dy3+/Eu3+ co–doped SrLaMgTaO6 phosphors
with different molar ratios of 7:0.5, 7:1, 7:3, 7:5, 7:7, 7:10 are placed
in Fig. 6(a, b). Under the excitation of 250 nm, the integrated
emission intensity of Eu3+ ions gradually increases (Fig. 6(a)),
whereas that of TaO6 group and Dy3+ ions are both decreased
monotonically. This phenomenon means that the gradually
increasing emission intensity of Eu3+ is based on the expense of
that of the host and Dy3+ ions under 250 nm stimulated, which
directly displays the energy transfer from host to Dy3+ and Eu3+
ions. Under the excitation of 354 nm, a similar phenomenon
happened, but the TaO6 group emission intensity was unchanged
(Fig. 6(b)), indicating that the energy transfer only occurred from
Dy3+ to Eu3+. Those phenomena in Figs 5 (a,b) and 6 (a,b) clearly
indicate the existence of energy transfer from host to Dy3+ and Eu3+
ions.
In order to illustrate the luminescence behaviors of SrLa0.933+
3+
phosphors, the schematic energy transyMgTaO6:0.07Dy , yEu
fer mechanism between TaO6 group and Dy3+, as well as Eu3+ ions
in SrLaMgTaO6 host is displayed in Fig. 7. Under the excitation of
250 nm, the host absorbs the UV light and transfers the energy to
TaO6 group, Eu3+, and Dy3+ ions, respectively. Then, the energy of
Dy3+ and Eu3+ ions undergoes a non–radiative process relaxing to
the 4F9/2 and 5D0 energy levels, respectively. By calculation, the 4F9/
3+
(21.144 103 cm1) and the 5D2 energy level
2 energy level of Dy
of Eu3+ (21.499 103 cm1) are close to each other. So the energy
can be transferred from 4F9/2 to 5D2 by a phonon assisted energy
transfer process, and then the 5D2 can relax to 5D0 energy level.
Simultaneously, all the TaO6 group, Dy3+ and Eu3+ ions give their
characteristic emissions.
Under the excitation of 354 nm, the energy transfer route shows
that the energy can be absorbed by both TaO6 group and Dy3+ ions.
But the energy migration only happened between Dy3+ and Eu3+,
no other energy migration related to TaO6 group due to the TaO6
group emission intensity unchanged (Fig. 6 (b)). When excited at
354 nm, the state 6P7/2 relax to the 4F9/2, then the energy is trapped
Fig. 5. (a) The PLE spectra of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors monitored at 577 nm of the Dy3+ ions. (b) The PLE spectra of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+
phosphors monitored at 615 nm of the Eu3+ ions.
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Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
Fig. 6. (a) The PL spectra of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphor with different Eu3+ concentrations under the excitation of 250 nm. (b) The PL spectra of SrLa0.930.07Dy3+, yEu3+ (y = 0.005, 0.01, 0.03, 0.05, 0.07, 0.10) phosphor under the excitation of 354 nm.
yMgTaO6:
by Eu3+ ions and give two characteristic emissions from Dy3+ and
Eu3+ ions.
To further identify the evidence of the energy transfer
from
Dy3+
to
Eu3+,
the
fluorescence
lifetimes
of
SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005–0.10) phosphors were
measured. The monitored excitation and emission wavelengths are
marked in Fig. 8(a). All decay curves (Fig. 8(a–g)) are found
following the double exponential equation as follows [33–36]:
t
t
þ A2 exp ð3Þ
IðtÞ ¼ y0 þ A1 exp t1
t2
where, I(t) is the phosphorescent intensity at certain time t; A1 and
A2 are the fitting parameters; t 1 and t 2 are the fast and slow decay
constants for the exponential components, which can determine
the attenuation rate. The average lifetimes (t ave) are evaluated by
the equation [36]:
A t 2 þ A2 t 22
ð4Þ
t ave ¼ 1 1
ðA1 t 1 þ A2 t 2 Þ
As Eu3+ concentration increases from 0 to 0.005, 0.01, 0.03, 0.05,
0.07 and 0.10 mol, the average lifetime t ave of the Dy3+ emission
monitored at 577 nm upon the excitation of 354 nm decreases from
0.221 to 0.218, 0.211, 0.199, 0.191, 0.183 and 0.176 ms, respectively.
