Study of the annealing temperature effect on the structural and luminescent properties of SrWO4:Eu phosphors prepared by a non-hydrolytic sol-gel process

Article abstract of DOI:10.1016/j.jallcom.2012.02.083

In this paper, we investigate the influence of temperature on the structural and luminescent properties of Eu³⁺-doped strontium tungstate oxide (SrWO₄) prepared by the non-hydrolytic sol-gel route. X-ray diffraction (XRD) analyses showed that the SrWO₄:Eu scheelite type structure was formed in a unique phase at 900 and 1000 °C over 2 h. Raman spectra indicated only one type of [WO₄] tetrahedron. The optical properties were investigated by ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements at room and liquid N ₂ temperatures. PL study showed ⁵D₀ → ⁷F₂ electric dipole transition is dominant when Eu ³⁺ occupies a non-centrosymmetric environment. The (FEG-SEM) images indicated that an increase in the annealing temperature contributed to the coalescence process which promoted the growth of aggregated particles of a polydisperse nature. A qualitative analysis of the powders obtained by dispersive X-ray detector (EDS) indicated that the samples are composed of Sr, W and O while Eu³⁺ ion was not observed due to it low concentration.

Full text of DOI:10.1016/j.jallcom.2012.02.083

Journal of Alloys and Compounds 526 (2012) 11–21
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Study of the annealing temperature effect on the structural and luminescent
properties of SrWO4:Eu phosphors prepared by a non-hydrolytic sol–gel process
P.F.S. Pereiraa, A.P. de Mouraa, I.C. Nogueiraa, M.V.S. Limaa, E. Longob, P.C. de Sousa Filhoc,
O.A. Serrac, E.J. Nassard, I.L.V. Rosaa,∗
a Universidade Federal de São Carlos/UFSCar, Via Washington Luís, Km 235, ZIP 13565-905, São Carlos, SP, Brazil
b Universidade Estadual Paulista Júlio de Mesquita Filho/UNESP, R. Francisco Degni, S/N, ZIP 14801-907, Araraquara, SP, Brazil
c Universidade de São Paulo/FFCLRP-USP, Via Bandeirantes, 3900, ZIP 14040-901, Ribeirão Preto, SP, Brazil
d Universidade de Franca/UNIFRAN, Via Dr. Armando Salles de Oliveira, 201, ZIP 14404-600, Franca, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we investigate the influence of temperature on the structural and luminescent properties
of Eu3+-doped strontium tungstate oxide (SrWO4) prepared by the non-hydrolytic sol–gel route. X-ray
diffraction (XRD) analyses showed that the SrWO4:Eu scheelite type structure was formed in a unique
phase at 900 and 1000 ◦C over 2 h. Raman spectra indicated only one type of [WO4] tetrahedron. The
optical properties were investigated by ultraviolet–visible (UV–vis) absorption spectroscopy and photo-
Received 28 October 2011
Received in revised form 18 January 2012
Accepted 13 February 2012
Available online 24 February 2012
luminescence (PL) measurements at room and liquid N2 temperatures. PL study showed 5D0
→
7F2 electric
Keywords:
dipole transition is dominant when Eu3+ occupies a non-centrosymmetric environment. The (FEG-SEM)
images indicated that an increase in the annealing temperature contributed to the coalescence process
which promoted the growth of aggregated particles of a polydisperse nature. A qualitative analysis of the
powders obtained by dispersive X-ray detector (EDS) indicated that the samples are composed of Sr, W
and O while Eu3+ ion was not observed due to it low concentration.
Non-hydrolytic sol–gel
Strontium tungstate
Scheelite
Europium
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
which facilitates their use as high frequency ultrasonic transduc-
ers [5]. Eu3+is one of the most interesting ions to study with site
Optical properties of trivalent rare earth (RE3+) ions in tungstate
materials with a scheelite structure have been widely investigated
[1]. There has been significant interest in studying the interactions
between (RE3+) ions in solids. This research is attributed in part
to the importance of rare earth doped materials in optical sys-
tem applications and also to advances in experimental techniques
which allow the investigation of the properties of the interactions
in greater detail than was previously possible [2]. The motivation
for these studies is based on: (i) their technological applications as
phosphor materials in fluorescent lamps, cathode ray tubes and
X-ray intensifying screens [1]; (ii) stimulated Raman scattering
(SRS) operations which have been thoroughly studied to develop
laser devices emitting in new spectral regions [3]; and (iii) dis-
play devices, transparent luminescence layers or markers on metal,
ceramics and plastics as well as biomaterials [4].
selection spectroscopy techniques because its optical properties
are very sensitive to its local environment. The intensities and
splitting of the spectral lines provide useful information concern-
ing the local site symmetry, sizes of cations and properties of the
chemical bonding [2]. Eu3+ is a preferable choice as an activator
7
ion with red emission via a 5D0 → F2 transition at 616 nm when
the ion is present in a non-centrosymmetric site which has been
used in most commercial red phosphors. The intensities of Eu3+
emissions at around 394 and 465 nm are improved in these mate-
rials as compared with most other Eu3+ doped phosphors [6]. Some
Eu3+ activated tungstates and molybdates with a scheelite struc-
ture have efficient red-light emission in the near-UV region [7,8].
These phosphors are relatively stable and have strong absorption
in the near-UV region; therefore, they are promising candidates as
a red component for white LED emission devices (WLEDs) [7].
However, most of these materials were prepared by a traditional
solid-state method which usually requires high temperatures, a
lengthy heating process and subsequent grinding. The grinding
process usually damages the phosphor surfaces which results in
the loss of emission intensity [6]. In addition, aggregation and
inhomogeneous shapes are also unavoidable and can inhibit the
absorption of the excitation energy and thereby reduce the emis-
sion intensity. Therefore, a simple and economical method for
Trivalent RE ions are used to dope inorganic oxides as a probe to
investigate local centers and energy promoting changes in optical
behavior or improving the capacitance response of these materials
∗
Corresponding authors at: Department of Chemistry, LIEC-UFSCar, Caixa Postal
676, 13560-905 São Carlos, SP, Brazil. Tel.: +55 16 3351 9308; fax: +55 16 3351 8214.
E-mail address: ilvrosa@ufscar.br (I.L.V. Rosa).
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.02.083

12
P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
making high-quality phoshors is desirable. The sol–gel method
has some advantages over the conventional solid-state reaction
method such as easy stoichiometric control, good homogeneity,
lower annealing temperatures and a shorter heating time; the
prepared samples show a small size and narrow particle size distri-
bution. Phosphor materials must have a narrow size distribution,
non-agglomeration properties and spherical morphology for good
luminescent characteristics [6,9–13].
d
c
In this paper, SrWO4 powders doped with Eu3+ were synthesized
by the non-hydrolytic sol–gel route; the powders were annealed
at 600, 800, 900 and 1000 ◦C for 2 h. The obtained powders were
analyzed by X-ray diffraction (XRD) and Raman spectroscopy. The
images were analyzed by FEG-SEM. A qualitative analysis of the
powders obtained was performed by using an energy dispersive X-
ray detector (EDS). Optical properties were investigated by UV–vis
absorption spectroscopy and photoluminescence (PL) measure-
ments at room and liquid N2 temperatures.
b
a
*
**
*
*
*
*
**
*
*
*
*
*
**
10
20
30
40
50
60
70
2θ (°)
Fig. 1. XRD of SrWO4:Eu samples annealed at (a) 600 ◦C, (b) 800 ◦C, (c) 900 ◦C and
2. Experimental
(d) 1000 ◦C for 2 h. JCPDS (85-0587).
2.1. Non-hydrolytic sol–gel synthesis of RE3+ doped SrWO4
3
˚
SrWO4:Eu powders were prepared by the non-hydrolytic sol–gel route [9–13].
