Synthesis of CaxSr1-xWO4 by the polymeric precursor method

Article abstract of DOI:10.1007/s10973-005-0073-3

In the present study, powders based on Ca_xSr _1-xWO₄ tungstates, with a scheelite structure were synthesized, using the polymeric precursor method. The powder precursors, calcined at 300°C, were thermally characterized by TG/DTA. It can be observed that the mass loss basically takes place in three stages: first - dehydration, second - pyrolysis and third - elimination of the remaining organic material. The structural characterization was performed by XRD, after thermal treatment between 300 and 700°C. The crystallization takes place at 500°C, but more defined and intense diffraction peaks appeared at 700°C. Based on these XRD results the lattice parameters, crystallinity and crystallite size were calculated.

Full text of DOI:10.1007/s10973-005-0073-3

Journal of Thermal Analysis and Calorimetry, Vol. 79 (2005) 401–406
SYNTHESIS OF CaxSr1–xWO4 BY THE POLYMERIC PRECURSOR METHOD
S. L. Porto1, M. R. Cassia-Santos1, IÃda. M. G. Santos1, S. J. G. Lima2, L. E. B. Soledade1,
A. G. Souza1*, C. A. Paskocimas3 and E. Longo4
1LTM, Departamento de Química, CCEN, Universidade Federal da Paraíba, 58.059-900 Jo±o Pessoa, PB, Brazil
2Departamento de Engenharia Mecânica, CT, Universidade Federal da Paraíba, 58.059-900 Jo±o Pessoa, PB, Brazil
3Departamento de Engenharia Mecânica, CT, Universidade Federal do Rio Grande do Norte, 59.072-970 Natal, RN, Brazil
4CMDMC, Departamento de Química, Universidade Federal de S±o Carlos, S±o Carlos, SP, Brazil
In the present study, powders based on CaxSr1–xWO4 tungstates, with a scheelite structure were synthesized, using the polymeric
precursor method. The powder precursors, calcined at 300°C, were thermally characterized by TG/DTA. It can be observed that the
mass loss basically takes place in three stages: first – dehydration, second – pyrolysis and third – elimination of the remaining
organic material. The structural characterization was performed by XRD, after thermal treatment between 300 and 700°C. The
crystallization takes place at 500°C, but more defined and intense diffraction peaks appeared at 700°C. Based on these XRD results
the lattice parameters, crystallinity and crystallite size were calculated.
Keywords: polymeric precursor method, scheelite, tungstates
Introduction
some samples excited in the short ultraviolet wave-
length at cryogenic temperatures (N2(l) for SrWO4
and H2(l) for BaWO4). The blue emission was only
observed for the samples calcined at a low tempera-
ture. Therefore, the study of the thermal and structural
properties of tungstate-based systems becomes more
and more important and necessary investigate their
optical properties.
This is the reason, why in the present work, the
synthesis of ceramic tungstates with molecular formula
of CaxSr1–xWO4 (with x=0, 0.2, 0.4, 0.6, 0.8 and 1.0)
with scheelite structure was carried out. The synthesis
has been carried out by the ‘Pechini’ method [7]. The
method was selected because of its efficiency, present-
ing some peculiar characteristics, such as a good ho-
mogeneity at the atomic level.
MWO4 tungstates (M=bivalent cations) are com-
pounds found frequently in nature, which crystallize
in alkaline solutions, obtained by the melting of the
binary oxide components. Similar case is known for
calcium tungstate.
The tungstates can be obtained in two iso-
structural groups: scheelite, CaWO4 and wolframite,
FeWO4 and MnWO4. The structure will depend on the
ionic radius of A2+ cations. For A2+ cations with cova-
lent character (such as Mg2+ and 3d orbital ions), the
wolframite structure is favored. Since Ca2+, Sr2+ and
Ba2+ cations have a stronger ionic character, the schee-
lite structure is more favorable. The transition from the
scheelite structure to the wolframite one can be stimu-
lated by high pressures – around 5 GPa for BaWO4 and
about 1.2 GPa for CaWO4 [1]. The crystalline structure
of scheelite is highly ionic, containing A2+ cations and
WO24– anions. Such structure presents the I41/a or C64h
space group in which A cations have an octahedral co-
ordination, W cations are tetrahedrally coordinated and
each unit cell has two AWO4 units [2–5].
In recent years, the scheelite structure reached
remarkable attention due to its attractive electro-opti-
cal properties. Blasse et al. [6] investigated the lumi-
nescence of barium and strontium tungstate synthe-
sized by solid-state reactions. They obtained a system
with scheelite structure, displaying luminescent prop-
erties (which varied with the concentration of these
metals). They observed a green and blue emission for
Experimental
Powder synthesis
All the tungstate-based powders were prepared keep-
ing the following stages: 1) preparation of tungsten
citrate; 2) Synthesis of the polymeric resin; 3) first
calcination, obtaining of the precursor powder;
4) second calcination, obtaining samples with differ-
ent crystallinity degrees.
Tungsten citrate was chosen as cationic precursor
in the preparation of the precursor resins for the syn-
thesis of the compounds displaying the molecular for-
mula CaxSr1–xWO4 (x=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0).
*
Author for correspondence: agouveia@quimica.ufpb.br
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

