BaMoO4 powders processed in domestic microwave-hydrothermal: Synthesis, characterization and photoluminescence at room temperature

Article abstract of DOI:10.1016/j.jpcs.2008.06.107

In this paper, BaMoO₄ powders were prepared by the coprecipitation method and processed in a domestic microwave-hydrothermal. The obtained powders were characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) spectroscopy, ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. The morphology of these powders were investigated by scanning electron microscopy (SEM). SEM micrographs showed that the BaMoO₄ powders present a polydisperse particle size distribution. XRD and FT-Raman analyses revealed that the BaMoO₄ powders are free of secondary phases and crystallize in a tetragonal structure. UV-vis was employed to determine the optical band gap of this material. PL measurements at room temperature exhibited a maximum emission around 542 nm (green emission) when excited with 488 nm wavelength. This PL behavior was attributed to the existence of intrinsic distortions into the [MoO₄] tetrahedron groups in the lattice.

Full text of DOI:10.1016/j.jpcs.2008.06.107

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Journal of Physics and Chemistry of Solids 69 (2008) 2674– 2680
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BaMoO powders processed in domestic microwave-hydrothermal:
4
Synthesis, characterization and photoluminescence at room temperature
a,Ã
a
a
a
a
b
b
L.S. Cavalcante , J.C. Sczancoski , R.L. Tranquilin , M.R. Joya , P.S. Pizani , J.A. Varela , E. Longo
a
LIEC-UFSCar, Departamento de Qu ´ı mica e F ´ı sica, P.O. Box 676, 13565-905 S ˜a o Carlos, SP, Brazil
b
LIEC-UNESP, Instituto de de Qu ´ı mica, P.O. Box 355, 14801-907 Araraquara, 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, BaMoO4 powders were prepared by the coprecipitation method and processed in a
domestic microwave-hydrothermal. The obtained powders were characterized by X-ray diffraction
Received 19 December 2007
Received in revised form
(
XRD), Fourier transform Raman (FT-Raman) spectroscopy, ultraviolet–visible (UV–vis) absorption
spectroscopy and photoluminescence (PL) measurements. The morphology of these powders were
investigated by scanning electron microscopy (SEM). SEM micrographs showed that the BaMoO
powders present a polydisperse particle size distribution. XRD and FT-Raman analyses revealed that the
BaMoO powders are free of secondary phases and crystallize in a tetragonal structure. UV–vis was
employed to determine the optical band gap of this material. PL measurements at room temperature
exhibited maximum emission around 542 nm (green emission) when excited with 488 nm
17 May 2008
Accepted 17 June 2008
4
Keywords:
4
A. Optical materials
B. Chemical synthesis
C. Electron microscopy
D. Luminescence
a
wavelength. This PL behavior was attributed to the existence of intrinsic distortions into the [MoO
tetrahedron groups in the lattice.
4
]
D. Optical properties
&
2008 Elsevier Ltd. All rights reserved.
1
. Introduction
complex and solvothermal methods to accelerate the reaction
process in the formation of molybdates with scheelite-type
structure [13,14]. In the case of microwave-hydrothermal proces-
sing, the high frequency electromagnetic radiation interacts with
Barium molybdate, BaMoO
4
, presents at room temperature a
ꢀ,
scheelite-type tetragonal structure with general formula ½AXO
4
where A ¼Ca, Sr, Ba and X ¼Mo [1,2]. This compound has been
2
the permanent dipole of the liquid (H O), which initiates a rapid
prepared by the conventional solid-state reaction between BaCO
3
heating from the resultant molecular rotation. Likewise, perma-
nent or induced dipoles in the dispersed phase cause rapid
heating of the particles. These result in a reaction temperature in
excess of the surrounding liquid-localized superheating [15].
Thus, microwave radiation is able to promote the formation of
materials with short processing time and reduced electric energy
cost [16].
The literature has reported the formation of several amorphous
compounds based on different metal oxianions with intense
photoluminescence (PL) at room temperature [17–19]. In parti-
cular, an important material with scheelite-type structure is the
ꢂ
and MoO
3
at high temperatures (ꢁ1200 C) [3]. BaMoO
4
obtained
for this method presents several problems, such as: inhomogene-
ity, contamination by impurities and nonuniform particle size
distribution [4]. Other methods also have been employed to
4
prepare the BaMoO , including as flux crystallization [5],
Czochralski technique [6] and spontaneous crystallization (SC) [7].