A methanolic europium chloride solution (0.1 mol L−1) was prepared from its oxide
(Eu2O3, 99.99%, Aldrich) which was first dissolved in concentrated HCl followed by
the evaporation of excess acid and the addition of anhydrous methanol. 1.0 g of
tungsten hexachloride (WCl6, Aldrich, 99.99%) was dissolved in 200 mL of methanol
(MeOH) which was used as a solvent and oxygen donor. In a subsequent step, acety-
lacetone (Hacac, Aldrich, 99.99%) in a 1:3 molar ratio related to the WCl6 was mixed
into this solution to prevent precipitation of the tungsten species. Strontium chlo-
ride (SrCl2, Aldrich, 99.99%) was added in a 1:1 W:Sr molar ratio with the addition
of 1.0% in mol of Eu3+ in relation to the Sr2+ ions. The mixture was kept in reflux for
4 h under an argon atmosphere. After the reaction, the mixture was cooled and aged
overnight at room temperature; the solvent was then removed under vacuum. The
SrWO4:Eu powder was dried and annealed at 600, 800, 900 and 1000 ◦C for 2 h.
and a cell volume of 350.56 A (JCPDS 85-0587) [20,21]. The corre-
sponding lattice sample parameters were obtained with a general
structure analysis system (GSAS) program and are listed in Table 2.
These powder phosphors fundamentally maintain characteristics
of a scheelite structure which obviously are not affected by doped
Eu3+ ions [22]. The highest intensity peak is visible at ca. 2ꢁ ≈ 27◦
(1 1 2). Fig. 1 does not show any differences regarding scheelite
diffraction patterns which indicates that doped Eu3+ ions do not
change the lattice structure [23,24]. Due to different valence states
and the difference in the ion size between W6+ (0.042 nm) and Eu3+
(0.107 nm), Eu3+ is expected to occupy the Sr2+ (0.113 nm) site in
this phosphor [23] which is reasonable because the electronic den-
sities of Eu3+ and Sr2+ at their coordination numbers are analogous
[25] (see Fig. 4). In this structure, W6+ occupies tetrahedral sites
constructed with O2− which compose the [WO4]2− clusters. Sr2+
is eight coordinated with O2− and forms a distorted dodecahedron
for the S4 point group [3,8,26,27]; Eu3+ doping ions occupy the Sr2+
site [8]. The absence of the peaks assigned to the europium oxide
(Eu2O3) indicates that SrWO4:Eu compounds were formed (see
Fig. 1). This absence is strong verification of successful substitution
of Sr2+ by the Eu3+ ions in the tungstate framework.
2.2. Measurements
The powders were structurally characterized by XRD using
a
Rigaku-
◦
˚
DMax/2500PC (Japan) with Cu K␣ radiation (ꢀ = 1.5406 A) in the 2ꢁ range from 15
to 75◦ with a 0.02◦ s−1 increment. Raman spectra were recorded using the Horiba
Jobin Yvon model LabRAM HR 800 mm. High resolution Raman spectra were taken
with a He–Ne laser at 632.81 nm (model CCD DU420A-OE-325) operating in the
50–1000 cm−1 range. UV–vis spectra were measured using Cary 5G (USA) equipment
in the reflection mode. The powder morphologies were verified using a FEG-SEM
(Supra 35-VP, Carl Zeiss, Germany). EDS results were obtained using a Noran Sys-
tem 7 model (Noran System Six version software). PL data were obtained under
continuous Xe lamp (450 W) excitation in a SPEX Triax 550 Fluorolog 3 spectroflu-
orometer at room and liquid N2 temperatures. The detection was performed with a
Peltier-cooled SPEX Synapse CCD. The emission was collected at 90◦ from the exci-
tation beam. The Eu3+ lifetime was evaluated through the decay curve set of the
excitation emission wavelength at 394 and 616 nm, respectively.
Average crystallite sizes were estimated by Scherrer’s equation
using the full width at half maximum (FWHM) of the most intense
peak (1 1 2). As reported in the literature, Scherrer’s equation (Eq.
(1)) is described as follows:
0.9ꢀ
B cos ꢁ
3. Results and discussion
D =
(1)
where D is average crystallite sizes, ꢀ is the X-ray wavelength
(0.15406 nm), ꢁ is the Bragg angle and B is the FWHM [17,28]. Based
on this equation, average crystallite sizes for the four SrWO4:Eu
powder samples were estimated in the range from 40 to 53 nm.
3.1. XRD
Fig. 1 shows a typical XRD pattern for SrWO4:Eu powders pre-
pared by the non-hydrolytic sol–gel route which were calcined at
600, 800, 900 and 1000 ◦C for 2 h.
Diffraction peaks can be used to evaluate the long-range struc-
tural order or the periodicity of the material [14,15]. According
to Fig. 1, the samples treated at 600 and 800 ◦C for 2 h showed a
phase mixture which can be ascribed to tungsten(VI) oxychloride or
tungsten(VI) oxide (WOCl4 or WO3), respectively. Diffractograms
indicate that the formation of a pure scheelite-like SrWO4:Eu does
not occur at these temperatures [16]. For the samples treated
at 900 and 1000 ◦C, the peaks indicate that pure scheelite-like
SrWO4:Eu powders were well crystallized which suggests a long-
range ordered structure [15,17–19]. All diffraction peaks can be
3.2. Rietveld refinement analysis
The Rietveld analysis was carried out on crystalline SrWO4:Eu
samples heat treated at 900 and 1000 ◦C for 2 h (see Figs. 2 and 3).
This analysis was performed using the fullprof package and
assuming a I41/a space group for a scheelite type tetragonal struc-
ture. In this scheelite type tetragonal structure, Sr, Eu, W and O
atoms occupy 4b, 4a and 16f sites with angles (˛ = ˇ = ꢂ = 90◦),
respectively. A typical example of this analysis is shown in
Figs. 2 and 3 which depicts the experimental and calculated XRD
patterns obtained by the refinement of the SrWO4 phase doped
with Eu3+. Coefficients of the Rietveld analysis are listed in Table 1.
indexed as a pure tetragonal structure with a space group of I41/a
6
˚
˚
(no. 88, C4h ) with cell parameters of a = b = 5.416 A and c = 11.951 A

P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
13
Table 4
SrWO4:Eu - 900ºC/2h
Bond length (W O) and angles of the [WO4] clusters.
˚
Yobs
SrWO4:Eu
˛
ˇ
Bond length (A)
900 ◦C for 2 h
1000 ◦C for 2 h
104.1
107.3
120.8
114.0
1.88
1.74
Ycalc
Yobs - Y calc
Bragg peak positions
Figs. 2 and 3 present minor deviations (Yobs − Ycalc), Rwp, RBragg
and ꢃ2 (see Table 1) which are attributed to the good quality of the
structural refinement [28]. These results indicate that SrWO4:Eu
samples treated at 900 and 1000 ◦C showed the desired crystalline
phase; no phase mixture was observed which confirms results
obtained by conventional XRD (see Fig. 1).
Table 2 shows lattice parameter values, angle and cell volume
of the unit cell for SrWO4:Eu obtained by Rietveld refinement.
Table 2 shows Rietveld refinement data for SrWO4:Eu powders
heat treated at 900 and 1000 ◦C which confirm a tetragonal struc-
ture pertaining to the I41/a space group. Lattice parameters and cell
volumes of the 900 and 1000 ◦C treated samples did not indicate
significant changes. However, only a slight environmental distor-
tion was noticed which is in agreement with the refinement from
the GSAS and the JCPDS card no. 85-0587.
10
20
30
40
50
60
2θ (º)
70
80
90
100
110
Fig. 2. Rietveld plot for crystalline SrWO4:Eu heat treated at 900 ◦C for 2 h.
SrWO4:Eu - 1000ºC/2h
Table 3 lists atomic coordinates for Sr, Eu, W and O atoms
obtained by the Rietveld refinement used to model the tetragonal
SrWO4:Eu unit cells which was heat treated at 900 and 1000 ◦C for
2 h. According to these data, the environment distortion is verified
mainly in the x atomic coordinate since its value is 0.2698 for the
O atom in the sample heated at 900 ◦C while in the sample heated
at 1000 ◦C, this coordinate was determined as 0.2343.
Y obs
Y calc
Y obs - Y calc
Bragg peak positions
3.3. Unit cell representation for SrWO4:Eu
Fig. 4 illustrates the schematic representation of a SrWO4:Eu
tetragonal unit cell with a I41/a space group which was modeled
by using the Diamond program (Version 3.2 DEMO) [29,30].
The construction of a 1 × 1 × 1 unit cell of SrWO4:Eu (see Fig. 4)
employed the structural parameters and atomic coordinates listed
in Tables 2 and 3. In Fig. 4 the Sr2+ and Eu3+ cations are coordi-
nated to the eight oxygen atoms considered as [SrO8] and [EuO8]
groups which form a distorted dodecahedron geometry [18,31].
