Synthesis and characterization of metal tungstates by novel solid-state metathetic approach

Article abstract of DOI:10.1016/j.jallcom.2007.10.089

Novel solid state metathetic approach (SSM) (ACl₂ + Na₂WO₄ → AWO₄ + 2NaCl, A = Ca, Sr, Ba, Zn, Mn, Ni) assisted by microwave energy has been successfully applied to the synthesis of tungstates of scheelite- and wolframite-type that are of technological importance. Well crystalline phases of scheelite-type tungstates, MWO₄ (M = Ca, Sr, Ba) have been synthesized where the characteristics of SSM reaction and the formation of high lattice energy by-product NaCl drives the reaction toward completion. Among wolframite-type tungstates, single-phase ZnWO₄ is synthesized by SSM reactions at ambient conditions and MWO₄ (M = Ni, Mn) are synthesized after subjecting the amorphous product to moderate temperature of heating (around 500 °C for 6 h). This alternative method of synthesis where the metathesis reactions proceed in solid state has features like: simple method of synthesis, cost-effectiveness, high yield, easy scale up, and thus has advantages over already known methods of synthesis.

Full text of DOI:10.1016/j.jallcom.2007.10.089

Journal of Alloys and Compounds 465 (2008) 380–386
Synthesis and characterization of metal tungstates by novel
solid-state metathetic approach
1
∗
Purnendu Parhi, T.N. Karthik , V. Manivannan
Department of Mechanical Engineering, Campus Delivery 1374, Colorado State University, Fort Collins, CO 80523, USA
Received 8 August 2007; received in revised form 16 October 2007; accepted 20 October 2007
Available online 26 October 2007
Abstract
Novel solid state metathetic approach (SSM) (ACl
2
+ Na
2
WO
4
→ AWO
4
+ 2NaCl, A = Ca, Sr, Ba, Zn, Mn, Ni) assisted by microwave energy
has been successfully applied to the synthesis of tungstates of scheelite- and wolframite-type that are of technological importance. Well crystalline
phases of scheelite-type tungstates, MWO (M = Ca, Sr, Ba) have been synthesized where the characteristics of SSM reaction and the formation of
high lattice energy by-product NaCl drives the reaction toward completion. Among wolframite-type tungstates, single-phase ZnWO is synthesized
(M = Ni, Mn) are synthesized after subjecting the amorphous product to moderate temperature
4
4
by SSM reactions at ambient conditions and MWO
of heating (around 500 C for 6 h). This alternative method of synthesis where the metathesis reactions proceed in solid state has features like:
simple method of synthesis, cost-effectiveness, high yield, easy scale up, and thus has advantages over already known methods of synthesis.
4
◦
©
2007 Elsevier B.V. All rights reserved.
Keywords: Ceramics; Solid-state reaction; Scanning electron microscopy; X-ray diffraction
1
. Introduction
co-precipitation [9], solvo-thermal process [10–12], sol–gel
reaction [13], reverse micellar reaction [14], microwave irra-
diation [15], and mechanochemical method [16]. Ryu et al.
have synthesized MWO4 (M = Ca, Ni) using citrate combustion
method [17]. The citrate complex route assisted by microwave
has been used by the same group for synthesis of MWO4
(M = Ca, Sr, Ba, Pb) [18]. Nanocrystalline particle synthesis of
MWO4 (M = Ca, Sr, Ba, Cd, Zn, Pb) has been carried out by
Chen et al. using ethylene glycol [19]. Polymethacrylic acid as
a template reagent has been used by Zhao et al. for synthesis of
SrWO4 hollow sphere [20]. Among these different methods the
solution based chemical synthetic methods play a crucial role in
the design and production of fine ceramics and have been suc-
cessful in overcoming many of the limitations of the traditional
solidstate, high-temperature methods. The useofsolutionchem-
istry can eliminate major problems such as long diffusion paths,
impurities and agglomeration which will result in products with
improved homogeneity.
AWO4 type of compounds (A = bi-valent element) are well
known. The most common are the minerals scheelite (CaWO4)
used for optical properties (scintillation counters, lasers, opti-
cal fibers) and wolframite (FeWO4, MnWO4) which is the
most common raw material for producing tungsten metal. In
the AWO4 type of compounds, if A2 has a small ionic radius,
+
˚
0.77 A (Mg, Zn) it will form the monoclinic wolframite struc-
<
2
+
˚
ture, but larger A cations >0.77 A (Ca, Ba) form the tetragonal
scheelite structure. Monoclinic ZnWO4 is also a known material
named sanmartinite.
AWO4-type have attracted a great deal of interest in the recent
years due to their use as laser host materials [1,2], as scintillator
[
3,4], oxide ion conductors [5], microwave applications [6] and
magnetic materials [7]. These tungstate materials doped with
rare-earth ions have optical and laser properties.
AWO4 powders have been prepared by a traditional solid-
state reaction [8] and by various wet methods such as
However, wet methods have disadvantages such as compli-
cated synthetic steps, use of expensive equipment, high synthetic
temperature and long sintering time. On the other hand, due
to excessive energy consumption, complex apparatus and tech-
niques the solid-state reaction becomes gradually unpopular
and unsatisfied. However, solid-state synthesis of materials by
∗
Corresponding author. Tel.: +1 970 491 2207; fax: +1 970 491 3827.
E-mail address: mani@engr.colostate.edu (V. Manivannan).
Summer intern from IIT, Chennai, India.
1
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.10.089