However, the main drawback of these techniques is the high
processing temperature. To minimize these problems, wet
chemical methods have been utilized, including polymerized
complex [8], citrate complex method [9] and electrochemical
method [10]. In addition, hydrothermal processing is considered
an efficient and versatile method for the formation of several
nanostructured materials with different morphologies and sizes.
Recently, methods as solvothermal [11] and conventional hydro-
4
BaMoO , which presents a high technological potential due to
their blue photoluminescence (PL) property [20]. Recently,
Marques et al. [8,21] reported the PL properties of amorphous
BaMoO
in well-crystallized BaMoO
Therefore, in this paper, we report on the preparation of
4
in thin film and powders. PL properties were also verified
thermal (CHT) [12] have been used to process BaMoO
temperatures in the range from 100 to 200 C for times from 12
to 24 h. The microwave radiation has been employed in the citrate
4
at
4
films [22].
ꢂ
BaMoO
4
powders by the coprecipitation method and processed in
ꢂ
a domestic microwave-hydrothermal (DMW-HT) at 140 C for 1 h.
These powders were structurally characterized by X-ray diffrac-
tion (XRD) and Fourier transform Raman (FT-Raman) spectro-
scopy. The morphology was investigated by scanning electronic
microscopy (SEM). The optical properties were analyzed by
Ã
Corresponding author. Tel.: +5516 33615215, Mob.: +5516 9176 4943;
fax: +5516 33518214.
E-mail address: laeciosc@bol.com.br (L.S. Cavalcante).
0
022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2008.06.107

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2675
ultraviolet–visible (UV–vis) absorption spectroscopy and photo-
luminescence (PL) measurements at room temperature.
collected and washed with deionized water several times and
then slowly dried at 85 C for some hours.
ꢂ
2
. Experimental details
.1. Synthesis and processing of BaMoO
BaMoO powders were synthesized by the coprecipitation
2.2. Characterizations
2
4
powders
BaMoO
4
powders were characterized by XRD patterns re-
radiation in
the 2y range from 5 to 75 with 0:02 =min. FT-Raman spectro-
corded on a (Rigaku-DMax 2500PC, Japan) with Cu-K
a
ꢂ
ꢂ
ꢂ
4
ꢂ
method [23] and processed in a DMW-HT at 140 C for 1 h in the
presence of polyethylene glycol (PEG). A typical experimental
procedure to obtain BaMoO
scopy was performed on Bruker-RFS 100, Germany. The spectra
were obtained using a 1064 nm line of an Nd:YAG laser, keeping
their maximum output power at 55 mW. The morphology of these
powders was observed through an scanning electron microscopy
[DSM940A, Carl Zeiss, Germany]. UV–vis spectroscopy was taken
using a (Cary 5G equipment, USA). PL spectra were taken with a
(U1000 Jobin-Yvon, France) double monochromator coupled to a
cooled GaAs photomultiplier with conventional photon counting
system. The 488.0 nm excitation wavelength of an argon ion laser
was used, keeping their maximum output power at 25 mW.
A cylindrical lens was used to prevent overheating of the powders.
4
powders is illustrated in Fig. 1.
ꢄ3
About 5 ꢃ10 mol of molybdic acid [H
2
MoO
] (99.5% purity,
, Synth) and 0.1 g of PEG (Mw
00) (99.9% purity, Sigma-Aldrich) were used as raw materials.
4
] (85% purity,
ꢄ3
Synth), 5 ꢃ10 mol of barium nitrate [Ba(NO
3 2
)
4 3
Sigma-Aldrich), NH OH (30% in NH
2
The solubilization of tungstic acid and barium salt in water can be
observed in Fig. 1. The insert shows the precipitation reaction
between H
particles of BaMoO
Ba(NO was stirred for 1 h. This procedure was employed to
accelerate the coprecipitation rate of BaMoO . Then, the following,
mL of NH OH was added in this solution to intensify the
hydrolysis rate between the metal acid and the salt in water. In
2
MoO
4
and Ba(NO
3
)
2
for the formation of the first
4
. This solution containing H
2
MoO and
4
)
2
The slit width used was 100
performed at room temperature.