The fluorescent lifetime of the Dy3+ ions significantly decreases
with Eu3+ concentration increasing as shown in Fig. 8(h). This
finding provides a visual evidence of the energy transfer from Dy3+
to Eu3+ ions. The similar phenomenon of the double exponential
decay energy transfer from donor to acceptor is observed in many
investigations [33–37].
Generally, the energy transfer from a donor to an acceptor in
phosphor may happen by exchange interaction or electric
multipolar interaction. According to Dexter’s energy transfer
formula of the multipolar interaction and Reisfeld’s approximation, the relationship can be obtained from the following equation
[38]:
PSA ¼
1
t s0
1
ts
n
/ C3
ð5Þ
where, t s0 and t s represent the lifetime of Dy3+ in the absence and
the presence of Eu3+, respectively; C is the concentration of Eu3+
ions; and PSA is the energy transfer probability; Eq. (5) with n = 3, 6,
8, and 10 corresponds to the exchange, dipole–dipole (d–d),
dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q)
interaction, respectively. In this case, the relationship of In(PSA )
and In(C =3 ) plotting with the linear fitting are illustrated in Fig. 9.
The value of n deduced from Eq. (5) is 3.38, which is close to 3. This
result clearly indicates that the energy transfer mechanism
between Dy3+ and Eu3+ in SrLaMgTaO6 is exchange interaction.
As we all known, the critical distance (RC) values between the
donor and the acceptor ions are able to identify by using the
concentration quenching method, on the basis of Blasse’s theory
[39], the average distance of RDy–Eu can be calculated by:
3V 1=3
ð6Þ
RDyEu 2
4pxN
1
Fig. 7. The energy transfer scheme between TaO6 group and Dy3+, as well as Eu3+
ions in SrLaMgTaO6 host.
where, N is the number of available sites of Eu3+ occupying per unit
cell; x is the total concentration of Eu3+ and Dy3+ ions; and V is the
volume per unit cell. For SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+
phosphors, N = 2, V = 253.73 Å3, then the RDy–Eu is calculated to
be 14.8, 14.5, 13.4, 12.6, 12.0, and 11.3 Å for x = 0.075, 0.08, 0.10, 0.12,
0.14, and 0.17, respectively. The RC between Eu3+ ions can be
obtained from the critical concentration (xc). xc means that the
luminescence intensity of Dy3+ doped sample is half of that
observed in the absence of Eu3+, which is calculated to be 0.074.
Fig. 10 clearly shows the dependence of the relative Dy3+ emission
Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
171
Fig. 8. The fluorescence lifetimes of Dy3+ (lex = 354 nm, lem = 577 nm) in the SrLa0.93-yMgTaO6: 0.07Dy3+, yEu3+ phosphor: (a) y = 0.00; (b) y = 0.005; (c) y = 0.01; (d) y = 0.03;
(e) y = 0.05; (f) y = 0.07; (g) y = 0.10. (h) The relationship of Dy3+ lifetimes and Eu3+ concentration.
intensity with different Eu3+ concentrations. Accordingly, the RC for
Dy3+ and Eu3+ in SrLaMgTaO6 is obtained about 14.8 Å.
show that the SrLa0.92MgTaO6: 0.07Dy3+, 0.01Eu3+ phosphor has
excellent thermal stability.
3.3. The thermally stable luminescence
3.4. Photometric characterization
Generally, the thermal stability of phosphor is an important
technological factor for LED application, especially in high–powder
LEDs, since it has a considerable impact on the color rendering
index and light output. In order to further evaluate the thermal
stability of the phosphor, the temperature–dependent spectra of
SrLa0.92MgTaO6:0.07Dy3+, 0.01Eu3+ sample excited at 354 nm with
temperature increasing from 30 C to 210 C is investigated and
shown in Fig. 11. As temperature increases from 30 C to 210 C, the
integrated PL intensity decreases progressively. As shown in the
inset of Fig. (11), the integrated PL intensity can retain 83.34% at
150 C of that measured at room temperature (30 C). The results
Fig. 12 presents the Commission Internationale de l'’Eclairage
(CIE) 1931 chromaticity diagram for SrLa0.93-yMgTaO6:0.07Dy3+,
yEu3+ phosphors excited at 250 and 354 nm. The calculated CIE
chromaticity coordinates and the correlated color temperature
(Tcct) for all the samples are summarized in Table 1. The color
coordinates are determined by the color calculator software. The
Tcct is obtained by the McCamy empirical formula [40]:
T cct ¼ 437n3 þ 3601n2 6861n þ 5514:31ð16Þ
ð7Þ
where, n ¼ ðx xe Þ=ðy ye Þ and the chromaticity epicenter is at
xe ¼ 0:3320 andye ¼ 0:1858.