The W atoms are coordinated to four oxygens which are [WO4]
groups with a slight distortion in the tetrahedral geometry [18,32].
This behavior was verified through the different bond angle values
between oxygen atoms when the heat treatment changes from 900
to 1000 ◦C (see Table 4).
10
20
30
40
50
60
2θ (º)
70
80
90
100 110
Fig. 3. Rietveld plot for crystalline SrWO4:Eu heat treated at 1000 ◦C for 2 h.
Table 1
Coefficients of the Rietveld refinement for SrWO4:Eu samples treated at 900 and
1000 ◦C for 2 h.
SrWO4:Eu
900 ◦
Rwp (%)
RBragg (%)
ꢃ2
C
9.45
7.74
3.72
2.42
7.648
5.395
Tables 4 and 5 show angle values and bond lengths of [WO4]
and [SrO8] clusters determined from Fig. 4 by simulation (Diamond
1000 ◦C
Table 2
Lattice parameters and volume cell of the samples SrWO4:Eu heat treated at 900 and 1000 ◦C for 2 h.
3
Unit cell angle (◦)
Cell volume (A )
˚
˚
SrWO4:Eu
Lattice parameters (A)
a = b
c
˛ = ˇ = ꢂ
900 ◦
C
5.419
5.417
5.416
11.961
11.957
11.951
90
90
90
351.246
350.949
350.559
1000 ◦C
JCPDS
Table 3
Atomic coordinates used to model the tetragonal SrWO4:Eu unit cell, heat treated at 900 and 1000 ◦C for 2 h.
Atom
Site
SrWO4:E (900 ◦C)
SrWO4:E (1000 ◦C)
x
y
z
x
y
z
O
16f
4b
4b
4a
0.2698
0.1164
0.2500
0.2500
0.2500
0.0475
0.6250
0.6250
0.1250
0.2343
0.1172
0.2500
0.2500
0.2500
0.0455
0.6250
0.6250
0.1250
Sr
Eu
W
0
0
0
0
0
0

14
P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
Fig. 4. Illustrations of the SrWO4:Eu coordination geometries of the [WO4]/[SrO8]/[EuO8] clusters treated at 1000 ◦C for 2 h.
Table 5
Bond length and angles of the Sr O clusters.
˚
SrWO4:Eu
˛
ˇ
ꢂ
ı
ε
ꢄ
Á
ꢁ
Bond length (A)
900 ◦C for 2 h
1000 ◦C for 2 h
69.9
67.8
71.3
73.0
73.6
76.6
76.5
79.1
97.8
97.5
132.0
128.0
136.9
137.7
146.5
148.6
2.52; 2.51
2.60; 2.63
program) through the Rietveld refinement data. A small variation
between angle values and bond lengths groups is observed. The
results showed that the SrWO4:Eu powder has a small degree of
distortion at short range in [WO4] and [SrO8] groups. The addi-
tion of 1.0% Eu3+ to SrWO4 and the annealing temperature effect
can promote some differences in the bond length due to group
rearrangement inside the lattice matrix.
between O W O; the lower energy modes are the torsion modes
between W O and Sr O. Among them, there are three Ag vibra-
tions (922.61, 337.93 and 190.33 cm−1), four Bg (839.75, 373.77,
337.93 and 76.05 cm−1) and five Eg (800.95, 384.10, 238.40, 135.09
and 102.62 cm−1) [35]. The Raman peak at 922.61 cm−1 could be
assigned as
of the W O symmetric stretching while the peak
at 337.93 cm−11 is assigned as
of the O W O symmetric bend-
2
The crystal structure of SrWO4:Eu is characterized by a cova-
lent/ionic character between Sr O and Eu O bonds whereas the
ing. The peaks at 839.75 and 800.95 cm−1 are designated as
3
of the W O anti-symmetric stretching, and peaks at 373.77 and
W
O bonds have a covalent nature along [WO4] clusters [18,33].
384.10 cm−1 are designated as
of the O O anti-symmetric
W
4
bending [36]. The peaks located at 238.40, 190.33, 135.09 and
102.62 cm−1 as well as at 76.05 cm−1are assigned to rotational (Ag,
Eg) and translational modes (Bg) [37]. External peak modes are
localized at a range from 76.05 to 135.09 cm−1 which corresponds
3.4. Micro-Raman spectroscopy analysis
Raman-active phonon modes can be employed to estimate the
short-range structural order of the material [14]. Vibrations of
AWO4 are classified into two types (internal and external modes)
[6] which occur due to weak coupling between the ionic group
[WO4] and the Sr2+ cation [28]. The first mode belongs to the vibra-
tion inside [WO4] molecular units where the centers of mass are
stationary. The second mode is a lattice phonon which corresponds
to the motion of A2+ cations and rigid molecular units. In free space,
[WO4] tetrahedra have a Td symmetry [34,35]. Their vibrations are
Symmetric Stretching - ν1
Anti-Symmetric Stretching - ν3
composed of four internal modes (
(A1),
(E),
(F2) and
1
2
3
(F2)), one free rotational mode (
(F1)) and one translational
4
f.r
d
c
mode (F2) [34]. In a lattice space, the [WO4] symmetry is reduced to
S4, and all degenerate vibrations are split [34,35] due to the crystal
field effect [34].
Stronger Raman-active vibrational modes indicate a strong
interaction between ions which is due mainly to stretching and
bending vibrations of shorter metal–oxygen bonds within anionic
groups [21].
Fig. 5 shows Raman spectra in the frequency ranging from 50 to
1000 cm−1 for SrWO4:Eu powders processed by the non-hydrolytic
sol–gel route and treated at 600, 800, 900 and 1000 ◦C for 2 h.
These spectra show 11 different energy vibrational modes
which are higher symmetric modes and anti-symmetric stretching
272.56
715.98
b
a
100 200 300 400 500 600 700 800 900 1000
Raman Shift (cm-1)
Fig. 5. Room temperature depolarized Raman spectra of SrWO4:Eu powders pro-
cessed by the non-hydrolytic sol–gel route and treated at (a) 600 ◦C, (b) 800 ◦C, (c)
900 ◦C and (d) 1000 ◦C for 2 h.

P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
15
Fig. 6. FEG-SEM micrographs of SrWO4:Eu heat treated at (a) 600 ◦C, (c) 800 ◦C, (e) 900 ◦C and (g) 1000 ◦C, respectively, processed by non-hydrolytic sol–gel route. (b, d, f, h)
EDS of the SrWO4:Eu sample heat treated in different temperatures.
at 102.62 and 76.05 cm−1 which are external modes. Broad peaks
at 715.98 and 272.56 cm−1 are probably due to additional phases
which disappear as the annealing temperature increases and indi-
cate that [WO4] clusters contain distortions on the short-range
tetrahedron group. These results seem to be in agreement with the
XRD (see Fig. 1(a) and (b)) which shows phase mixtures. Therefore,
well defined Raman active modes indicate that SrWO4:Eu powders
processed by the non-hydrolytic sol–gel method and at annealing
temperatures of 900 and 1000 ◦C are ordered at short range (see
to the stretching and flexion mode of Sr O. Free rotation modes of
[WO4] are visible at 190.33 and 238.40 cm−1. Vibrational modes are
in accordance with analyzed Raman vibrations [35]. Spectra pro-
vide evidence for the scheelite structure for all products. According
to literature data, all Raman modes observed for SrWO4:Eu
obtained in this work are characteristics of the tetragonal structure.
The compounds treated at 600 and 800 ◦C (see Fig. 5(a) and
(b)) show some large peaks of
related to internal modes; peak characteristics of
(Eg) at 800.95 cm−1 which are
3
are visible
f.r.

16
P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
gap was determined by extrapolation of the linear portion of the
70
60
50
40
30
20
10
0
SrWO4:Eu
Linear Fit of SrWO4:Eu
curve or tail. The combination of absorbance and PL measurements
reveals energy levels of the materials and the optical band gap value
[17].
Fig. 7 shows the UV–vis spectral dependence on absorbance
for SrWO4:Eu samples, and these values are listed in Fig. 7; sam-
ples were processed by the non-hydrolytic sol–gel route and heat
treated at 600, 800, 900 and 1000 ◦C for 2 h.
