Preparation and luminescence property of red-emitting hardystonite phosphors by near-ultraviolet irradiation

Article abstract of DOI:10.1111/j.1551-2916.2010.04261.x

A_1.95ZnSi₂O₇:Eu_0.05 ³⁺ (A=Ca, Sr, Ba) red phosphors were prepared by combustion-assisted synthesis method and their efficient red emission under near ultraviolet (UV) were observed. The luminescence and crystallinity were investigated using luminescence spectrometry and X-ray diffractometer, respectively. The emission spectrum shows that the most intense peak is located at 614 nm, which corresponds to the ⁵D₀→⁷F₂ transition of Eu³⁺. These phosphors have two main excitation peaks located at 394 and 465 nm, which match the emission of UV and blue light-emitting diodes, respectively. Thus, these phosphors could be used as red components for white light-emitting diodes.

Full text of DOI:10.1111/j.1551-2916.2010.04261.x

J. Am. Ceram. Soc., 94 [5] 1477–1482 (2011)
DOI: 10.1111/j.1551-2916.2010.04261.x
r 2010 The American Ceramic Society
ournal
J
Preparation and Luminescence Property of Red-Emitting Hardystonite
Phosphors by Near-Ultraviolet Irradiation
Shanshan Yao, Lihong Xue,w Youwei Yan, and Mifang Yan
State Key Laboratory of Materials Processing and Die & Mould Technology, College of Materials Science and
Engineering, Huazhong University of Science & Technology, Wuhan 430074, China
31
A1.95ZnSi2O7:Eu
0.05
(A 5 Ca, Sr, Ba) red phosphors were
order to obtain red phosphors with high color purity, it is nec-
essary to increase the relative intensity of 5D0-7F2 to 5D0-7F1
transitions. It is well known that the relative intensities of the
5D0-7F2 to 5D0-7F1 transitions strongly depend on the local
symmetry of Eu31 ions.17–19 The peak at 616 nm is due to the
forced electric dipole transition (5D0-7F2), in which transition
is associated with a relatively low local symmetry of Eu31 ions.
The better color purity might result from the more distorted
lattices and relatively lower crystal symmetry. Consequently, a
change of the crystal structure of host materials can produce a
good red phosphor.20
prepared by combustion-assisted synthesis method and their
efficient red emission under near ultraviolet (UV) were observed.
The luminescence and crystallinity were investigated using
luminescence spectrometry and X-ray diffractometer, respec-
tively. The emission spectrum shows that the most intense peak
is located at 614 nm, which corresponds to the D0-7F2 tran-
5
sition of Eu31. These phosphors have two main excitation peaks
located at 394 and 465 nm, which match the emission of UV and
blue light-emitting diodes, respectively. Thus, these phosphors
could be used as red components for white light-emitting diodes.
Recently, silicates phosphors have become important lumi-
nescent materials because of its excellent thermal and charge
stabilization. The hardystonites are represented as A2ZnSi2O7,
where A is an alkaline earth. There are only a few studies rela-
ted to their optical properties. Herein, we synthesized the
A2ZnSi2O7:Eu31 (A5 Ca, Sr, Ba) red-emitting phosphors by
the combustion-assisted synthesis method and investigated their
luminescent properties.
I. Introduction
HE white light-emitting diodes (WLEDs) have attracted con-
siderable attention for use an illuminator for solid-state
T
lighting. WLEDs show high potential for replacement of con-
ventional incandescent and fluorescent lamps, the advantages
being it’s longtime, saving energy, reliability, and safety.1
Up to now, three-band WLEDs were proposed to achieve the
combination of the blue GaN-based LED with green and red
phosphors, or the pumping of tri-color (red, green, and blue)
phosphors with ultraviolet (UV) or violet LED.2–7 Three-band
WLEDs maintain a very high color-rendering index and were
believed to offer the greatest potential for high-efficiency solid-
state lighting.8 For excellent color-rendering index, both meth-
ods need efficient red phosphors that should have the excitation
wavelength matching with the emission wavelength of the blue
LEDs (lem 5 440–470 nm) or the near UV-LEDs (lem 5 350–
420 nm). The presently used red phosphors for blue and
GaN-based LED are commercially still limited to divalent Eu
II. Experimental Procedure
A(NO3)2 (A5 Ca, Sr, Ba)(AR), Zn(NO3)2 ꢀ 6H2O(AR), Si(OC2
H5)4(AR), Eu2O3 (99.99%) were employed as the raw materials
(all these materials being analytical grade). NH2CO
NH2(AR) was added as a fuel and small quantities of H3
BO3(AR) as a flux, respectively. Eu2O3 was dissolved in
HNO3 to convert into Eu(NO3)3 solution completely. The raw
materials were weighed as the optimal nominal composition as
reported previously.21 Then a suitable volume of deionized mat-
ter and stoichiometric amount of A(NO3)2, Zn(NO3)2 ꢀ 6H2O,
NH2CONH2 and small quantities of H3BO3 were added to form
a clear solution. A stoichiometric amount of Si(OC2H5)4 dis-
solved in ethanol was added dropwise into the above solution
under vigorous stirring. Si(OH)4 was formed by the hydrolysis
of Si(OC2H5)4 as follows:
ion activated alkaline earth binary sulfides and Y2O3S:Eu31
,
respectively. However, these sulfides-based phosphors are chem-
ically unstable and the lifetimes of these materials are inade-
quate, and their luminescent intensities are very low relative to
blue and green phosphors. The luminescence intensities of these
materials are very low relative to blue and green phosphor.
Hence, the search for stable red phosphors with intense excita-
tion in the near-UV/blue spectra region is an urgent need to
increase the overall white light efficiency and lifetime.9
nSiðOC2H5Þ4 þ 4nH2O ! nSiðOHÞ4 þ 4nC2H5OH
The trivalent europium ion as a red luminescent activator in
many host materials has been studied intensively.10–15 Major
emissions are centered at 590 nm (5D0-7F1) and 616 nm
(5D0-7F2), corresponding well to orange and red color, respec-
tively. So, the relative intensities of 5D0-7F2 to 5D0-7F1 tran-
sitions in phosphors are important for their applications.16 In
The mixture solution was allowed to react at 801C for 2 h to
obtain a homogenous solution. And then the solution was in-
troduced into a muffle furnace preheated at 6001C. Within a few
minutes, the solution boiled and was ignited to produce a self-
propagating flame. The product obtained was postannealed at
10001C for 3 h.
The synthesized phosphors were ground to a powder and
passed through a 200-mesh sieve before the characterization.
The crystal phase of the synthesized powders prepared was
characterized by X-ray powder diffraction using an X0Pert
PRO (Almelo, the Netherlands) X-ray diffractometer having a
J. Ballato—contributing editor
˚
Manuscript No. 28257. Received June 29, 2010; approved October 19, 2010.
This work was financially supported by the Natural Science Foundation of China
(No.51002054), the Natural Science Foundation of Hubei Province of China (No.2008CB
252), the Fundamental Research Funds for the Central Universities (C2009Q012) and by
the Scientific Research Foundation for the Returned Overseas Chinese Scholar, State
Education Ministry.
CuKa radiation (l 5 1.5406 A) at 40 kV tube voltage and 40 mA
tube current. The XRD patterns were collected in the range of
101r2yr901. The emission spectrum was measured on a RF-
5301 fluorescence spectrophotometer (Shimadzu, Tokyo, Japan)
equipped with a xenon discharge lamp as an excitation source.
wAuthor to whom correspondence should be addressed. e-mail: xuelh@mail.hust.edu.cn
1477

