Photoluminescence in the CaxSr1-xWO4 system at room temperature

Article abstract of DOI:10.1016/j.jssc.2008.04.015

In this work, a study was undertaken about the structural and photoluminescent properties, at room temperature, of powder samples from the Ca_xSr_1-xWO₄ (x=0-1.0) system, synthesized by a soft chemical method and heat treated between 400 and 700 °C. The material was characterized using Infrared, UV-vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the sample calcined at 600 °C, which is neither highly disordered (400-500 °C), nor completely ordered (700 °C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not observed for pure CaWO₄ or SrWO₄, but for Ca_0.6Sr_0.4WO₄. The PL emission spectra could be separated into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO₄ and SrWO₄.

Full text of DOI:10.1016/j.jssc.2008.04.015

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 1876– 1881
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Photoluminescence in the Ca Sr WO system at room temperature
x
1ꢀx
4
a
b
c
c
d
e
a,f
S.L. P oˆ rto , E. Longo , P.S. Pizani , T.M. Boschi , L.G.P. Sim o˜ es , S.J.G. Lima , J.M. Ferreira ,
a
a
a
a
g
L.E.B. Soledade , J.W.M. Espinoza , M.R. C a´ ssia-Santos , M.A.M.A. Maurera , C.A. Paskocimas ,
I.M.G. Santos , A.G. Souza
a,
Ã
a
a
Laborat o´ rio de Combust ´ı veis e Materiais (LACOM/DQ/CCEN), Universidade Federal da Para ´ı ba, Campus I, Cidade Universit a´ ria, Jo ˜a o Pessoa, PB, CEP 58059-900, Brazil
CMDMC/LIEC, Instituto de Qu ´ı mica, UNESP-Araraquara, Rua Prof. Francisco Degni s/n, Araraquara, SP, CEP 14800-900, Brazil
b
c
Departamento de F ´ı sica, Universidade Federal de S ˜a o Carlos, S ˜a o Carlos, Rodovia Washington Luiz km 235, SP, CEP 13565-905, Brazil
d
Centro Multidisciplinar de Desenvolvimento de Materiais Cer aˆ micos (LIEC/DQ), Universidade Federal de S ˜a o Carlos, Rodovia Washington Luiz km 235, S ˜a o Carlos, SP,
CEP 13565-905, Brazil
e
Laborat o´ rio de Solidifica c- ˜a o R a´ pida, Departamento de Tecnologia Mec aˆ nica (LSR/DTM/CT), Universidade Federal da Para ´ı ba, Campus I, Cidade Universit a´ ria, Jo ˜a o Pessoa, PB,
CEP 58059-900, Brazil
f
´
COAMA, Area de Meio Ambiente, Centro Federal de Educa c- ˜a o Tecnol o´ gica da Para ´ı ba, Av. 11 de Maio 720, Jaguaribe, Jo ˜a o Pessoa, PB, CEP 58015-430, Brazil
g
Departamento de Engenharia Mec aˆ nica, Universidade Federal do Rio Grande do Norte, Natal, RN, CEP 59072-970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a study was undertaken about the structural and photoluminescent properties, at room
Received 21 December 2007
Received in revised form
temperature, of powder samples from the Ca Sr WO4 (x ¼ 0–1.0) system, synthesized by a soft
x
1ꢀx
chemical method and heat treated between 400 and 700 1C. The material was characterized using
Infrared, UV–vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the
sample calcined at 600 1C, which is neither highly disordered (400–500 1C), nor completely ordered
13 March 2008
Accepted 10 April 2008
Available online 30 April 2008
(
700 1C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not
observed for pure CaWO or SrWO , but for Ca0.6Sr0.4WO . The PL emission spectra could be separated
into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher
wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO and
SrWO
Keywords:
4
4
4
Tungstates
Scheelite
4
Photoluminescence
Disorder
4
.
&
2008 Elsevier Inc. All rights reserved.
1. Introduction
majority of the crystalline materials are only photoluminescent at
cryogenic temperatures.