These spectra verify that the optical band gap is depen-
dent on the annealing temperature and the degree of structural
order or distortion of the lattice. According to the literature
[18,20,21,28,31,39–41], the Egap is associated with intermediary
energy levels within the material band gap. Fig. 7(a–d) shows that
an increase in processing temperature leads to a reduction in the
Egap. Wood and Tauc [38] associated this decrease in the gap energy
value with intermediate localized states in the band gap due to
structural defects. This behavior indicates that the density of local-
ized states in the band gap value of the sample heat treated at
1000 ◦C is greater than the value obtained for samples treated at
other temperatures. The energy gap decreases as structural disor-
ders or distortions increase which is in accordance with Rietveld
data.
c
d
b
a
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
Fig. 7. UV–vis absorbance spectra for the SrWO4:Eu sample heat treated at (a) Egap
(600 ◦C) = 4.55 eV, (b) Egap (800 ◦C) = 4.28 eV, (c) Egap (900 ◦C) = 4.08 eV and (d) Egap
(1000 ◦C) = 3.71 eV.
Fig. 5(c) and (d)). Raman spectra for these samples did not rep-
resent any significant change which indicates that the addition of
1.0% europium was unable to modify stretching, torsion and bend-
ing vibrational modes of W O bonds since the W atom is located at
the B site of the scheelite structure. This behavior is possible only
because Eu3+ is replacing the Sr2+ located at the A site in the SrWO4
matrix.
The absorbance from sample curves of Fig. 7(a) 600 ◦C and (b)
800 ◦C reveal a smoother slope than sample curves of Fig. 7(c) 900 ◦C
and (d) 1000 ◦C which shows absorption tails below the exponential
part of the edge. Absorbance measurement for samples annealed
at 900 and 1000 ◦C suggests the absence of a uniform band gap
structure with a tail of localized states (see Fig. 7(c) and (d)) and a
well defined inter-band transition with a quasi-vertical absorption
which is probably related to crystalline effects.
3.5. FEG-SEM: morphology
Results between Egap (eV) values of SrWO4 obtained in this work
were analyzed and compared with results reported in the literature
by different methods. Santos et al. [20] obtained Egap values of 4.7
and 4.5 eV for thin films annealed at 700 and 600 ◦C for 2 h, respec-
tively. Sczancoski et al. [21] showed Egap values of 4.36, 4.49, 4.59,
4.68, 4.43 eV to powders obtained by a microwave method at 140 ◦C
at 0.5, 1, 2, 5 and 8 h, respectively. Orhan et al. [31] reported Egap
values of 5.76 and 4.61 eV for thin films annealed at 600 and 400 ◦C
for 4 h, respectively. Maurera et al. [39] and Lou and Cocivera [40]
obtained Egap values of 5.78 and 4.9 eV for thin films annealed at
400 ◦C for 4 h and 700 ◦C for 1 h, respectively. We believe that Egap
values can also be related with factors such as particle morphology,
annealing temperature, processing time, preparation method and
shape (thin films or powders) as well as doping.
Fig. 6(a), (c), (e), and (g) shows FEG-SEM micrographs and EDS
spectra ((b), (d), (f), and (h)) of SrWO4:Eu samples heat treated
at 600, 800, 900 and 1000 ◦C, respectively, processed by the non-
hydrolytic sol–gel route.
The increase in the annealing temperature contributes to the
coalescence process which promotes the growth of aggregated par-
ticles of a polydisperse nature [21,33]. According to Fig. 6(a), (c),
(e), and (g), an increase in particle sizes and shapes results in self-
assemby which originates from the characteristic morphology of
the material as plates having different particle size distributions.
A qualitative analysis of the powders obtained was performed
using EDS data (see Fig. 6(b), (d), (f), and (h)) which clearly indicates
that the samples are composed of Sr, W and O since their peaks
were observed as expected. The presence of the C peak due to the
upper lying conductive carbon tape is obvious. Samples were doped
with 1.0% of Eu3+; thus, due to its low concentration, no europium
peak was observed. The Sr signal overlaps with the W signal in
the energy scale of ca. 1.9 keV. Fig. 6(b) and (d) shows a peak of Cl
which is in agreement with the mixture phase which is ascribed
to the tungsten oxychloride (WOCl4) as verified by XRD data (see
Fig. 1(a) and (b)). Also in agreement with XRD data and Raman
spectra, Fig. 6(f) and (h) did not manifest these signals (see Fig. 1(c)
and (d)) which verifies the presence of only SrWO4:Eu crystals.
In SrWO4:Eu phosphors, the Eu3+ ion is expected to replace the
Sr2+ ion it would be difficult to keep a charge balance in the phos-
phor. When a trivalent metallic ion such as Eu3+ is incorporated into
a host lattice and substitutes for a divalent metallic ion, charge bal-
ancing is necessarily required. The equations to solve the problem
of the charge compensation are suggested below [15,42–44]:
[SrO8]x + [SrO7·V0x] → [SrO8]ꢀ + [SrO7·V0
[SrO8]x + [SrO7·V0•] → [SrO8] + [SrO7·V0
[EuO8]• + [SrO8]ꢀ → [EuO8]x + [SrO8]x
]
(3)
(4)
(5)
(6)
(7)
(8)
•
ꢀ
••
]
[WO4]x + [WO3·V0x] → [WO4]ꢀ + [WO3·V0
[WO4]x + [WO3·V0x] → [WO4]ꢀ + [WO3·V0
[EuO8]• + [WO4]ꢀ → [EuO8]x + [WO4]x
]
•
3.6. UV–vis absorption spectroscopy analyses
••
]
The optical band gap energy (Egap) values obtained from the
UV–vis spectroscopy are shown in Fig. 7. According to Wood and
Tauc [38], the optical band gap energy is related to the absorbance
intensity and photon energy:
In disordered tungstate powders, oxygen vacancies can occur in
different charge states such as [SrO7·V0x], [WO3·V0x] which capture
electrons and are neutral in relation to the lattice as well as singly
ionized [SrO7·V0•], [WO3·V0•] states and doubly positively charged
states in the lattice [SrO7·V0••], [WO3·V0••] which do not trap any
electrons. These oxygen vacancies induce the appearance of new
1/n
˛hꢅ = A(hꢅ − Egap
)
(2)
where ˛ is the absorbance, h is the Planck constant, ꢅ is the fre-
quency and Egap is the optical band gap. Therefore, the optical band

P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
λ= 436
17
254
220
230
240
250
260
270
280
Wavelength (nm)
250
275
300
325
350
375
400
425
450
475
400
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 8. Excitation spectrum of SrWO4:Eu3+ samples heat treated at (a) 600 ◦C, (b)
Fig. 9. Emission spectrum (ꢀexc .= 254 nm) of the non-doped SrWO4 sample treated
at 800 ◦C for 2 h. Insert: Excitation spectrum (ꢀem. = 436 nm) recorded at a liquid N2
temperature.
800 ◦C, (c) 900 ◦C and (d) 1000 ◦C for 2 h which monitors the 5D0
616 nm performed at a liquid nitrogen temperature of 77 K.
→
7F2 emission at
energy states in the band gap which can be attributed to tungsten
and strontium oxygen centers. Before donor excitation, a hole in
the acceptor and an electron in a donor are created, according to
the Eqs. (3)–(8) above. These equations suggest that the oxygen
vacancy trapped electron in the valence band is necessary for the
transition of a valence band hole in the conduction band. It must be
understood that this is not a band-to-band transition and that the
charge transfer depends on intrinsic structural defects in the lattice
[15,42–44].
Different mechanisms are responsible for the PL behavior in
SrWO4 such as Eu3+ disorder or distortion caused by [SrO7·V0x],
[WO3·V0x], [SrO7·V0•], [WO3·V0•] and [SrO7·V0••], [WO3·V0••] clus-
ters in disordered structures and an intrinsic distorted [WO4]2−
tetrahedral in a short-range ordered structure. In this case the con-
centration of the clusters [EuO8]• is higher than the [SrO8]x one.
luminescent centers and the crystal lattice because these couplings
mainly depend on the distance from metal ions to ligands [7].