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Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
Fig. 9. The relationship of In(PSA ) and In(C =3 ) plots with the linear fitting in
SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.01, 0.03, 0.05, 0.07, 0.10) phosphors.
1
Fig. 12. CIE chromaticity diagram of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005,
0.01, 0.03, 0.05, 0.07, 0.10) phosphors excited at 250 nm (points a1 to a6) and 354 nm
(points b1 to b6).
Fig. 10. Dependence of the relative Dy3+ emission intensity in SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005, 0.01, 0.03, 0.05, 0.07, 0.10) on the concentration of Eu3+.
Usually, the calculated Tcct values under 3300 K are called warm
light; the calculated Tcct values from 3300 K to 6000 K are defined
as warm white light; and the calculated Tcct values over 6500 K are
known as cool white light. Different color temperatures have
individual applications. From the chromaticity diagram (Fig. 12),
we found that the CIE chromaticity coordinates for SrLa0.933+
3+
phosphors show the location of color
yMgTaO6:0.07Dy , yEu
coordinates (x, y) from blue to white region under 250 nm
excitation and from white to red region under 354 nm excitation.
By comparison, the color coordinate point of b2 (x = 0.359,
y = 0.335) is particularly close to the National Television Standards
Committee white (x = 0.330, y = 0.330). When excited at 354 nm,
the warm white light can be obtained at 0.005, 0.01, and 0.03 mol
Eu3+ co–doped samples, whereas the cold white light is generated
at higher Eu3+ contents. The results show that it is possible to tune
the emission color by varying the excitation wavelength or by
adjusting the Dy3+/Eu3+ molar ratios.
4. Conclusions
Fig. 11. The temperature–dependent spectra of SrLa0.92MgTaO6:0.07Dy3+, 0.01Eu3+
sample excited at 354 nm with temperature increasing from 30 C to 210 C.
In conclusion, we have developed energy transfer phosphors
SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005–0.10) and investigated
the luminescence properties as a function of Eu3+ content. The
obtained solid solutions could be effectively excited by the near–
UV region from 350 to 400 nm, revealing that these solid solution
can be well applied to the output wavelength of near UV used
phosphor–converted white LEDs. A significant spectral overlap in
the range of 400–500 nm among the PLE spectra of Dy3+/Eu3+
single–doped SrLaMgTaO6 and the PL spectra of un–doped/Dy3+
doped SrLaMgTaO6 phosphor, suggesting the presence of energy
transfer from the host to Dy3+ and Eu3+ ions. The energy transfer
mechanism has been studied by Dexter’s theory, demonstrating to
be exchange interaction. Moreover, by using the concentration
quenching method, we found that the critical distance for energy
transfer between Dy3+ and Eu3+ was 14.8 Å. The white light
emission of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005–0.10)
phosphors can be realized by methodically adjusting the dopant
concentration of activator and utilizing the energy transfer under
near UV pumped. In addition, the temperature–dependent spectra
Y. Guo et al. / Materials Research Bulletin 88 (2017) 166–173
173
Table 1
Calculated CIE chromaticity coordinates and correlated color temperature (Tcct) of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ (y = 0.005, 0.01, 0.03, 0.05, 0.07, 0.10) phosphors excited at
250 and 354 nm.
Points
Samples
a1
a2
a3
a4
a5
a6
b1
b2
b3
b4
b5
b6
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
SrLaMgTaO6:0.07Dy3+,
0.005Eu3+
0.01Eu3+
0.03Eu3+
0.05Eu3+
0.07Eu3+
0.10Eu3+
0.005Eu3+
0.01Eu3+
0.03Eu3+
0.05Eu3+
0.07Eu3+
0.10Eu3+
of SrLa0.92MgTaO6:0.07Dy3+, 0.01Eu3+ sample shows that the
phosphor has excellent thermal stability. All the results indicate
that the single–phase white–emitting SrLa0.93-yMgTaO6:0.07Dy3+,
yEu3+ phosphors have a potential application for white light LEDs.
Acknowledgment
This work was supported by Research Grant of Pukyong
National University (2016).
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