Photoluminescence research has been focused on crystalline
materials, either pure or doped, mainly with rare earth ions such
as Eu , Er , Nd , due to the potential in optical-electronic
applications [1].
4
CaWO (CWO) is well known for its strong visible lumines-
cence. It has been used for 75 years in X-ray photography
as intensifying screen. This is due to its capability to absorb
X-rays and convert their energy into radiation, which is able
3+
3+
3+
Nevertheless, photoluminescence (PL) was recently observed
to blacken the photographic film [11]. Conversely, SrWO
4
in disordered perovskite materials in systems such as SrTiO
CaTiO [4], BaTiO [5], (Ba,Sr)TiO [6], PbTiO [7], LiNbTiO
ZrTiO [9,10], which displayed intense PL emission at room
3
[2,3],
(SWO) does not display room temperature PL. Nowadays the
challenge is to use these tungstates as solid-state optoelectronic
devices as lasers, optical fibers components or scintillators
[12–15].
3
3
3
3
3
[8],
4
temperature, at the visible region. Such discovery raises a great
technological interest, once the crystalline systems need high
temperatures for preparation, whereas the disordered systems
can be obtained at lower temperatures (below 500 1C), thus
reducing the processing cost. Therefore, oxides with low degree of
organization are candidates to replace crystalline oxides in a
series of optical-electronic applications. Moreover, disordered
materials are photoluminescent at room temperature, while the
Cho and Yoshimura [16] investigated the structural evolution
and the PL properties, at 77 K, of Sr1ꢀxCa
heat treated from 400 to 1000 1C. Cho et al. [17] also investigated
the PL of highly crystalline CaWO , films at room temperature and
x
WO
4
(0pxp1) films,
4
at 77 K. These films presented a blue emission at 456 nm (25 1C)
and at 465 nm (77 K), with the excitation lines of 254 nm (25 1C)
and 251 nm (77 K). The blue emission was ascribed to the WO
3
group defects. Blasse and Schipper [18] studied the PL of barium
tungstate and strontium tungstate, both synthesized by solid-
state reaction, calcined between 850 and 1100 1C.
Ã
Ciaco et al. [1] studied the room temperature PL of amorphous
calcium tungstate. Their theoretical calculations results indicate
Corresponding author. Fax: +55 83 32167441.
E-mail address: ieda@quimica.ufpb.br (I.M.G. Santos).
0
022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.04.015

ARTICLE IN PRESS
S.L. P oˆ rto et al. / Journal of Solid State Chemistry 181 (2008) 1876–1881
1877
that the formation of threefold coordination in the amorphous
3. Results
The photoluminescence results of the system Ca Sr WO are
system may introduce localized electronic levels in the HOMO and
the LUMO, which are related to the formation of the tail observed
in the UV–vis spectra.
Orhan et al. [19,20] carried out a joint experimental and
theoretical study of the photoluminescence properties of dis-
x
1ꢀx
4
presented in Fig. 1. These results were evaluated as a function of
the heat treatment temperature and the Ca/Sr molar ratio.
In relation to temperature, it can be clearly noticed that the
maximum PL intensity was obtained for the sample heat treated
at 600 1C. A significantly lower PL was seen for the sample heat
treated at 500 1C, whereas the heat treatment temperatures of 400
and 700 1C yielded small PL intensities. It may be observed that
the sample Ca0.6Sr0.4WO4 presents the highest PL intensity,
indicating that the use of two different modifiers leads to an
increase in the PL intensity. According to XRF results, the real
composition of this sample is Ca0.6Sr0.45W0.95O4.
4 4
ordered CaWO (CWO) thin films and SrWO powders. First
principles quantum mechanical calculations were employed to
study the electronic structure of a crystalline (CWO-c) and an
asymmetric (CWO-a) periodic models. The authors related that
the symmetry breaking process, on going from CWO-c to CWO-a,
creates localized electronic levels above the valence band and that
a negative charge transfer process takes place from threefold,
[
3 4
WO ], to fourfold, [WO ], tungsten coordinations. As a conse-
quence, polarons are formed in the structure, due to the presence
of a disorder–order system. In highly disordered or highly ordered
systems, strong polarons are not observed and the photolumines-
cence vanishes [21].