Fig. 9 shows the emission spectrum of the non-doped SrWO4
sample treated at 800 ◦C for 2 h and monitored under ꢀexc. = 254 nm.
The insert shows the excitation spectrum of the same sample
monitored at 436 nm; both assays were performed at liquid N2 tem-
peratures. According to Fig. 9, the undoped SrWO4 sample produces
a broad blue emission under UV-light excitation. However, Fig. 10
shows that the blue emission is quenched, and narrow lines are
ascribed to Eu3+ emission spectra which are intensified due to the
introduction of this ion into the SrWO4 matrix [49].
Fig. 9 insert shows a broad and highly intense band at around
254 nm which is assigned to the CCCT of the O → W from [WO4]
units in the pure SrWO4 matrix. In comparison with doped com-
pounds (see Fig. 8), the excitation bands ascribed to the CCCT are
shifted to lower energies (∼280 nm). This behavior could be asso-
3.7. PL
ciated with the coordination of WO4 groups with Eu3+ ions.
2−
Eu3+ PL is sensitive to local lattice symmetry and can be used
as an intrinsic probe to map out changes in chemical surroundings
regarding lattice dimensions and symmetry for different kinds of
nanocrystals [45,46].
The emission spectrum of the pure SrWO4 sample excited at
254 nm (see Fig. 9) shows a broad blue emission band centered at
around 436 nm. This spectrum exhibits a similar behavior when
compared to other reported tungstates [47,50].
Fig. 8 shows excitation spectra of a Eu3+-doped SrWO4 matrix
7
which monitor the Eu3+ 5D0 → F2 emission at 616 nm at a liquid
nitrogen temperature of 77 K.
578,2
In the 310–475 nm spectral range depicted in Fig. 8, sharp lines
are visible due to 4f ↔ 4f transitions assigned to the 7F0 → LJ
5
576,5
577,7
(L = D, G, H, L where J = 0–7) of the Eu3+ ion transitions for all
samples. The main peak is assigned to the 7F0 fundamental to
the 5L6 excited state transition (electric dipole) at 393 nm. These
narrow lines are assigned to transitions from the 7F0 to the 5GJ
576
577
578
579
580
5D0
7F4
d
5
5
(365–385 nm), L6,7 (381 and 393 nm), 5H3 (317 nm) and D2–4
(360, 415 and 462 nm) levels. Spectra have a broad band in the
range of 230–320 nm which is attributed to the cluster–cluster
charge transfer (CCCT) of oxygen to tungsten and europium ions
[47]. Furthermore, it is remarkable that synthesized phosphors can
strongly absorb not only in short (∼280 nm) and lower (393 nm)
UV ranges, but also in the visible (462 nm) energy range [48]. As
the annealing temperature increases, the intensity of the f–f tran-
sitions increases for the CCCT which indicates a higher crystalline
organization of materials. In SrWO4:Eu phosphors, the CCCT from
O2− to W6+ is ascribed to electron transitions from the oxygen
2p orbital to the empty tungsten 5d orbital or to an empty Eu3+
4f orbital. They are closely related to the coupling between the
5D0
7F3
c
b
a
550
575
600
625
650
675
700
Wavelength (nm)
Fig. 10. Emission spectra of SrWO4:Eu3+ treated at (a) 600 ◦C, (b) 800 ◦C, (c) 900 ◦
C
and (d) 1000 ◦C for 2 h with excitation of 393 nm (5L6) at 77 K.

18
P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
7
7
The ratio of the (5D0 → F2)/(5D0 → F1) emission intensity pro-
vides valuable information about the symmetry of the site where
Eu3+ ions are situated. Emission spectra of powders showed a
marked increase in intensities as the heat temperature increases
during the synthesis process.
Fig. 10 shows emission spectra of SrWO4:Eu treated at (a) 600,
(b) 800, (c) 900 and (d) 1000 ◦C for 2 h with excitation at 393 nm
(5L6) recorded at 77 K.
Emission spectra are similar for all samples and correspond to
typical 4f levels specific transitions of Eu3+. The emission spec-
tra were recorded at room temperature and have essentially the
same emission profile. However, a higher resolution of the emission
bands is observed at 77 K because at this temperature the higher
vibrational levels of the lattice are almost depopulated and thus
result in a less intense vibronic coupling between matrix vibra-
tional modes and metal electronic states. In these emission spectra,
multifold components are attributed to (2J + 1) Stark levels of J-
A ratio between the integrated intensity these two transitions,
I0–2/I0–1, is used in lanthanide-based systems as a probe to evaluate
local cation surroundings [54] (see Table 6). The higher variation of
the I0–2/I0–1 ratio of the sample annealed at 600 ◦C as compared
to the other samples is an indication of the Eu3+ structural change
to a higher symmetry which generates breaks in symmetry sys-
tems and produces intermediary levels within the band gap which
causes differences in the luminescence behavior of these materi-
als. The I0–2/I0–1 ratio for samples treated at 800, 900 and 1000 ◦C
did not show variations between these values which indicates that
Eu3+ ions have lower symmetry in relation to sample heated at
600 ◦C for 2 h. The I0–0/I0–2 intensity parameter ratio between inten-
7
degeneracy. The 5D0 → F2 electric dipole transition is dominant
when Eu3+ occupies a non-centrosymmetric environment such as
in the scheelite framework [51]. Emission spectra of all samples
revealed transitions from 5D0 excited levels to ground states 7FJ
(J = 0, 1, 2, 3 and 4) of the Eu3+ at around 578, 589, 616, 652 and
699 nm, respectively. The levels with J = 0 are not degenerate, and
sities of the 5D0 → F0 and 5D0 → F2 transitions was determined
for SrWO4:Eu samples annealed at different temperatures as com-
pared to Refs. [55,56]. The I0–0/I0–2 parameter may give information
7
7
7
7
the 5D0 → F0 transition shows no more than one band; 5D0 → F1
three bands; 5D0 → F2 five bands; 5D0 → F3 seven bands and
7
7
5D0 → F4 nine bands, if the Eu3+ is located in sites having only
7
on the J-mixing effect associated with the 5D0 → F0 transition as
7
one kind of symmetry. The sample heated at 600 ◦C shows three
peaks at 576.5, 577.7 and 578.2 nm (see Fig. 10 insert) ascribed
described in Ref. [57]. This effect is mainly due to mixing between
the 7F2 manifold and the 7F0 level through two rank components
of the chemical environment around the rare earth ion. SrWO4:Eu
samples annealed at 800, 900 and 1000 ◦C had similar I0–0/I0–2
value-based complexes and polymers (these systems show a much
smaller value than the value obtained in Refs. [55,56]). However,
for SrWO4:Eu samples annealed at 600 ◦C for 2 h, a higher value
is observed than the value for SrWO4:Eu samples annealed at dif-
ferent temperatures as well as different complexes and polymers
[55,56] which indicates a much higher magnitude of the J-mixing
effect.
to 5D0 → F0 transitions which suggests more than one site of dif-
7
ferent symmetry for the chemical environment around Eu3+ ions.
Samples treated at 800, 900, and 1000 ◦C show only one peak at
578.8 nm which indicates only one kind of Eu3+ symmetry site.
Emission spectra with excitation at 462 nm (5D2) recorded at 77 K
and room temperature (not illustrated) have the same emission
profile which indicates only one kind of Eu3+ symmetry site.
In the tetragonal SrWO4 system, Eu3+ ions partially substitute
Sr2+ ions. The strongest peaks are visible at 616 nm because Eu3+
ions are located at non-centrosymmetrical sites [52]. SrWO4:Eu
powders show a predominant red emission of the characteris-
The intensity of a forced electric dipole 5D0 → F2 transition is
7
strongly sensitive to the nature of the Eu3+-ligand surroundings
and increases as the site symmetry of the Eu3+ center decreases.
The increase in the hypersensitive I0–2/I0–1 ratio was ascribed pre-
viously to an increase in both the covalence and the polarization in
the local vicinities of the Eu3+ cations in short-range effects.
tic Eu3+ 5D0 → F2 transition when excited at 393 nm. Eu3+ is
7
a good probe for the chemical environment of rare-earth ions
7
since the 5D0 → F2 transition (electric dipole) is hypersensitive to
the surrounding Eu3+environment while the 5D0 → F1 transition
7
(allowed by magnetic dipole) is insensitive to the environment. In a
site with inversion symmetry, the 5D0 → F1 transition dominates
7
3.7.1. Lifetime
7
while in a site without inversion symmetry, the 5D0 → F2 transi-
On the basis of emission spectra and lifetimes of the 5D0 emitting
level, the quantum efficiency, Á, of 5D0 for Eu3+ ions in SrWO4:Eu
nanocrystals can be determined.