P. Parhi et al. / Journal of Alloys and Compounds 465 (2008) 380–386
381
metathetic route is emerging as a viable alternative approach
to synthesize high-quality novel inorganic materials in a short
amount of time.
Solid-state metathetic approach has been successfully
applied for the synthesis of many oxide materials. For example,
Gopalakrishnan et al. have synthesized oxides of K2La2Ti3O10,
Ca2La2CuTi2O10 belonging to Ruddleson-Popper type of mate-
rials, ABO3 perovskite type of materials like LaMO3 (M = Co,
Mn), ATiO3 (A = Ca, Sr and Ba) and double perovskites like
ꢀ
ꢀ
Ba3MM 2O9 (M = Mg, Ni, Zn; M = Nb, Ta) by this approach
21–24]. In addition, Kaner et al. have synthesized oxides of Zr,
[
Hf and Cu using this approach [25,26]. Recently we have syn-
thesized Zn3(PO4)2 at room-temperature using this procedure
[
27].
Synthesis of scheelite-type ABO4 (A = Ca, Sr, Pb; B = Mo,
W) has been reported by solution-based metathetic approach
where the desired product was precipitated after dissolv-
ing high purity alkaline earth and lead chloride in distilled
water, and adding this solution to the solution of appropri-
ate amounts of sodium molybdate or tungstate [28]. Synthesis
of wolframite-type MnWO4 was reported by solution based
metathesis reactions, when the precipitate obtained from mix-
ingequimolarsolutionsofcorrespondinghydratedmetalnitrates
◦
and hydrated sodium tungstates was heated to 800 C for 15 h
[
29].
Here we report the synthesis of scheelite-type ABO4 (A = Ca,
Fig. 1. XRD pattern showing the formation of NaCl (marked *) along with the
desired products CaWO4 (marked #) (a) and ZnWO4 (marked #) (b), validating
the solid-state metathesis approach of synthesis of making tungstate materials.
Sr, Ba) by novel solid-state metathetic approach assisted by
microwave energy radiation and extended the same approach
to the successful synthesis of tungstates of wolframite-type
MWO4, (M = Zn). To the best of our knowledge such an
approach towards the synthesis of metal tungstates has not been
reported thus far.
◦
ter with the capability to measure reaction temperatures up to 3200 C was
used.
2
. Experimental
3. Results and discussion
Na2WO4, CaCl2, SrCl2, BaCl2, ZnCl2, MnCl2, NiCl2 obtained from Alfa
Fig. 1a shows the powder XRD of scheelite-type calcium
Aesar, USA, were used as precursors for the preparation of the metal tungstate
compounds. Preparation of calcium tungstate was carried out by reacting a well-
ground mixture of CaCl2 and Na2WO4 in a molar ratio of 1:1. Sample mixtures
were put into crucibles and then exposed to domestic microwave operating at a
frequency of 2.45 GHz and a power of 1100 W for 10 min duration. The samples
were washed with distilled water to remove the sodium chloride reaction by-
tungstate product as a result of solid-state metathesis reaction
between calcium chloride and anhydrous sodium tungstate. The
XRD pattern contains peaks corresponding to two phases, NaCl
(
marked with *) and that of metal tungstate (marked with #).
◦
The formation of NaCl as a by-product confirms the reaction is
characteristic of solid-state metathesis reactions: fast, energetic
involving exchange of reacting partners and driven by high lat-
tice energy of the co-produced salt (NaCl), as established in the
literature [21–27,30–37]. Microwave radiation has provided the
required energy to overcome the energy barrier which precludes
spontaneous reaction and helped to heat the bulk of the material
uniformly resulting in fine particles of controlled morphology
and forming the product in a “green” manner without generation
of any solvent wastes.
The as-prepared sample is thoroughly washed with distilled
water and dried with acetone to dissolve NaCl and get the desired
product CaWO4. XRD (Fig. 2a) showed a well crystalline,
single-phase nature for the product with yield >80%. Calcium
tungstate crystallizes in tetragonal crystal structure. Scheelite
structures of strontium and barium tungstates were synthesized
in the method described above and their single-phase nature of
product and dried at 80 C. Preparation of strontium, barium and zinc tungstate
powders were carried out in a similar manner by reacting to a well-ground
mixture of ACl2 (A = Sr, Ba Zn) and Na2WO4, as mentioned above. For the syn-
thesis of MnWO4 and NiWO4 the microwave synthesized product after washing
◦
is heated at 500 C for 6 h.
Powder X-ray diffraction (XRD) measurements were carried out using Scin-
tag X2 diffractometer with Cu K␣ radiation and Peltier detector. A scan rate
◦
◦
of 1 /min with a step size of 0.02 was employed to obtain the XRD spec-
tra. Scanning electron microscope (SEM) characterization was performed on
the JSM-6500F, a field emission system with the In-Lens Thermal Field Emis-
sion Electron Gun (TFEG). Diffuse reflectance (DR) spectra were recorded in
the wavelength range 250–2500 nm using Varian Associates Cary 500 double
beam spectrophotometer equipped with Praying mantis. X-ray photoelectron
spectroscopy (XPS) experiments were performed on a Physical Electronics
5
(
800 spectrometer. This system has a monochromatic Al K␣ X-ray source
h␣ = 1486.6 eV), hemispherical analyzer, and multichannel detector. A low
energy (30 eV) electron gun was used for charge neutralization on the non-
conducting samples. The binding energy (BE) scales for the samples were
referenced to the C 1s peak at 284.8 eV. To determine the SSM reaction final
adiabatic temperatures Pyro manufactured calibrated Micro Optical Pyrome-