In order to identify the reasons for this behavior, the samples
were characterized by Raman and UV–vis spectroscopy (Figs. 2
and 3), X-ray diffraction (Figs. 4 and 5) and FEG-SEM (Fig. 6).
In this work, a study was undertaken about the structural and
photoluminescent properties of powder samples from the
Ca
x
Sr1ꢀxWO
4
(x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) system, synthesized by
the polymeric precursor method and heat treated from 400 to
7
00 1C.
2
. Experimental
The polymeric precursor method [22–26] was used to
synthesize Ca
x
Sr1ꢀxWO
4
(x ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0) powders.
Tungsten citrate was formed by dissolution of tungstic acid
(
(
H
2
WO
Cargil—purity 99%), under constant agitation. Strontium carbo-
) (Alfa AESAR—purity 99.99%) and/or calcium carbo-
) (Reagen purity 99.66%) were slowly added to the
solution. After homogenization, ethylene glycol (Synth—purity
9.5%) was added to promote the citrate polymerization by the
4
) (Vetec—purity 97%) in an aqueous solution of citric acid
nate (SrCO
nate (CaCO
3
3
9
polyesterification reaction. The molar ratio between citric acid
and the sum of metals was 3:1. The citric acid/ethylene glycol
mass ratio was fixed as 60:40.
After the primary heat treatment at 300 1C, the powders were
primarily heated at 300 1C for 24 h and then at temperatures
ranging from 400 to 700 1C for 4 h. The final composition was
evaluated by X-ray fluorescence (XRF 1800—Shimadzu). The
samples were structurally characterized, using XRD and Raman
spectroscopy. The morphology was evaluated by FEG-SEM.
The diffraction patterns were recorded in a Siemens D-5000
diffractometer, using the FeKa radiation source. The Raman
spectra were taken with a Bruker model RFS/100/S FT-Raman
spectrometer, using the 1064 nm line of a Nd:YAG laser, with a
6
0 mW power.
Field-emission scanning electron microscopy (FEG-SEM) (FEI-
Sirion), using a lens secondary electron detector, was used to
observe the microstructure of the powders.
The spectral dependence of optical absorbance for the
Ca
x
Sr1ꢀxWO
4
powders, heat treated at different temperatures,
was measured in the wavelength range from 200 to 800 nm, using
a Cary 5G spectrophotometer.
The PL spectra of the Ca
x
4
Sr1ꢀxWO powder samples were taken
with a U1000 Jobin-Yvon double monochromator coupled to a
cooled GaAs photomultiplier and a conventional photon counting
system. The 488.0 nm exciting wavelength of an argon ion
laser was used, with the output power of 60 mW. A cylindrical
lens was used to prevent the sample from overheating. The slit
width was 100 mm. All the measurements were taken at room
temperature.
Fig. 1. (a) PL spectra of the Ca0.6Sr0.4WO
4
samples heat treated at different
temperatures. The inset shows the magnification of the PL spectra, with emphasis
in the PL emission after calcination at 400, 500 and 700 1C; (b) PL spectra of the
Ca
—Ca0.2Sr0.8WO
6—CaWO
x
Sr1ꢀxWO
4
samples (x ¼ 0–1) heat treated at 400 1C for 4 h. Legend: 1—SrWO
4
,
,
2
4
,
3—Ca0.4Sr0.6WO 4—Ca0.6Sr0.4WO 5—Ca0.8Sr0.2WO
4
,
4
,
4
4
.

ARTICLE IN PRESS
1878
S.L. P oˆ rto et al. / Journal of Solid State Chemistry 181 (2008) 1876–1881
Fig. 2. Raman spectra of the Ca0.6Sr0.4WO
4
sample, as a function of temperature.