mm. All measurements were
3
4
5
4
2
þ
this case, the Ba
cations are accept species of electron pairs
3. Results and discussion
2
4
ꢄ
(Lewis acid) while MoO
are donor species of electron pairs
2þ
(Lewis base). The reactions between these two species Ba
:
3.1. X-ray diffraction analysis
2ꢄ
MO4 form a covalent bond. The covalent bond occurs due to the
Lewis acid to occupy the lowest molecular orbital (LUMO), which
interacts with the highest molecular orbital (HOMO) of the Lewis
base. Afterwards 0.1 g of PEG was added into this solution to
Fig. 2 shows the XRD patterns of BaMoO
4
powders processed in
ꢂ
DMW-HT at 140 C for 1 h.
The presence of diffraction peaks can be used to evaluate the
structural order at long-range or periodicity of the material. All
diffraction peaks can be indexed to the scheelite-type tetragonal
promote
a interaction between the small particles. In the
sequence, the chemical solution was stirred for 30 min in
ultrasound at room temperature was transferred into a Teflon
autoclave, which was sealed and placed into
1
structure with space group I4 =a [24]. No additional phase peaks
a
DMW-HT
were observed. The lattice parameters were calculated using
the least square refinement from the UNITCELL-97 program [25].
(2.45 GHz, maximum power of 800 W). This system was kept at
ꢂ ꢂ
40 C for 1 h, using a heating rate fixed at 25 C=min. The
1
˚
The obtained lattice parameters were a ¼b ¼5:5696 A and
pressure into the autoclave was stabilized at 3.92 bar. After
microwave-hydrothermal processing, the autoclave was cooled
to room temperature naturally. The obtained powders were then
3
˚
˚
c ¼12:7865 A with an unit cell volume of 396:642ð8ÞA . These
values are in agreement with those reported in the literature and
with the respective JCPDS (Joint Committee on Powder Diffraction
Standards) card No. 29-0193 [26].
Fig. 2. XRD patterns of BaMoO
4
powders processed in a domestic microwave-
ꢂ
Fig. 1. Flow-chart illustrating the experimental procedure for the formation of
hydrothermal at 140 C for 1 h. The vertical dashed lines indicate the position and
relative intensity of the respective JCPDS card no. 29-0193.
4
BaMoO powders.

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L.S. Cavalcante et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2674–2680
Table 1
Comparative results between the lattice parameters of BaMoO
4
obtained in this work with those reported in the literature by different methods
T
Time
(h)
Average lattice
Parameter
Method
Reference
(
1C)
a ¼b ð A˚ Þ
˚
c ðAÞ
1
9
7
200
40
72
2
5.62
12.82
Czochralski
[6]
50
00
–
–
SC
[7]
5.579(1)
5.5479(9)
–
12.811(4)
12.743(2)
–
CPM
[8]
1
150
50
40
12
12
1
Precipitation/calcination
Molten flux reaction
DMW-HT
[31]
5
[37]
1
5.569(6)
12.786(5)
[This work]
T; temperature; CPM; complex polymerization method; SC; spontaneous crystallization and DMW-HT; domestic microwave-hydrothermal.
Table 2
4
Atomic coordinates used to model the BaMoO unit cell
Atom
Site
x
y
z
Molybdenum
Barium
4a
0
0
0
4b
16f
0
0
0.5
Oxygen
0.76731
0.14013
0.08188
a ¼b ¼5:5696 A˚ and c ¼12:7865 A˚ .
Fig. 3. 1 ꢃ1 ꢃ1 unit cell of BaMoO
4
tetragonal structure.
Table 1 shows a comparative between the lattice parameters
values of BaMoO obtained in this work with other methods
4
reported in the literature.
4
As can be seen in Table 1, the preparation of BaMoO powders
in a DMW-HT system leads to a reduction of temperature and
processing time. Compared with the usual methods, the micro-
wave processing is able to promote the formation of materials
with small particle sizes, narrow particle size distribution and
high purity. Jansen et al. [27] reported that these advantages could
be attributed to the fast homogeneous nucleation and easy
dissolution of the particles in the sol–gel system.
Fig. 4. Raman spectrum of BaMoO4 powders processed in a domestic microwave-
ꢂ
hydrothermal at 140 C for 1 h.