3
82
P. Parhi et al. / Journal of Alloys and Compounds 465 (2008) 380–386
Fig. 2. XRD pattern showing the single-phase formation for scheelite-type (a)
CaWO4, (b) SrWO4, and (c) BaWO4.
Fig. 3. XRD pattern showing the pure-phase formation of (a) NiWO4, (b)
MnWO4, and (c) ZnWO4.
material is confirmed by XRD (Fig. 2b and c). Crystal struc-
ture details about the scheelite-type of tungstates synthesized
by solid-state metathesis approach are summarized in Table 1.
All the peaks of the XRD are indexed and well matched with
the reported phases of JCPDS [38].
after subjecting the as-prepared product to high-temperature
sintering.
3.1. Thermodynamics behind metathesis
SSM approach has been extended to the synthesis of
tungstates of wolframite-type as well. Fig. 1b shows the XRD of
as-preparedmaterialthatcontainsbothNaClandthedesiredzinc
tungstate phases. Presence of sodium chloride once again con-
firms the reaction has proceeded in solid-state metathesis way.
The desired product is obtained after dissolving sodium chlo-
ride and the XRD showed well crystalline single-phase nature
with yield >80%. Intensities of the peaks (Fig. 3c) and the cell
parameters clearly confirmed the phases belong to wolframite
ZnWO4 (Table 1). For NiWO4 and MnWO4, XRD showed
amorphous nature for the as-washed products which were sub-
Solid state metathesis reactions, an example of which
is shown in this study (ACl2 + Na2WO4 → AWO4 + 2NaCl,
2+
A = M ) involves exchange of atomic/ionic species, where the
driving force being the formation of thermodynamically stable
alkali or alkaline earth halides with high lattice energy. Ther-
modynamic basis for such metathetic reactions are reported
in the literature [23,40–42]. SSM reactions are characterized
by a large enthalpy change (ꢀHm) and high adiabatic tem-
perature (Tm). The final adiabatic temperature measured was
1623 K for ZnWO4. Using thermodynamic data available in
the literature [43], we have calculated the enthalpy (ꢀH) and
free energy changes (ꢀG) associated with the formation of
tungstates, and presented the result in Table 2. The result
show, (ꢀH −36.17 KJ/mol for ZnWO4) that both enthalpy and
free energy favors the metathesis reaction and the enthalpy
change is indeed the driving force for the metathesis involv-
ing the formation of NaCl. We believe the final adiabatic
◦
jected to moderate temperature of heating, i.e. around 500 C
for 6 h in air to obtain single-phase crystalline phases (Fig. 3a
and b). All the peaks of the XRD are indexed and well matched
with the reported phase of JCPDS [39]. The lattice parameters
for these compounds are also given in Table 1. It is possible that
the formation of amorphous products for Ni and Mn could be
due to insufficient and inhomogeneous transfer of microwave
energy to form the product. Varying the microwave conditions
◦
temperature for scheelite tungstates to be below 650 C. Due
(
power and duration of the reaction), within the limitations of
to restricted wavelength response of the device to the red
region of the visible, optical pyrometers can only be used
to measure objects that are hot enough to be incandescent
or glowing, which limited the lower end of the temperature
the microwave equipment yielded only amorphous products. It
is interesting to note that the previous report of synthesis of such
compounds, including that of low temperature methods (except
hydrothermal method) [15,28] obtained crystalline product only
◦
measurement range of the devices to be about 650 C. SSM
Table 1
Crystal structure details of metal tungstates synthesized in this study
Composition
a ( A˚ )
b ( A˚ )
c ( A˚ )
α
β
γ
Volume ( A˚ 3)
Z
Space group
CaWO4
SrWO4
BaWO4
ZnWO4
MnWO4
NiWO4
5.216(0)
5.376(0)
5.570(2)
4.720(0)
4.798(0)
4.599(2)
5.216(0)
5.376(0)
5.570(2)
5.700(0)
5.710(6)
5.660(6)
11.313(0)
11.876(0)
12.637(0)
4.950(0)
4.973(8)
4.906(8)
90
90
90
90
90
90
90
90
90
89.66(7)
91.22
90.03
90
90
90
90
90
90
307.79
350.66
400.86
133.17
136.26
127.74
4
4
4
2
2
2
I 4/a
I 4/a
I 4/a
P 2/c
P 2/c
P 2/c