PORTO et al.
Powder characterization
This citrate was obtained by dissolving tungstic acid in
a citric acid solution, under constant agitation at 70°C.
Ammonium hydroxide was slowly added to this aque-
ous solution in order to provide the complete dissolu-
tion of the tungstic acid at pH»8. After some hours of
continuous agitation, a limpid and transparent tungsten
citrate was obtained.
The thermal characterization of the powder precursors
was done by TG/DTA. In the TG experiments, a
Shimadzu model TGA-50 thermobalance was used,
with air atmosphere (flow rate of 30 mL min–1) heating
rate: 10 K min–1 between 30 and 900°C the initial sam-
ple mass was about 10 mg. For DTA measurements, a
TA Instruments equipment was used, with a synthetic
air (flow rate of 30 mL min–1) and under the above
written conditions.
The formation of the crystalline phase as a
function of the temperature and the amount of added
calcium was followed by X-ray diffraction (XRD),
using a D 5000/Siemens diffractometer.
After the synthesis of this citrate, the SrCO3 and
CaCO3 salts were slowly added to the citrate, till their
total dissolution. 3:1 citric acid:metal cation molar ra-
tio was used in order to guarantee the complete metal
chelation. Then ethylene glycol was added to the solu-
tion. 40:60 citric acid:ethylene glycol mass ratio was
used. The reaction temperature was raised to approxi-
mately 90°C to promote the esterification and the
evaporation of the water excess. Limpid solutions were
obtained, which were heated to 300°C, at air resulting
a semi-carbonized material displaying a foam-type
structure, called to powder precursor. These precursors
were de-agglomerated in a porcelain mortar and were
ground until passing through a 200 mesh-sieve. The
thermal characterization of these precursors was per-
formed by thermogravimetry (TG) and differential
thermal analysis (DTA).
Results and discussion
The TG/DTG curves representing the thermal behavior
of CaxSr1–xWO4 (0£x£1) powder precursors are pre-
sented in Fig. 1. Basically, the mass losses appear in
three stages: 1) up to 300°C, which is attributed to wa-
ter elimination, corresponding to the partial dehydra-
tion of the compounds; 2) from 270 up to 620°C, two
consecutive processes occur, corresponding to the
polymer degradation, with the elimination of CO2 and
The powder precursors were calcined, at air
atmosphere at 300°C for 2 h and then at 400, 500, 600
and 700°C for 4 h, at a heating rate of 5 K min–1.
Fig. 1 TG/DTG curves of the CaxSr1–xWO4 powder precursor system; a – x=0.0, b – x=1.0, c – x=0.8
402
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SYNTHESIS OF CaxSr1–xWO4
H2O, and the loss of the remaining organic material, re-
sample, showing a better defined diffraction peaks,
evidenced by the decrease of the full width at half
maximum values of the peaks. After calcination
at 700°C, the formation of a crystalline phase is ob-
served, resulting even more defined diffraction peaks
compared to the ones observed after calcination
at 600°C.
As carbon loss starts at about 270°C, there is
enough time for its elimination followed by sample
crystallization.
All the diffraction peaks were indexed as repre-
sentatives of the tetragonal scheelite unit cell.
At 500°C the samples already display a scheelite phase
and the increase of the calcination temperature favors
the organization of this structure, eliminating structural
defects, as a consequence of atomic movement by a
solid-state diffusion process (Fig. 3). These results sug-
gest the formation of a single-phase substitutional solid
solution. This behavior was expected as the system sat-
isfies the three basic conditions for substitutional solid
solution formation: the Ca2+ ionic radius (1.12 )
is 11.6% smaller than the ionic radius of Sr2+ (1.25 );
the crystalline structures of CaWO4 and SrWO4 are the
same; and the electropositive characters of the two
cations are similar.
spectively; 3) between 620 and 720°C the small mass
loss observed may representative for the decomposi-
tion of the carbonate formed during the pyrolysis of the
material, as it will be shown by DTA [8, 9].
The DTA curves of the same three compounds are
presented in Fig. 2. Exotherm and endotherm transi-
tions can be observed for all samples. The first
exotherm peak is attributed to the polymer combustion.
This stage was supported by the TG curves, indicating
a mass loss at about 550°C. The second endotherm
peak is associated to the carbonate decomposition,
possibly formed during the second event – the polymer
pyrolysis. Small endotherm peaks due to dehydration
are observed only for samples containing Ca and Sr,
which present a higher mass loss at low temperatures.
The diffraction patterns of the CaxSr1–xWO4 sys-
tem are presented in Fig. 3. After calcination
at 400°C, a diffuse band is observed, indicating the
predominance of the amorphous phase. After calcina-
tion at 500°C the diffraction peaks related to the crys-
talline phase are observed. Nevertheless, these peaks
are still wide, showing that a major part of the mate-
rial is still amorphous. The increase of the calcination
temperature to 600°C increases the crystallinity of the
Fig. 2 DTA curves of the CaxSr1–xWO4 powder precursor system; a – x=0.0, b – x=1.0, c – x=0.8
J. Therm. Anal. Cal., 79, 2005
403