3.3. FT-Raman spectroscopy analysis
ꢄ1
Fig. 4 shows the Raman spectrum in the range from 68 cm to
3
.2. Unit cell representation for BaMoO
4
ꢄ1
1
1
000 cm for the BaMoO
40 C for 1 h.
4
powders processed in a DMW-HT at
ꢂ
Fig. 3 illustrates the schematic representation of BaMoO
tetragonal unit cell with I4 =a space group. The Java Structure
Viewer Program (Version 1.08lite for Windows) [28] was used for
modeling this unit cell.
4
6
BaMoO
4
presents a tetragonal structure with symmetry C at
4
h
1
room temperature. According to group theory calculation, the
BaMoO presents 26 different vibrations as described by the
following equation:
4
For construction of 1 ꢃ1 ꢃ1 unit cell of BaMoO
4
as showed in
Fig. 3, we employed the structural parameters and atomic
coordinates listed in Table 2 [29,30]. Nassif et al. [31] showed
that Ba
forming a scalenohedra configuration. The oxygen coordination
G
¼3A þ5A þ5B þ3B þ5E þ5E ,
(1)
g
u
g
u
g
u
2
þ
cations are coordinated to the eight oxygen atoms,
where all the even vibrations (A
modes, while the odd modes (4A
the infrared spectra. The three B
Eq. (1) also includes one A and one E
u
g
; B
and 4E
vibrations are silent modes.
acoustic vibrations [6].
g
and E
g
) are Raman-active
u
u
) can be only verified in
6
þ
polyhedra of Mo
cations are slightly distorted tetrahedral,
u
ꢂ
ꢂ
presenting the angles between oxygen of 108:3 and 111:83 . For
visual effect, our unit cell shows only the coordination between
Mo–O atoms.
u
According to Porto and Scott [32] the molybdates are classified
into two groups, the external and internal modes. The first is

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2677
Table 3
Comparative results between the Raman-active modes of BaMoO
4
obtained in this work with those reported in the literature by different methods
M
T
t
B
g
E
g
B
g
E
g
A
g
B
g
B
g
E
g
E
g
B
g
A
g
Ref.
(
1C)
h
*
*
*
|
.
.
ꢅ
ꢅ
’
’
E
DMW-HT
CZ
140
1
78
76
78
–
109
110
107
–
140
137
141
–
190
188
190
184
325
234
325
–
327
324
326
325
346
345
346
–
340
358
359
362
791
791
791
780
838
837
838
829
892
892
891
871
[This work]
1200
700
165
40
2
1
3
[6]
CPM
[8]
MWS
[14]
2
ꢄ
2þ
M; method; T; temperature; t; time; assignments modes: ꢆ;
E;
n
ext-external modes MoO4 and Ba motions; |;
n
f:rðF
1
Þfree rotation; .;
n
2
ðEÞ; ꢅ;
n
4
ðF
2
Þ; ’;
n
3
2
ðF Þ;
n
1
ðA Þ; methods of preparation: DMW-HT; domestic microwave-hydrothermal; CPM; complex polymerization method; CZ; Czocharalski method and
1
MWS; microwave-solvothermal process and Ref:; references.
2
þ
called lattice phonon, which corresponds to the motion of Ba
The average particle size of BaMoO
m to 3:1 m present as tendency a regular lognormal
distribution. The higher frequency (%) of particles was approxi-
mately 1:3 m. This distribution is asymmetrical on the logarith-
mic scale of average particle size. This system presents the
microcrystallites with an octahedron-like morphology and poly-
disperse particle size distribution.
4
powders in the range from
cations and the rigid molecular units. The second belong to the
0:7
m
m
2ꢄ
vibration inside ½MoO
stationary. In free space, ½MoO
point symmetry T [6]. Their vibrations are composed by four
4
ꢀ
molecular units with the center mass
2ꢄ
4
ꢀ
tetrahedrons present a cubic
m
d
internal modes (
mode and one translation mode (F
Table 3 shows a comparative between the Raman modes of
BaMoO obtained in this work with those reported in the
n
1
ðA
1
Þ;
n
2
ðE
1
Þ,
n
3
ðF
2
Þand
n
4
ðF
2
Þ), one free rotation
2
) [6].
4
3.5. UV– vis absorption spectroscopy analysis
literature by different methods.
In Table 3, it was verified that all Raman-active modes are
Fig. 7 shows a typical UV–vis spectrum of BaMoO
processed in a DMW-HT at 140 C for 1 h. The obtained optical
band gap value is shown in Fig. 7 and listed in Table 4.