According to literature publication [47], the lifetime (ꢆ), non-
radiative (Anrad) and radiative (Arad) rates are related through the
following equation: Atot = 1/ꢆ = ARAD + ANRAD, where the ARAD rate
tion dominates. The dominant emission emanates mainly from the
parity forbidden electric dipole transition rather than the magnetic
dipole transition which indicates that Eu3+ ions are located at a
non-site symmetry [8,22,53].
As shown in Fig. 10, the blue emission is quenched, and narrow
lines ascribed to the Eu3+ emission spectra related to the introduc-
tion of this ion into the SrWO4 matrix are intensified. We believed
that this behavior is due to defects in the framework which forms
the vacancy that probably contributes to the arrangement of the lat-
tice. Consequently, the emission ascribed to the matrix is quenched
by removing the emission broad band at 436 nm. Oxygen vacan-
cies can occur through the [EuO8]•, [SrO8]ꢀ and [WO4]ꢀ which are
neutralized in the lattice as shown in Eqs. (5) and (8).
was obtained by summing over the radiative rates A0J for each
ꢀ
5D0 → FJ transition is given by ARAD
=
JA0J. The emission quan-
7
tum efficiency of the emitting 5D0 level is given by:
ARAD
Á =
(9)
ARAD + ANRAD
Fig. 11 shows decay curves of the Eu3+ 5D0 → F2 transition in
SrWO4 samples heat treated at (a) 600, (b) 800, (c) 900 and (d)
1000 ◦C where excitation and emission wavelengths were fixed at
394 and 616 nm, respectively.
7
7
7
Table 6 shows the relative area of 5D0 → F0/5D0 → F2 and
5D0 → F2/5D0 → F1 transitions on SrWO4:Eu samples.
7
7
The curve (a) shows the fit of the bi-exponential function to a
sample heat treated at 600 ◦C (ꢆ1 = 0.84 and ꢆ2 = 0.05 ms, second
order) and mono exponential function to samples heat treated at
800, 900 and 1000 ◦C (ꢆ = 0.71, ꢆ = 0.64 and ꢆ = 0.65 ms), respectively
which verified that these values are in agreement with emission
spectra (see Fig. 10). Changes in the environment can cause only
a slight change in the position of electronic transition lines of rare
earth. However, different values in its luminescence lifetime have
been observed [58]. Eu3+ decay times for samples were evaluated
from these decay curves (see Table 7). Table 7 shows the lifetime
Table 6
Relative area of 5D0
→
7F0/5D0
→
7F2 and 5D0
→
7F2/5D0
→
7F1 transitions on the
SrWO4:Eu samples heat treated at 600, 800, 900 and 1000 ◦C for 2 h.
SrWO4:Eu
5D0
→
7F0/5D0
→
7F2
5D0
→
7F2/5D0
→
7F1
600 ◦C
800 ◦C
900 ◦C
1000 ◦C
0.039
5.8
0.0003
0.0006
0.0008
11.7
11.2
9.8

P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
19
Table 7
Experimental intensity parameters (˝ꢀ), emission quantum efficiencies (Á), lifetime (ꢆ), non-radiative (Anrad), radiative (Arad) and chromaticity (x and y) for the SrWO4:Eu
sample at room temperature.
ꢆ (ms)
ARAD (s−1
)
ANRAD (s−1
)
Á (%)
˝
(×10−20 cm2)
˝
(×10−20 cm2)
Chromaticity
2
4
x
y
600 ◦
C
–
–
792
777
693
–
617
785
845
–
56
50
45
–
–
0.669
0.673
0.683
0.681
0.328
0.326
0.315
0.318
800 ◦C
900 ◦C
1000 ◦
0.71
0.64
0.65
25.7
24.8
21.7
7.12
7.51
7.12
C
tendency due to temperature. A decrease in the lifetime of the
to determine the ˝ꢀ (ꢀ = 2 and 4) experimental intensity param-
7
7
5D0 → F2 transition was observed due to temperature which can
eters by taking the 5D0 → F1 transition as the reference. Thus
be explained by sample sinterization.
experimental intensity parameters (˝ꢀ) of the SrWO4:Eu systems
Quantum efficiencies for samples heat treated at 800, 900 and
1000 ◦C were evaluated as 56%, 50% and 45%, respectively. It is
clear that the quantum efficiency of samples heat treated at 800 ◦C
is higher the quantum efficiency for samples at 900 and 1000 ◦C
which seems to be consistent with the longer lifetime of the sam-
ple at 800 ◦C (ꢆ = 0.71 ms) (see Table 7). The enhanced quantum
efficiency and the increased lifetime for a sample at 800 ◦C could
be interpreted as a decrease in the non-radiative combination cen-
ters of the Eu3+ ions which strongly contributes to multiphonon
deactivation at the 5D0 level. This level is almost depopulated which
lowers the rate of vibronic coupling and results in an increase in the
Eu3+ emission luminescence lifetime. The relatively short lifetime
determined for the samples heat treated at 900 and 1000 ◦C at room
temperature (ꢆ = 0.64 and 0.65 ms), respectively, is a consequence
of the large number of high energy vibrational oscillators around
Eu3+ ions. Non-radiative combination centers can be produced by
factors such as defects and surface states where these defects may
serve as non-radiative recombination centers for decreased quan-
tum efficiency which is in accord with the Egap (see Fig. 7) that
decreases with a temperature increase.
were determined through the ratio between the intensities of the
7
7
7
5D0 → F2,4 and 5D0 → F1 transitions. The 5D0 → F1 transition is
taken as the reference due to its predominant magnetic dipole
character. The hypersensitive 5D0 → F2 transition is forbidden by
7
magnetic dipole and allowed by electric dipole mechanism.
The calculations were obtained based on Refs. [47,59], through
the equation:
ꢁ
4e2ꢇ3
3h¯ c3
2
ꢃ
ꢃ
ꢃ
ꢃ
ꢂ
ꢄ
(ꢀ)
A0−ꢀ
=
ꢃ
˝
5D0
U
7FJ
,
(10)
ꢀ
ꢀ=2,4
where ꢃ = n0(n20 + 2)2/9 is the Lorentz local field correction. The
ꢃ
ꢃ
ꢃ
ꢃ
ꢂ
ꢄ
2
square reduced matrix elements are 5D0
U
7F2 = 0.0032
(2)
ꢃ
ꢃ
ꢃ
ꢂ
ꢄ
2
and 5D0
U
7F4 = 0.0023, while the average index of refrac-
(4)
ꢃ
tion is 1.5. The A0–ꢀ values are obtained according the relation
presented at Eq. (11):
ꢅ
ꢆꢅ
ꢆ
S0−ꢀ
S0−1
ꢈꢀ
ꢈ1
A0−ꢀ = A0−1
(11)
where S0–ꢀ is the area of the band related to the 5D0 → Fꢀ transition
obtained from the spectral data, ꢈꢀ is the barycenter of the 0–ꢀ
transition and A0–ꢀ is Einstein’s coefficient for the 0–1 magnetic
7
3.7.2. Intensity parameters
The luminescence behavior of the Eu3+ ion into SrWO4 can be
investigated by radiative and non-radiative contributions to decay
dipole transition. The A0,1 value is estimated to be 50 s−1
.
rates from the emitting level such as 5D0 using the 5D0 → F2 and
7
Tables 7 and 8 shows ˝ꢀ intensity parameter values (ꢀ = 2
and 4) for SrWO4:Eu compounds. When a comparison is made
between ˝2 parameters for all compounds, the values are very
close which indicates that the Eu3+ ion is in a similar polarizable
environment and that the covalent character of the metal–donor
atom interaction is also similar for these ions. Comparing ˝ꢀ
intensity parameters values with other RE3+-complexes and other
RE3+-matrices, the chemical environment around rare earth ions
in tungstate systems is less polarizable than the Eu(TTA)3·2H2O,
Eu(TTA)3·2DBSO, polymer which suggests a smaller electric dipole
7
5D0 → F4 transitions [47]. It is important to study the influence
of the tungstate anion on the europium luminescence behavior by
determining the radiative contribution of the emitting level depop-
ulation, 5D0, and consequently for the emission quantum efficiency.