In Fig. 2, the Raman spectra of the Ca0.6Sr0.4WO
treated at 500, 600 and 700 1C for 4 h are showed. No vibration
mode is noticed at 500 1C. At 600 1C, the peaks at 332 n (A +B ),
) are the first ones to appear
groups [27–30]. These peaks indicate
the onset of short range order, in the network former region. The
4
samples heat
2
g
g
ꢀ
1
7
99 for n
3 g 1 g
(E ) and 919 cm for n (A
and are related to the WO
4
samples heat treated at 700 1C display vibration modes at 83 (B
07 (E ), 135 (E ), 200 (A ), 268 (E ), 332 (A +B ), 389 (E ), 806 (E
) and 912 cm
g
),
),
1
8
7
g
g
g
g
g
g
g
g
ꢀ
1
37 (B
g
ꢀ
(A
g
). The additional vibration mode at
1
11 cm
could not be identified, as it was not cited in the
literature. These experimental values are in agreement with the
values reported by Pontes et al. [27], Basiev et al. [28], Desgreniers
et al. [29] and Hardcastle et al. [30].
These results are confirmed by the UV–vis spectra, which show
the decrease of the Urbach tail with the temperature increase
(
Fig. 3a). It can be noted that from 400 to 600 1C, the absorbance
increases sharply, followed by a slight decrease from 600 to
00 1C. The samples heat treated at 500 and 600 1C showed a
7
spectral dependence on absorbance similar to that found in
amorphous semiconductors, such as amorphous silicon (Si) and
insulators, with the presence of exponential optical edges and
tails. The nature of these exponential optical edges and tails may
be associated with localized defect states promoted by the
disordered structure. It can be observed that the absorbance
Fig. 3. (a) Absorbance of Ca0.6Sr0.4WO
00 1C; (b) influence of composition and heat treatment temperature on the
optical bandgaps of the samples.
4
powders heat treated at 500, 600 and
7
spectra indicate that the sample heat treated at
a lower
temperature displays a more disordered structure. On the other
hand, the sample heat treated at 700 1C showed an interband
transition, typical of crystalline materials. The sample heat treated
at 400 1C presented high absorbance values throughout the whole
photon energy range, due to the presence of a high amount of
carbonaceous materials.
As a consequence of the short range disorder, an increase in the
bandgap values [31] with temperature occurs (Fig. 3b). In relation
to the composition, it can be observed that, for all the heat
treatment temperatures, the bandgap values reach a minimum for
In the XRD patterns of the samples calcined at 500 1C, the
presence of a crystalline phase was clearly observed, identified by
the presence of several broad peaks. With the temperature
increase, a better definition of the diffraction peaks is observed.
Peaks were indexed to the tetragonal scheelite unit cell (space
1 4
group I4 /a), according to the JCPDS 41-1431 file, for CaWO and
JCPDS 8-490 for SrWO
4
.
According to Fig. 4b, a shift in the XRD peak is observed as
calcium substitutes strontium in the lattice. This behavior is due
to the change in the lattice parameter. It should be observed that
the profiles of the XRD patterns are the same suggesting the
formation of a substitutional solid solution, without a change in
the crystalline structure, with XRD patterns typical of a single
phase system.
the composition Ca0.6Sr0.4WO
4
.
Fig. 4a shows the XRD patterns of the Ca0.6Sr0.4WO
4
powders
after calcination at several temperatures, while Fig. 4b shows the
XRD patterns of samples after calcination at 700 1C. A similar
behavior was observed for all the different compositions investi-
gated. A diffuse XRD pattern is observed at 400 1C, indicating the
formation of an inorganic disordered phase. The increase of
temperature favors the long range order, eliminating structural
defects.
For pure CaWO
4 4
and SrWO , the experimental values of the
parameters a and c, as well as the unit cell volume (Fig. 5), were
3
˚
smaller than the values of the respective JCPDS files (312.63 A for

ARTICLE IN PRESS
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1879
Fig. 5. Deviation from the Vegard Law observed for the variation of the unit cell
volume, as a function of calcium mole fraction (x), for the samples calcined at
600 1C.
4
Fig. 6. FEG-SEM micrograph of CaWO calcined at 500 1C.