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Journal of the American Ceramic Society—Yao et al.
Vol. 94, No. 5
31
Figure 2 shows the XRD patterns of A1.95ZnSi2O7:Eu
0.05
The excitation and emission slits were set to 3.0 nm. All the
above measurements were taken at room temperature.
The chromaticity coordinates were obtained according to the
Commission International de l0Eclairage (CIE) using a Spectra
Lux Software v.2.0 Beta.22
(A5 Ca, Sr, Ba) ceramic phosphors. Figures 2(a) and (b) pat-
terns obtained can be indexed according to the JCPDS files card
no.: 75-0916 and 39-0235 for Ca2ZnSi2O7 and Sr2ZnSi2O7 hav-
ing hardystonite-type structure (tetragonal) with the space
group P421m (no. 113). Figure 2(c) shows the XRD patterns
for Ba1.95ZnSi2O7:Eu , which is indexed according to JCPDS
31
0.05
file card no.: 86-0769 for Ba2CoSi2O7 having a monoclinic struc-
ture with the space group C2/c (no. 15).26 The lattice parameters
listed inTable I shows that there is good agreement between
III. Results and discussion
The tetragonal crystal structure of A2BSi2O7 was first reported by
Kimata, where A 5 Ca and B 5Co.23 The compositions Sr2Mg
Si2O7 , Ca2ZnSi2O7, Sr2ZnSi2O7, etc., belonging to the A2BSi2O7
series have a tetragonal crystal structure.24,25 Some of the other
silicates in A2BSi2O7 series like Ba2ZnSi2O7, Ba2MgSi2O7,
and Ba2CuSi2O7 crystallize in the monoclinic structure.26–28 The
crystal structure of the tetragonal form of A2ZnSi2O7 (A 5Ca,
Sr) is shown in Figs. 1(a) and (b). It contains discrete [Si2O7]6ꢁ
units formed by two corner-sharing SiO4 tetrahedrals. The Zn
atoms lie in the tetrahedral formed by oxygen atoms while the
A21 (A 5Ca, Sr) cations have an eightfold oxygen coordination.
The crystal structure of the monoclinic form of Ba2ZnSi2O7
is shown in Fig. 1(c). It consists of [ZnO4] tetrahedral, which
share their oxygen atoms with those of the [Si2O7] groups, thus
forming two-dimensionally infinite sheets that are separated by
the Ba atoms.
the literature and the prepared A2ZnSi2O7 and A1.95ZnSi2O7:
31
Eu
0.05
(A5 Ca, Sr, Ba) samples values, suggesting that the
method starting from the silicates is successfully applied here.
Meanwhile, it is clear that the Eu21 doping ions do not change
the general structure.
An acceptable percentage difference in ion radii between doped
and substituted ions must not exceed 30%.29 The calculations of
the radius percentage difference (Dr) between the doped Eu31
ions and the possible substituted ions (A21 [A 5 Ca, Sr, Ba],
Zn21) in A2ZnSi2O7 (A 5 Ca, Sr, Ba) are summarized in Table II.
The values are based on the following formula30:
½RmðCNÞ ꢁ RdðCNÞꢂ
Dr ¼
(1)
RmðCNÞ
Fig. 1. The crystal structure of (a, b) the tetragonal form of A2ZnSi2O7 (A5 Ca, Sr) and (c) the monoclinic form of Ba2ZnSi2O7.