In this case, spectral data at room temperature must be consid-
7
7
ered by using 5D0 → F2 and 5D0 → F4 transitions of the Eu3+ ion
7
character for the 5D0 → F2 transition. Parra and Malta [55,56]
observed high ˝2 values (20–40 × 10−20 cm2) in epoxy resin and
rare earth complexes which reflects the hypersensitive behav-
ior of the 5D0 → F2 transition. Nevertheless, the europium ion
7
in SrWO4:Eu (see Table 7) is in a more polarizable environment
than YAG:Eu3+ samples (see Table 8) because ˝2 parameter values
are ∼25 × 10−20 cm2 and 1.64 × 10−20 cm2, respectively which indi-
cates the significant covalent character of the metal–donor atom
interaction in europium tungstate compounds. The ˝2 value which
d
c
Table 8
b
a
Experimental intensity parameters (˝ꢀ) for the Eu(TTA)3·2H2O, Eu(TTA)3·2DBSO,
polymer 10% and YAG:Eu3+ samples at room temperature.
˝
(×10−20 cm2)
˝
(×10−20 cm2)
Ref.
2
4
1
2
3
4
5
Eu(TTA)3·2H2O
Eu(TTA)3·2DBSO
Polymer
33
29
35.4
1.64
4.6
3.5
9.9
[55,56]
[56]
[55]
Time (ms)
Fig. 11. Decay curves of the Eu3+ emission (ꢀem. = 616 nm, ꢀex. = 393 nm) for SrWO4
YAG:Eu3+
5.61
[12]
samples heat treated at (a) 600 ◦C, (b) 800 ◦C, (c) 900 ◦C and (d) 1000 ◦C.

20
P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
depends on the degree of covalence experienced by the metal
(higher ˝2 values correspond to higher covalences) attests to a
highly covalence character for bonds established in the compounds
studied in this work.
values than the Eu(TTA)3·2H2O, Eu(TTA)3·2DBSO, polymer which
reflect a highly polarizable chemical environment around Eu3+ ion
in these samples.
Tables 7 and 8 show the ˝4 experimental intensity parameter
values with the highest value for the SrWO4:Eu sample which indi-
Acknowledgements
7
cates the high sensitivity behavior of the 5D0 → F4 transition. In
The authors acknowledge to FAPESP, CNPq and CAPES (Brazilian
research funding agencies) for financial support.
the tungstate system, ˝2 > ˝4 parameters suggest that the coordi-
nation geometry is higher, and the site symmetry occupied by the
Eu3+ ion in the tungstate system does not have a centrosymmetric
References
7
chemical environment character since the 5D0 → F2 transition is
[1] C.A. Kodaira, H.F. Brito, E.E.S. Teotônio, M.C.F.C. Felinto, O.L. Malta, G.E.S. Brito,
J. Braz. Chem. Soc. 15 (2004) 890.
[2] J.K. Tyminski, C.M. Lawson, R.C. Powell, J. Chem. Phys. 77 (1982) 4318.
[3] F. Cornacchia, A. Toncelli, M. Tonelli, E. Cavalli, E. Bovero, N. Magnani, J. Phys.
Condens. Matter 16 (2004) 6867.
[4] Y. Su, L. Li, G. Li, Chem. Mater. 20 (2008) 6060.
[5] T.M. Mazzo, M.L. Moreira, I.M. Pinatti, F.C. Picon, E.R. Leite, I.L.V. Rosa, J.A. Varela,
L.A. Perazolli, E. Longo, Opt. Mater. 32 (2010) 990.
[6] C. Guo, T. Chen, L. Luan, W. Zhang, D. Huang, J. Phys. Chem. Solids 69 (2008)
1905.
[7] S. Shi, J. Gao, J. Zhou, Opt. Mater. 30 (2008) 1616.
[8] S. Shi, X. Liu, J. Gao, J. Zhou, Spectrochim. Acta Part A 69 (2008) 396.
[9] E.J. Nassar, L.R. Ávila, P.F.S. Pereira, C. Mello, O.J. Lima, K.J. Ciuffi, L.D. Carlos,
Lumin. J. 111 (2005) 159.
[10] E.J. Nassar, L.R. Ávila, P.F.S. Pereira, O.J. Lima, L.A. Rocha, C. Mello, K.J. Ciuffi, L.D.
Carlos, Quim. Nova 28 (2005) 238.
formally forbidden due to the electric dipole selection rule [47,59].
Table 8 shows that YAG:Eu3+ has lower ˝2 values which indicates
a weak chemical Eu O polarization environment.
The chromaticity coordinates at room temperature for
SrWO4:Eu compounds heat treated at 600, 800, 900 and 1000 ◦C are
(x = 0.669, y = 0.328), (x = 0.673, y = 0.326), (x = 0.683, y = 0.315) and
(x = 0.681, y = 0.318), respectively (see Table 7). The sample color
coordinates are well fitted in the red region which corresponds to
the 5D0 → F2 europium ion emission. These chromaticity coordi-
7
nates are excellent for a red phosphor and indicate that they are
comparable to commercially available red phosphor coordinates
used for cathode ray tubes as Y2O2S:Eu3+; where x = 0.64, y = 0.34)
[47].
[11] P.F.S. Pereira, J.M.A. Caiut, S.J.L. Ribeiro, Y. Messaddeq, K.J. Ciuffi, L.A. Rocha, E.F.
Molina, E.J. Nassar, J. Lumin. 126 (2007) 378.
[12] E.J. Nassar, P.F.S. Pereira, E.C.O. Nassor, L.R. Ávila, K.J. Ciuffi, P.S. Calefi, J. Mater.
Sci. 42 (2007) 2244.
4. Conclusions
[13] P.F.S. Pereira, M.G. Matos, L.R. Ávila, E.C.O. Nassor, A.C. Cestari, K.J. Ciuffi, P.S.
Calefi, E.J. Nassar, J. Lumin. 130 (2010) 488.
[14] L.S. Cavalcante, J.C. Sczancoski, J.W.M. Espinosa, J.A. Varela, P.S. Pizani, E. Longo,
J. Alloys Compd. 474 (2009) 195.
Crystalline scheelite SrWO4:Eu phosphors were prepared by
using a non-hydrolytic sol–gel route; this methodology is more
efficient in the doping of Eu3+ ions than a traditional solid-state
reaction because it promotes the mixture of the starting materials
at the molecular level. Besides SrWO4:Eu was successfully pre-
pared by this method at temperatures below those mentioned in
the literature.
[15] A.B. Campos, A.Z. Simões, E. Longo, J.A. Varela, V.M. Longo, A.T. Figueiredo, F.S.
Vicente, A.C. Hernandes, Appl. Phys. Lett. 91 (2007) 051923–51931.
[16] Q. Shao, H. Li, K. Wu, Y. Dong, J. Jiang, J. Lumin. 129 (2009) 879.
[17] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.A. Varela, P.S. Pizani, E. Longo, Chem.
Eng. J. 140 (2008) 632.
[18] J.C. Sczancoski, M.D.R. Bomio, L.S. Cavalcante, M.R. Joya, P.S. Pizani, J.A. Varela,
E. Longo, M.S. Li, J.A. Andrés, J. Phys. Chem. C 113 (2009) 5812.
[19] M.A. Santos, E. Orhan, M.A.M.A. Maurera, L.G.P. Simões, A.G. Souza, P.S. Pizani,
E.R. Leite, J.A. Varela, J. Andrés, A. Beltrán, E. Longo, Phys. Rev. B 75 (2007)
165105–165111.