Table 1
Values of FWHM for XRD patterns and Raman spectra, for samples calcined at
00 1C
6
Fig. 4. (a) XRD patterns of the sample Ca0.6Sr0.4WO
4
calcined at different
Sample
FWHM
Relative crystallinity (%)
x 4
temperatures; (b) XRD patterns of the Ca Sr1ꢀxWO system (x ¼ 0–1) heat treated
at 700 1C.
ꢀ1
)
XRD (deg)
Raman (cm
3
SrWO
Ca0.2Sr0.8
4
0.2870.01
0.3570.01
0.3570.01
0.5270.01
0.4870.01
0.3870.01
10.470.1
11.970.1
12.870.6
15.470.5
13.270.2
13.170.4
100.070.2
15.970.3
8.470.2
˚
CaWO
4
and 350.66 A for SrWO
4
), indicating that a lower amount
O
4
of defects may be present.
The XRD results were used to calculate the relative crystallinity
Table 1) [22], which is much smaller in the range of x varying
Ca0.4Sr0.86
Ca0.6Sr0.4
O
4
O
4
4
5.370.1
Ca0.8Sr0.2
CaWO
O
15.270.3
30.670.6
(
4
from 0.2 to 0.8, than for pure CaWO
4
4
and SrWO . This suggests
that crystallization of Ca Sr1ꢀxWO
x
4
system is more difficult
2+
2+
when Ca and Sr are simultaneously present in the lattice.
The analysis of the unit cell volume points out that the sample
with x ¼ 0.6 presents the highest deviation from the Vegard Law
indicate that samples with the concomitant presence of two
modifiers present a higher amount of disordered material and
their crystals present a higher disorder at short and long ranges.
The FEG-SEM micrograph (Fig. 6) shows that samples present a
high degree of sintering, even after calcination at 500 1C. This
result is related to the high amount of carbon which is not
eliminated, even after calcination at 300 1C for 24 h. This organic
material is only eliminated by combustion reaction at higher
(
Fig. 5).
Values of full width at half maximum (FWHM) were calculated
for the (112) peak of the XRD patterns and for the n (A ) peak of
the Raman spectra (Table 1). Both results indicate that the
samples with x ¼ 0.6 and 0.8 present the highest disorder,
moreover after heat treatment at 600 and 700 1C. These results
1
g

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S.L. P oˆ rto et al. / Journal of Solid State Chemistry 181 (2008) 1876–1881
temperatures, which is highly exothermic, leading to a high
amount of energy release. As a consequence, a local increase of
temperature occurs, and the particles are sintered.
4
crystallization is reached, only WO clusters exist and the PL
drastically decreases, showing that a complete order is not
suitable for a good PL emission, similarly to what was previously
observed for the case of titanates [6,7,10,36–38] and tungstates
[1,19–22]. The best PL emission is obtained for the structure
(600 1C) that is neither highly disordered (400 and 500 1C) nor
highly ordered (700 1C). Calcinations at higher temperatures
would lead to a further increase in the short and long range
order, and photoluminescence emission would not occur.
In FEG-SEM micrographs, crystallites may also be observed,
displaying an average size of about 12 nm. This result is in
agreement with the crystallite size calculated by the XRD
patterns, using the Scherrer equation. For calcium tungstate
calcined at 500 1C, a crystallite size of about 17 nm was obtained.
Thus, in the present work, micrometric particles were obtained,
with nanometric crystallites. As a consequence, nano properties
do not affect the photoluminescence.
According to the theoretical study of Orhan et al. [19], the
4 3
charge difference between WO and WO clusters is 0.43 e. This
polarization also favors the creation of trapped holes and
electrons, giving origin to the room temperature photolumines-
4
. Discussion
x 1ꢀx
cence in the investigated powder samples from the Ca Sr WO4
system. The coexistence of localized levels and this charge
gradient creates favorable conditions for the self-trapping of
excitons before the emission of luminescence photons [1,19]. Thus,
As explained by Blasse [11], the PL arises from a radiative
return of the excited state to the ground state, phenomenon that is
in concurrence with the non-radiative return to the ground state,
in which the energy of the excited state is used to excite the
vibrations of the host lattice. The radiative emission process
occurs more easily if trapped holes or electrons exist in the
structure [19].