4
powders
characteristic of BaMoO
the literature [6,8]. Therefore, these results confirm the tetragonal
structure for the BaMoO powders. The small variations can be
4
phase in agreement with the reported in
ꢂ
4
g
The optical band gap (E ) was estimated by the method
associated with the preparation method, average crystal size and
structural order degree. The presence of Raman-active modes can
be used to evaluate the structural order at short-range of the
materials. Thongtem et al. [14] observed the presence of only six
proposed by Wood and Tauc [36]. According to these authors, the
optical band gap energy is associated with absorbance and photon
energy by the following equation:
opt 1=2
g
Raman-active modes for the BaMoO
microwave radiation. Thus, our results show the presence of
Raman-active modes indicated that the BaMoO powders are
4
formed through cyclic
h
na /ðh
n
ꢄE Þ
,
(2)
where
a
is the absorbance, h is the Planck constant,
n
is the
4
opt
frequency, and Eg is the optical band gap.
completely ordered at short-range. Possibly, these differences are
caused by the preparation method, geometry and/or particle size.
In this case, E value was determinate extrapolating the linear
g
portion of the curve or tail. The combination between the optical
band gap and PL measurements allows to correlated that
electronic transitions in the materials. UV–vis measurements
3
.4. Scanning electron microscope analysis
4
revealed a typical value of 4.10 eV for the BaMoO powders. This
observed behavior can be associated with the energy difference
between the valence band and conduction band for this material.
The obtained result was similar to that reported by Afanasiev [37]
and Eng et al. [38]. The fast processing in microwave-hydro-
BaMoO
through the scanning electron microscope (SEM), as shown in Fig. 5.
Fig. 5(a) shows the SEM micrographs of the BaMoO powders.
The micrographs revealed the presence of BaMoO powders with
agglomerate nature and with different particle size distribution.
A projection set of BaMoO powders is indicated by the dotted
4
powders with octahedral morphology was observed
4
4
thermal can lead to the formation of possible defects in BaMoO
4
lattice, thus promoting the appearance of intermediate electronic
levels in the band gap. Table 4 shows a comparative between
4
white circle (see Fig. 5(b)). Therefore, obtained micrographs show
that the microwave irradiation contribute significantly for the
g
4
E values of BaMoO obtained in this work with those reported in
the literature by different methods.
4
formation of BaMoO powders with short processing time. The PEG
The differences verified in the optical band gap values can be
related with the different preparation methods, shape, average
crystal size and structural order–disorder degree in the lattice. As
promoted a increase in the aggregation process of small particles or
nucleation seeds on the surface through the lateral interaction of
hydrogen bonding of water with the OH groups of this polymer. In
Fig. 5(c) the dotted white circles show four different regions of
can be seen in Table 4, the E
by Marques et al. for BaMoO
et al. [38] the E values in material is dependent upon the
g
value is slightly lower than obtained
4
thin films [21]. According to Eng
4
BaMoO powders. As can be seen in this figure, initially several
g
small particles or seeds act as small nucleation centers during the
DMW-HT processing (see Fig. 5(d)). Fig. 5(e) shows the poly-
electronegativity of the transition metal ion, connectivity of the
polyhedra and deviations in the O–Mo–O bonds.
4
disperse particle size distribution of BaMoO powders processed in
a DMW-HT system. The coalescence process is indicated by dotted
white ellipses. A possible growth mechanism occurs through the
coalescence process between small and large particles during the
DMW-HT processing (arrows in Fig. 5(f)). Our proposed mechanism
for the formation of particles with octahedron-like morphology is
in agreement with those reported in the literature [33–35].
The average particle size distribution were obtained through
the SEM micrographs by the counting of approximately 102
particles, as illustrated in Fig. 6.
3.6. PL analysis
Fig. 8 shows the PL spectrum of BaMoO
a DMW-HT at 140 C for 1 h. The maximum (PL) emission was
observed at around 542 nm (green emission).
PL measurements were realized at room temperature
because this optical property behavior can be influenced by
the temperature. Moreover, PL measurements at different
4
powders processed in
ꢂ

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L.S. Cavalcante et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2674–2680
ꢂ
Fig. 5. SEM micrographs: (a) BaMoO
4
powders processed in DMW-HT at 140 C for 1 h, (b) selected region of an agglomerated of BaMoO
4
powders, (c) polydisperse particle
size distribution, (d) nucleation process by small seeds, (e) coalescence process and (f) possible growth mechanism of BaMoO
hydrothermal.