XRD and vibrational and optical studies showed that the struc-
ture of the synthesized SrWO4:Eu scheelite type was formed at
900 and 1000 ◦C for 2 h. Raman spectra indicated only one kind of
[WO4] tetrahedra; lattice parameters and the cell volume of sam-
ples heat treated at 900 and 1000 ◦C for 2 h verify that an increase in
the temperature decreases lattice parameters and cell volumes and
cause only a slightly environment distortion rather than significant
changes. The ionic radius of Eu3+ (0.107 nm) is slightly lower than
the ionic radius of Sr2+ (0.113 nm) which results in lower lattice
parameters. This small difference in lattice parameters is an indi-
cation that the addition of Eu3+ ion leads to a substitution in the
A site occupied by the Sr atoms. PL is a complementary technique
for XRD and Raman spectroscopy on the characterization of these
materials in a short-, medium- and long-range order. Excitation
and emission spectra, decay curves and the dependence of lumi-
nescence on the temperature were studied. In emission spectra,
the strongest emission is the electric dipole transition red emission
[20] M.A. Santos, F.C. Picon, M.T. Escote, E.R. Leite, P.S. Pizani, J.A. Varela, E. Longo,
Appl. Phys. Lett. 88 (2006) 211913–211921.
[21] J.C. Sczancoski, L.S. Cavalcante, M.R. Joya, J.W.M. Espinosa, P.S. Pizani, J.A. Varela,
E. Longo, J. Colloid Interface Sci. 330 (2009) 227.
[22] J. Liu, H. Lian, C. Shi, Opt. Mater. 29 (2007) 1591.
[23] L. Zhou, J. Wei, J. Wu, F. Gong, L. Yi, J. Huang, J. Alloys Compd. 476 (2009) 390.
[24] X. Li, Z. Yang, L. Guan, Q. Guo, Mater. Lett. 63 (2009) 1096.
[25] X. He, M. Guan, N. Lian, J. Sun, T. Shang, J. Alloys Compd. 492 (2010) 452.
[26] J.P. Sattler, J. Nemarich, Phys. Rev. B 1 (1970) 4249.
[27] K.-S. Hwang, S. Hwangbo, J.-T. Kim, Ceram. Int. 35 (2009) 2517.
[28] L.S. Cavalcante, J.C. Sczancoski, V.C. Albarici, J.M.E. Matos, J.A. Varela, E. Longo,
Mater. Sci. Eng. B 150 (2008) 18.
[29] http://www.crystalimpact.com/diamond/,. Accessed 14 February 2011.
[30] G. Bergerhoff, M. Berndt, K. Brandenburg, J. Res. Natl. Inst. Stand. Technol. 101
(1996) 221.
[31] E. Orhan, M.A. Santos, M.A.M.A. Maurera, F.M. Pontes, C.O.P. Santos, A.G. Souza,
J.A. Varela, P.S. Pizani, E. Longo, Chem. Phys. 312 (2005) 1.
[32] L.S. Cavalcante, J.C. Sczancoski, R.L. Tranquilin, M.R. Joya, P.S. Pizani, J.A. Varela,
E. Longo, J. Phys. Chem. Solids 69 (2008) 2674.
7
5D0 → F2 (616 nm) while the magnetic dipole transition orange
[33] L.S. Cavalcante, J.C. Sczancoski, L.F. Lima, J.W.M. Espinosa, P.S. Pizani, J.A. Varela,
E. Longo, Cryst. Growth Des. 9 (2009) 1002.
7
emissions 5D0 → F1,3 (589 and 652 nm) are subordinated. PL prop-
7
7
erties, the calculus of the relative area of the 5D0 → F2/5D0 → F1
transitions of the Eu3+ ion and the calculus of the emission quan-
tum efficiencies (Á) and lifetimes (ꢆ) prove that the materials were
formed because the strongest peak at 616 nm is due to Eu3+ ions
in a site without inversion symmetry. These results are in agree-
ment with XRD, refinement data, Raman spectroscopy and Egap
values. A rationale for observed changes related to UV–vis and lumi-
nescence spectra is proposed to study the nanocrystals obtained.
Luminescence data and CIE chromaticity coordinates show that the
SrWO4:Eu material is a potential red phosphor for the fabrication of
lighting devices based on near-UV or blue LED which can be excited
at around 363 and 462 nm. SrWO4:Eu compounds have higher ꢉ2
parameter values compared with the YAG:Eu3+ system but lower
[34] S. Nishigaki, S. Yano, H. Kato, T. Nonomura, J. Am. Ceram. Soc. 71 (1988) C11.
[35] A.P. de Moura, R.C. Lima, M.L. Moreira, D.P. Volanti, J.W.M. Espinosa, M.O.
Orlandi, P.S. Pizani, J.A. Varela, E. Longo, Solid State Ionics 181 (2010) 775.
[36] Z.C. Ling, H.R. Xia, D.G. Ran, F.Q. Liu, S.Q. Sun, J.D. Fan, H.J. Zhang, J.Y. Wang, L.L.
Yu, Chem. Phys. Lett. 426 (2006) 85.
[37] Y. Mao, S.S. Wong, J. Am. Chem. Soc. 126 (2004) 15245.
[38] D.L. Wood, J. Tauc, Phys. Rev. B 5 (1972) 3144.
[39] M.A.M.A. Maurera, A.G. Souza, L.E.B. Soledade, F.M. Pontes, E. Longo, E.R. Leite,
J.A. Varela, Mater. Lett. 58 (2004) 727.
[40] Z. Lou, M. Cocivera, Mater. Res. Bull. 37 (2002) 1573.
[41] S.K. Arora, B. Chudasama, Cryst. Res. Technol. 41 (2006) 1089.
[42] D.P. Volanti, I.L.V. Rosa, E.C. Paris, C.A. Paskocimas, P.S. Pizani, J.A. Varela, E.
Longo, Opt. Mater. 31 (2009) 995.
[43] A.P.A. Marques, V.M. Longo, D.M.A. de Melo, P.S. Pizani, E.R. Leite, J.A. Varela, E.
Longo, J. Solid State Chem. 181 (2008) 1249.
[44] L.S. Cavalcante, M.F.C. Gurgel, A.Z. Simões, E. Longo, J.A. Varela, M.R. Joya, P.S.
Pizani, Appl. Phys. Lett. 90 (2007) 011901–11911.

P.F.S. Pereira et al. / Journal of Alloys and Compounds 526 (2012) 11–21
21
[45] F. Zhang, M.Y. Sfeir, J.A. Misewich, S.S. Wong, Chem. Mater. 20 (2008) 5500.
[46] F. Zhang, Y. Yiu, M.C. Aronson, S.S. Wong, J. Phys. Chem. C 112 (2008) 14816.
[47] C.A. Kodaira, H.F. Brito, M.C.F.C. Felinto, J. Solid State Chem. 171 (2003) 401.
[48] M.M. Haque, H.-I. Lee, D.-K. Kim, J. Alloys Compd. 481 (2009) 792.
[49] Y. Hu, W. Zhuang, H. Ye, D. Wang, S. Zhang, X. Huang, J. Alloys Compd. 390
(2005) 226.
[54] L.-Y. Zhou, J.-S. Wei, F.-Z. Gong, J.-L. Huang, L.-H. Yi, J. Solid State Chem. 181
(2008) 1337.
[55] D.F. Parra, H.F. Brito, J.D. Matos, L.C. Dias, J. Appl. Polym. Sci. 83 (2002)
2716.
[56] O.L. Malta, H.F. Brito, J.F.S. Menezes, F.R.G. Silva, S.A. Junior, F.S.F. Junior, A.V.M.
de Andrade, J. Lumin. 75 (1997) 255.
[50] C. Zhu, S. Xiao, J. Ding, X. Yang, R. Qiang, Mater. Sci. Eng. B 150 (2008) 95.
[51] F.-B. Cao, Y.-W. Tian, Y.-J. Chen, L.-J. Xiao, Q. Wu, J. Lumin. 129 (2009) 585.
[52] J. Wang, X. Jing, C. Yan, J. Lin, F. Liao, J. Lumin. 121 (2006) 57.
[53] F. Lei, B. Yan, J. Phys. Chem. C 113 (2009) 1074.
[57] O.L. Malta, W.M. Azevedo, E.G. Araújo, G.F. Saˇı, J. Lumin. 26 (1982) 337.
[58] I.L.V. Rosa, A.P.A. Marques, M.T.S. Tanaka, D.M.A. Melo, E.R. Leite, E. Longo, J.A.
Varela, J. Fluoresc. 18 (2008) 239.
[59] C.A. Kodaira, H.F. Brito, O.L. Malta, O.A. Serra, J. Lumin. 101 (2003) 11.