When the synthesis of tungstates is done using the polymeric
precursor method, the structural transformations occurring from
disordered to ordered phases start from the early stage of the
polyesterification of the citrate solution, containing tungsten and
calcium and/or strontium ions. Tungsten, which is the network
former, tends ideally to be coordinated by four oxygen atoms.
However, before it reaches this ideal configuration there are
various coordination numbers, mainly 3 and 4, in the structure.
Before the crystallization, i.e. before the heat treatment reaches
4 3
the charge transfer between the WO and WO clusters is related
to the presence of ordered and disordered clusters within the
microstructure, which are responsible for the photoluminescence
emission.
Corroborating the role of order/disorder in the PL phenomen-
on, the most intense PL response was not observed for pure
4 4 4
CaWO or SrWO , but for Ca0.6Sr0.4WO , which presents the
highest amount of disorder. This result is confirmed in all analyses
carried out in this work. The FWHM values (Table 1), for samples
heat treated at 600 1C, show that the highest short range (Raman
data) and long range (XRD data) disorder are observed for this
sample. Moreover, this sample has the lowest crystallinity (Table
1), the lowest bandgap (Fig. 3b) and the highest deviation from
the Vegard Law (Fig. 5). In spite of the PL emission being related to
7
00 1C, the Ca
mixture of WO
tetrahedra (WO
19–21,32–35].
In the present work, at 400 1C, samples present a high disorder
x
Sr1ꢀxWO
tetrahedra (WO
clusters) linked by the Ca and/or Sr ions
4
structure is composed of a random
4 3
the presence of WO –WO polyhedra, a change of the network
4
4
clusters) and WO truncated
3
modifier region induces a short range disorder in the network
former region, increasing photoluminescence.
The PL emission spectra could be separated into three of four
Gaussian curves (Table 2). The results were obtained for the
2+
2+
3
[
at short and long ranges and no PL emission is observed. At 500 1C,
4
sample Ca0.6Sr0.4WO , heat treated at different temperatures. The
the long range ordering starts, but a very high short range disorder
peak of highest energy is placed around 2.34 eV, corresponding to
529 nm (green), and the lower energy peak at 1.69 eV, correspond-
ing to 733 nm (red). The other peaks are placed at different
regions, according to the heat treatment temperature at 400 and
500 1C, a red emission is observed at 638 and 621 nm, respectively,
while an orange emission is observed at 600 and 700 1C, at 590
and 602 nm, respectively.
The values obtained from these Gaussian deconvolution curves
agree with the results of other researchers investigating the PL in
disordered tungstates. The PL emission observed by Orhan et al.
ꢀ
1
is still observed, with only one small broad peak at 914 cm in
the Raman spectra and a big Urbach tail. The PL emission
increases but it is still very low.
At 600 1C, the Raman spectra present more peaks, all related to
WO
4
vibrations, but with low definition and low intensity,
whereas the Urbach tails display a further decrease. These results
indicate that a higher short range order is observed in the network
former region, but the material is not completely ordered,
especially when the network modifier region is considered. In
the XRD patterns, broad peaks are still observed. These samples
present a great increase in the PL emission.
At 700 1C, well defined peaks are observed in the Raman
spectra, with an even smaller Urbach tail in the UV–vis spectra.
The PL emission vanishes. Comparing the Orhan results with the
present work, it may be observed that the higher the heat
treatment temperature, the more ordered the structure and the
4
[19] in amorphous CaWO thin film can be decomposed into three
components: three broad bands at about 530 nm (green), 580 nm
(yellow) and 650 nm (red). Orhan et al. [20] also carried out an
investigation on the PL in amorphous SrWO
4
. In this case, two
wide bands were observed, at 530 and 600 nm.