P. Parhi et al. / Journal of Alloys and Compounds 465 (2008) 380–386
383
Table 2
4
3.2. Characterization of MWO materials
Thermodynamic data for CaWO4 and ZnWO4 synthesized by metathesis reac-
◦
◦
◦
◦
tions. ꢀH and S were calculated using the expression, ꢀH = ΔH
+
T
T
T
298
ꢀ
ꢀ
Among scheelite tungstates, SEM showed spherical shape
particles for Ca, Sr compounds Fig. 4a and b and rod-like fea-
tures for Ba materials (Fig. 4c). Zn compounds have well defined
prism-type morphology and the particles are sub-micron in size
T
T
◦
◦
◦
◦
2
98 (C /T)dT and S = S + 298 (C /T)dT, together with the expres-
p
T
298
p
◦
◦
sion of C as a function of T. ꢀG values were calculated by using the expression
p
T
◦
◦
◦
◦
◦
◦
ꢀ
G
= ꢀHT − TꢀS ꢀH , S , C values were taken from reference [43]
T
T
298 298
p
Composition
ZnWO4
−97.8
(Fig. 4d). For other wolframite tungstates Ni and Mn, SEM
◦
◦
−1
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
H
(KJ mol
)
)
2
98
showed the particles were agglomerated in nature as shown in
Fig. 4e and f.
−1
−1
S
(KJ mol
(KJ mol
)
0.0167
−102.77
−36.17
0.095
2
98
◦
G
2
98
◦
−1
−1
H
S
(KJ mol
(KJ mol
(KJ mol
)
For tungstate materials to be used as practical phosphor as
one of the potential technological applications, a dense pinhole
free coating of the same has to be produced on the desired sub-
strate. The quality of coatingis determinedmainlybythe particle
size distribution and morphology of the particles. For example,
in case of a fluorescent lamp phosphor, the optimum coating
thickness is roughly proportional to its mean particle size: that
is, the smaller the particle size, the thinner the coating can be.
Fine-particle phosphors also yield denser coatings. The well-
defined particle features of the tungstates synthesized by SSM
reactions showed that these reactions have control over morphol-
T
◦
)
T
◦
−1
)
G
−190.35
T
reactions occur so rapidly that all of the released enthalpy is
essentially used to heat up the solid products, usually rais-
ing the alkali halide near or above its normal boiling point
and have been recognized as approximately adiabatic in nature
[
23].
Fig. 4. SEM pictures of (a) CaWO4, (b) SrWO4, (c) BaWO4, (d) ZnWO4, (e) MnWO4 and (f) NiWO4 showing particle features.