May 2011
Luminescent Properties of A2ZnSi2O7: Eu3+(A5 Ca, Sr, Ba)
1479
31
31
Fig. 2. X-ray diffraction patterns of (a) Ca1.95ZnSi2O7:Eu , (b) Sr1.95ZnSi2O7:Eu
0.05
31
and (c) Ba1.95ZnSi2O7:Eu
0.05
ceramic phosphors.
0.05
where CN is the coordination number, Rm(CN) is the radius of
host cation, and Rd(CN) is the radius of doped ion. We take the
data of Eu31 with CN 56 as a responsible approximation.32 The
values of Dr between Eu31 occupied alkaline earth metal ions
(A 5Ca, Sr, Ba) sites are 4.46%, 15.08% and 24.65%, respec-
tively, while Dr between Eu31 and Zn21 (or Si41) is ꢁ58.33%
(or ꢁ265.38%). Obviously, the doping ions of Eu31 will clearly
substitute the alkaline earth metal ions (A 5Ca, Sr, Ba) sites.
As trivalent Eu31 ions are doped into A2ZnSi2O7 (A5 Ca,
Sr, Ba), they would nonequivalently replace the alkaline earths
ions. In order to keep the charge balance, two Eu31 ions would
be needed to substitute for three alkaline earths ions (the total
charge of two trivalent Eu31 ions is equal to that of three alka-
line earths ions). Hence, one vacancy defect of VA00 (A5 Ca, Sr,
Ba) with two negative charges, and two positive defects of EuAꢃ
(A5 Ca, Sr, BA) would be created by each substitution
of every two Eu31 ions in the compound. The vacancy VA00
would act as a donor of electrons, while the two EuꢃA defects
become acceptors of the electrons. Consequently, the negative
charges in the vacancy defects of VA00 would be transferred to
the Eu31 sites. The whole process can be expressed by the
following equation33:
3A2þðA ¼ Ca; Sr; BaÞ þ 2Eu3þ ! VA00 þ 2EuꢃA
VA00 ! Vx þ 2e
A
2EuꢃA þ 2e ! 2EuxA
31
The excitation spectra of A1.95ZnSi2O7:Eu
0.05
are measured in
the wavelength range of 300–500 nm by monitoring with the in-
tense red emission located at 614 nm (Fig. 3). The excitation
spectra consist of two intense bands at 394 and 465 nm in ad-
dition to four relatively weak bands peaking about 320, 362, 383,
and 416 nm. The bands peaking around 320, 362, 394 and 465
nm are assigned to transition from the 7F0 level to the 3H4, 5D4,
5L6, and 5D2 levels of f–f transitions of Eu31, respectively. On the
other hand, rest of the bands peaking around 383 and 416 nm are
assigned to the transitions from the thermal populated 7F1 level
to the 5F4 and 5L6.34,35 The strong broad band peaking at 394 nm
and the narrow band at 465 nm correspond to the characteristic
Table II. Ionic Radii Difference Percentage (Dr) Between
Matrix Cations and Doped Ions
31
Table I. Lattice Parameters Values of A2ZnSi2O7:Eu
0.05
(A5 Ca, Sr, Ba) Phosphors Calculated from the XRD Pattern
Dr 5 [Rm(CN)–Rd (CN)]/Rm (CN) (%)
3
V (A )
˚
a (A)
˚
b (A)
˚
c (A)
˚
Samples
21
21
21
41
R (4) 5
Si
˚
0.26 (A)
Doped
ions
Rd
R2C1a(8) 5
R
Sr
(8) 5
˚
R
(8) 5
˚
R
Zn
(4) 5
˚
Ba
31
˚
(CN) (A) 1.12 (A) 1.26 (A) 1.42 (A) 0.60 (A)
˚
Ca0.95Zn2Si2O7:Eu
JCPDS (75-0916)
Sr0.95Zn2Si2O7:Eu
JCPDS (39-0235)
Ba0.95Zn2Si2O7:Eu
JCPDS (86-0769)
7.649
7.830
7.763
8.002
8.338
8.434
7.649
7.830
7.763
8.002
10.560
10.722
5.078
4.990
5.130
5.170
8.338
8.436
297.10
305.93
309.16
331.10
696.33
710.75
0.05
Eu31
0.95 (6)
1.07 (8) 4.46
ꢁ58.33 ꢁ265.38
31
0.05
15.08 24.65
31
0.05
CN stands for coordination number, Rm (CN) and Rd (CN) for the radii of
matrix and doped cations, respectively, and the data of the effective ionic radii are
from Liebau.31