In the present work, it may also be observed that the emission
energies of all three peaks increase with temperature up to 600 1C,
followed by a decrease at 700 1C. This may be related to the higher
4
higher the concentration of the WO clusters. When the complete
Table 2
4
Deconvolution of the PL spectra of the sample Ca0.6Sr0.4WO heat treated at different temperatures
Calcining
Peak 1
Peak 2
Peak 3
Peak 4
temperature
(
1C)
Position (nm)
Area (%)
Position (nm)
Area (%)
Position (nm)
Area (%)
Position (nm)
Area (%)
4
5
6
7
00
00
00
00
–
–
556
546
529
534
46.2
54.5
41.2
58.2
638
621
590
602
30.1
29.9
43.6
32.6
733
706
693
704
23.6
15.6
11.9
9.2
–
–
502
–
3.2
–

ARTICLE IN PRESS
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1881
short range order in the lattice, leading to higher energy
transitions. At 600 1C, one more small peak was necessary to fit
the emission band, with energy of 2.47 eV (502 nm), being
ascribed to a green emission. For this sample, the highest
contribution for the emission band comes from the orange
emission, while the other samples have a higher contribution of
the green emission.
[3] L.E.B. Soledade, E. Longo, E.R. Leite, F.M. Pontes, F. Lanciotti, C.E.M. Campos,
P.S. Pizani, J.A. Varela, Appl. Phys. A 75 (2002) 629–632.
[4] F.M. Pontes, C.D. Pinheiro, E. Longo, E.R. Leite, S.R. Lazaro, J.A. Varela, P.S.
Pizani, T.M. Boschi, F. Lanciotti, Mater. Chem. Phys. 78 (2003) 227–233.
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disordered structures of CaWO
4
and SrWO
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4
, by displacing two
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3
both theoretical studies of tungstates, the authors ascribed the
bottom of the conduction band to W* (5d) orbitals, whereas the
top of the valence band was occupied by O* (2p), with the bonded
O (2p) orbitals lying at a lower energy position within the valence
band.
Accordingly, the authors suggest that the presence of various
bands in the PL spectra of the CWO thin films is related to these
orbitals. The highest energy bands, green and yellow, may be due
to the radiative decay occurring from the W* (5d) orbitals to the O
[
[
[
[
[
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[
[
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(
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5
. Conclusions
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The polymeric precursor method was successfully employed in
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Santos, A.G. Souza, J.A. Varela, P.S. Pizani, E. Longo, Chem. Phys. 312 (2005)
the synthesis of powder samples from the Ca
x
Sr1ꢀxWO (0pxp1)
4
1
–9.
system, producing fine single phase powders at a relatively low
temperature. A disordered material was obtained for the heat
treatment temperatures up to 400 1C. Starting from 500 oC, the
[
21] M. Anicete-Santos, E. Orhan, M.A.M.A. de Maurera, L.G.P. Sim o˜ es, A.G. Souza,
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Prasad, J.G. Lima, C.A. Paskocimas, E. Longo, J. Therm. Anal. Calorim. 67 (2002)
%
[
crystallization of the system takes place. The most intense room
temperature PL emission (600 1C) was obtained for the structure
that is neither highly disordered (400 and 500 1C), nor completely
ordered (700 1C).
4
59–464.
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As for the influence of composition on the optical bandgap, at
[
[
[
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6
00 1C, the highest short and long range disorder was observed for
, the same compositions that displayed the highest
Ca0.6Sr0.4WO
4
photoluminescent emission. This result corroborates the role of
disorder in the PL phenomenon.
The PL emission spectra could be separated into three of four
Gaussian curves, with a red, red/orange and green emission. The
lower wavelength peak is placed at around 540 nm, and the higher
wavelength peak at about 700 nm. Similar results were reported
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M.A.C. Machado, P.S. Pizani, J.A. Varela, J. Eur. Ceram. Soc. 23 (2003)
3001–3007.
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205–216.
[
[
[
[
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4
in the literature for CaWO .
Acknowledgments
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[
[
[
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This work was supported by the Brazilian funding programs
PRONEX/CNPq/ Fapesq and PADCT/CNPq/MCT.
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