PORTO et al.
Fig. 3 X-ray diffraction patterns of the CaxSr1–xWO4 system as a function of calcination temperature; a – x=0.0, b – x=1.0, c – x=0.8
in the present work are very close to the values
Comparing the diffractograms of Fig. 3, it is ob-
reported in the JCPDS index card for CaWO4
(JCPDS 41-1431: a=5.24294 , c=11.373  and
served that the strontium tungstate (SrWO4) also pres-
ents a band after calcination at 400°C. At 500°C,
SrWO4 presents higher diffraction peaks than
CaWO4, indicating that it crystallizes at a lower tem-
perature. This evidences that in the presence of cal-
cium the crystallization process more hindered.
V=312.63 3) and for SrWO4 (JCPDS 8-490:
a=5.4168 , c=11.951  and V=350.66 3).
The unit cell microstrain can be evaluated on the
base of full width at half maximum values (FWHM) of
the most intense peak (112), Fig. 4d. The peak is getting
With the data obtained by XRD and using the
narrower (112) as the calcination temperature is in-
Rede 93 program, it was possible to determine the
creased. This behavior indicates a reciprocal relation-
lattice parameters a and c of the system CaxSr1–xWO4
ship between temperature and the microstrain of the
crystalline structure. The influence of the network modi-
(0£x£1), as a function of calcination temperature
(Fig. 4). The calcium addition led to a decrease in the
fier on the MeWO4 structure was also observed. Cal-
lattice parameters (a and c), and consequently a
cium and strontium act as modifiers, promoting or in-
reduction in the unit cell volume of the SrWO4 lattice.
creasing the formation of non bridging oxygens (NBO)
This result is attributed to the difference in the ionic
within the three-dimensional lattice, generating struc-
tural defects, which favors the system disorder and con-
sequently, the microstrain. The presence of calcium
concomitant and strontium in the lattice increases the
radii of the cations. The replacement of the bigger
Sr2+ cation, (ionic radius of 1.25 ) by a smaller Ca2+
cation (ionic radius of 1.12 ) results the shrinkage of
the unit cell. The experimental values of the parame-
disorder, comparing to undoped SrWO4 and CaWO4.
ters a and c, and of the unit cell volume (V) obtained
After calcination at 500 and 600°C, strontium tungstate
404
J. Therm. Anal. Cal., 79, 2005