4
powders in a domenstic microwave-
temperatures generally require expensive and sophisticated
equipments. Also, it is technologically more interesting the
materials with PL emissions at room temperature for the
development of electrooptic devices, including DVD player lasers,
cell phone displays, photocopying lamps and so on. The literature
has reported explanations on PL emissions of molybdates. Lan and
emission in calcium molybdate and attributed this behavior to the
intrinsic distortions into the [MoO ] tetrahedron groups. In other
work, Wu et al. [20] investigated the blue PL emission at room
temperature of BaMoO microcrystals and show that slightly
distorted tetrahedral symmetry leads to an structured absorption
band for the A
!T1ð2Þtransition. Marques et al. [40] attributed
the green PL emission to structural disorder in the MoO cluster
for the BaMoO powders obtained by the complex polymerization
method. Liu et al. [41] attributed to the charge-transfer transitions
4
4
1
Blasse [39] prepared BaMoO
solutions. These authors observed that BaMoO
4
using Ba(NO
3
)
2
and Na
exhibit a weak
2
MoO
4
4
4
4
ꢂ
green emission (around 530 nm) at ꢄ268:8 C. This green emission
2
ꢄ
was also verified by Spassky et al. [7], with the maximum
within the ½MoO
4
ꢀ
complex to mainly factor to green PL
ꢂ
emission around 520 nm at ꢄ263 C. Both authors attributed the
emission in barium molybdate. We believe that the green PL
emission at room temperature in barium molybdate is linked a
2
4
ꢄ
green luminescence of BaMoO with the intrinsic MoO group.
4
Ryu et al. [9] reported the great dependence of the PL properties
on the morphology and crystallinity of the BaMoO powders.
Recently, Campos et al. [17] observed and reported the green PL
several factors, such as: distortions on the [MoO
4
] tetrahedron
ꢂ
4
groups caused by the different angles between O–Mo–O (108:3
ꢂ
and 111:8 ), particle sizes, crystallinity degree, morphology and

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2679
Fig. 6. Average particle size distribution of BaMoO
4
powders processed in a
ꢂ
domestic microwave-hydrothermal at 140 C for 1 h.
Fig. 8. PL spectrum at room temperature of BaMoO
4
powders processed in a
ꢂ
domestic microwave-hydrothermal at 140 C for 1 h excited with the 488 nm line
of an argon ion laser.
spectroscopy revealed a characteristic optical band gap of 4.10 eV.
4
SEM micrographs showed that the BaMoO powders present a
polydisperse particle size distribution. These micrographs also
showed that the coalescence mechanism is responsible by the
growth process of BaMoO
Intense PL emission at room temperature was attributed to the
distortions on the [MoO ] tetrahedron groups, particle sizes,
4
with octahedron-like morphology.
4
crystallinity degree, morphology and surface defects.
Acknowledgments
The authors thank the financial support of the Brazilian
research financing institutions: CAPES, FAPESP, CNPq and also
Prof. Dr. Dawy Keyson-UFPB and Dr. Diogo P. Volanti by adaptation
of the hydrothermal reactor in the domestic microwave.
References
Fig. 7. UV–vis absorbance spectra for the BaMoO
4
powders processed in a
ꢂ
domestic microwave-hydrothermal at 140 C for 1 h.
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Table 4
Comparative results between the optical band gap energy of BaMoO
4
obtained in
this work with those reported in the literature by different methods
Temperature (1C)
Time (h)
140
600
2
200
4
200
16
550
12
1300
12
1
[
[
Optical gap (eV)
Reference
4.10
4.88
[21]
4.73
[21]
4.82
[21]
4.30
[37]
4.00
[38]
This work
[
[
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(
(
surface defects. In this case, these factors promote the formation
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[
[
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4
. Conclusions
In summary, BaMoO
4
powders were obtained by the copreci-
ꢂ
pitation and processed at 140 C for 1 h in a domestic microwave-
hydrothermal system. XRD patterns and FT-Raman revealed that
the BaMoO
4
powders present a scheelite-type tetragonal structure
without the presence of secondary phases. UV–vis absorption

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