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Journal of the American Ceramic Society—Yao et al.
Vol. 94, No. 5
31
Fig. 3. Excitation intensity (lem 5 614 nm) of A1.95ZnSi2O7:Eu
(A5Ca, Sr, Ba) phosphors.
0.05
4
f–f transitions of Eu31 within its f6 configuration. From above
observations, the codopant Eu31 ions are substituted for the
alkaline earth metal ions (A 5 Ca, Sr, Ba) sites.
excitation peaks indicate that these phosphors are very suitable
as a color converter using UV lights as the primary light source.
It can be used as red phosphors excited by UV-LED chip and
would have applications in the solid-state lighting field.
Figure 4 shows the emission spectral of as-synthesized Eu31
-
doped A2ZnSi2O7 phosphors. The spectrum exhibits five emis-
sion peaks at 579, 591, 614, 652, and 703 nm. These five emission
peaks can be attributed to the 5D0-7F0, 1, 2, 3, 4 transitions of
The main highest emission peak is located at around 614 nm
for all samples without a visible shift, caused by 5D0-7F2,
which is a hypersensitive forced electric dipole transition.39 It is
known that the emission wavelengths of Eu31 are determined
primarily by their local environment in host crystallites. The in-
tensity ratio of 5D0-7F2 to 5D0-7F1, also called the asymmet-
Eu31 36,37
All of them consist of two main peaks centered at 591
.
and 614 nm, which come from the transitions of 5D0-7F1 and
5D0-7F2, respectively. The most intense emission is the
5D0-7F2 transition located in 614 nm, corresponding to the
red emission, in good accordance with the Judd–Ofelt theory.38
Therefore, strong red emission can be observed. The main
ric ratio, is close to being related to the local environment of
.
5
Eu31 40
Generally, the larger the intensity ratio of D0-7F2 to
5D0-7F1, the lower the local symmetry.41 The asymmetric
31
Fig. 4. Emission intensity (lex 5 394 nm) of A1.95ZnSi2O7:Eu
(A5 Ca, Sr, Ba) phosphors.
0.05

May 2011
Luminescent Properties of A2ZnSi2O7: Eu3+(A5 Ca, Sr, Ba)
1481
Table III. Asymmetric Ratios of I [5D0-7F2]/I [5D0-7F1]
The results indicate that these hardystonite phosphors are very
suitable for color converter using UV lights as the primary light
source.
31
of A1.95ZnSi2O7:Eu
0.05
(A5 Sr, Ca, Ba)
Ã
Samples
Asymmetric ratio I [5D0-7F2]/I [5D0-7F1]
31
Sr1.95ZnSi2O7:Eu
0.05
870/808 5 1.08
906/815 5 1.11
964/820 5 1.18
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31
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31
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Fig. 5. CIE diagram of (a) Ca1.95ZnSi2O7:Eu , (b) Sr1.95ZnSi2O7:
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31
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Eu301.05, and (c) Ba1.95ZnSi2O7:Eu
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