SYNTHESIS OF CaxSr1–xWO4
Fig. 4 Structural parameters calculated from the X-ray diffraction data for the CaxSr1–xWO4 system as a function of x at different
calcination temperatures: a – lattice parameter a, b – lattice parameter c, c – unit cell volume, d – full width at half
maximum (FWHM) of the 100% peak
presents a lower degree of microstrain than the calcium
tungstate but at 700°C the degrees of microstrain are
much closer, as it is indicated in Fig. 4.
Relative crystallinity was calculated using the
100% peak intensities and Eq. (1).
ing both modifiers, showing their influence in the nu-
cleation and crystal growing. It is also observed that
the relative crystallinity decreases up to 60 mol% of
calcium, and for higher amounts of Ca the relative
crystallinity increases again.
This finding can be attributed to the presence of
the two different network modifiers (Ca and Sr), with
different ionic radii, favoring the lattice disorder. This
behavior is compatible with FWHM results, which
indicate a higher microstrain for samples containing
both modifiers.
In Fig. 5b, the results of the crystallite sizes as a
function of the calcium content and of the calcination
temperature are presented. It can be observed the
higher the calcination temperature, the larger the
crystallite size. This effect is attributed to the pheno-
mena of nucleation and grain growth which is a ther-
mally activated process. From the crystallite size val-
ues, it is also verified that the increase of the calcium
concentration in the CaxSr1–xWO4 system promotes a
decrease of the crystallite size up to the molar per-
centage of 60%. For a higher calcium content, an in-
crease of the crystallite size is observed, which can be
better visualized at 700°C. This result is in agreement
with the previous findings.
I – I0
CR(%) =
×100
(1)
I100 – I0
where I=intensity of the (110) plane peak;
I0=intensity of the (110) plane peak of the less
crystalline sample; I100=intensity of (110) plane peak
of the most crystalline sample.
Relative crystallinity of samples of the
CaxSr1–xWO4 system for different (x) values and dif-
ferent calcination temperatures are presented in
Fig. 5a. It can be observed that the increase in the
temperature favors the crystallization, except for
SrWO4 calcined at 600°C. It could be observed that
undoped SrWO4 crystallizes at lower temperature
than CaWO4 under the same conditions. The increase
of crystallinity was ascribed to the incorporation of
the atoms in the lattice which is a thermally activated
process and originates from the atomic movement by
diffusion. Undoped SrWO4 and CaWO4 present a
higher relative crystallinity than the samples contain-
J. Therm. Anal. Cal., 79, 2005
405

PORTO et al.
Conclusions
Polymeric precursor method was used in the synthesis
of the CaxSr1–xWO4 (0£x£1) system and resulted sin-
gle-phase powders at relatively low temperatures.
Their thermal characterization by TG/DTG showed
three well-defined thermal decomposition stages: de-
hydration; pyrolysis, carbonate decomposition. X-ray
diffraction indicated the presence of an amorphous
phase at 400°C and the increase of the relative
crystallinity starting from 500°C, for the CaxSr1–xWO4
system studied. All the samples presented a tetragonal
scheelite-type structure.
Acknowledgements
The authors thank the Brazilian agencies CAPES, FINEP and
CNPq for financial support.
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