3
84
P. Parhi et al. / Journal of Alloys and Compounds 465 (2008) 380–386
Fig. 6. (a) Diffuse reflectance spectra of (1) CaWO4, (2) SrWO4, and (3) BaWO4
in the wavelength range 250–2500 nm. (b) Plot of F(R∞) vs. E (eV) for the
estimation of the optical absorption edge energy.
Fig. 5. XPS of (a) scheelite-type (a) CaWO4 and (b) wolframite-type MnWO4.
ogyofthefinalparticlesandcouldbeusedforsuchtechnological
applications.
Fig. 5a shows the XPS of CaWO4 (scheelite) and Fig. 5b
shows the XPS of ZnWO4 (wolframite). XPS provides valu-
able information concerning the elements in the near surface
region. Examination of the binding energies of the core-
level electronic states of the elements in the surface region
provides qualitative, semi-quantitative and chemical-state infor-
mation. Since the electron binding energy of elements differs
from each other, a full-scan spectrum was given for an over-
all understanding on surface elemental constituents of the
tested sample. In the spectra we can see peaks correspond-
ing to Ca2s (442.0 eV), Ca2p (351.6 eV), W4s (598.2 eV),
O1s (534.8 eV), W4d (250.8 eV), W4f (39.6 eV). Similarly in
the spectra of ZnWO4 we can see peaks corresponding to
Zn2p1/2 (1047.6 eV), Zn2p3/2 (1024.4 eV), Zn3s (142.0 eV),
Zn3p (91.6 eV), W4s (587.6 eV), W4p3/2 (429.2 eV), W4p1/2
(
477.2 eV), W4d (252.0 eV), W4f (37.6 eV), O1s (533.2 eV).
The C1s peak at 284.8 eV is due to adventitious carbon present
in the surface.
3
.3. Physical property measurement of MWO4 compounds
In order to determine the optical band gap for the techno-
logically important tungstate materials, DR measurements were
carried out. Fig. 6a shows the diffuse reflectance spectra of the
CaWO4, SrWO4, BaWO4 samples in the UV–vis–NIR range
whereas Fig. 7a shows the diffuse reflectance spectra of the
MnWO4, NiWO4, and ZnWO4. The diffuse reflectance data was
Fig. 7. (a) Diffuse reflectance spectra of (1) ZnWO4, (2) NiWO4, and (3)
MnWO in the wavelength range 250–2500 nm. (b) Plot of F(R ) vs. E (eV)
4
∞
for the estimation of the optical absorption edge energy.

P. Parhi et al. / Journal of Alloys and Compounds 465 (2008) 380–386
385
Table 3
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continued help, encouragement and support.

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