Preparation, characterization and electrical conductivity studies of nanocrystalline la doped BaMoO4

Article abstract of DOI:10.1016/j.materresbull.2010.10.001

Different compositions of scheelite type nanocrystalline La doped BaMoO₄ [Ba_1-xLa_xMoO_4+x/2, where x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5] samples were prepared by acrylamide assisted sol-gel combustion process. Dried gels prepared at 60 °C were heated at different temperatures and characterized using TG/DTA, XRD, FTIR and SEM-EDX techniques. From XRD patterns, crystalline phases for La doped BaMoO₄ samples were confirmed and their average crystallite sizes were calculated using Scherrer's formula and it was found to be less than 80 nm. Structure and thermal behavior of scheelite type nanocrystalline La doped BaMoO₄ samples were identified respectively using FTIR and TG/DTA measurements. Microstructure and existence of O, La, Ba and Mo elements in the La doped BaMoO₄ samples were obtained from SEM-EDX and HRTEMtechniques. The 'd' spacing values were obtained for different (h k l) planes and were well matched with the standard BaMoO₄. (h k l) values for different directions of planes were assigned for the observed HRTEM images and were matched with standard BaMoO₄. Grain and grain boundary conductivities were evaluated by analyzing the impedance data, using the Winfit software, measured at different temperatures.

Full text of DOI:10.1016/j.materresbull.2010.10.001

Materials Research Bulletin 46 (2011) 32–41
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Preparation, characterization and electrical conductivity studies
of nanocrystalline La doped BaMoO4
a
a
a,
b
N. Nallamuthu , I. Prakash , N. Satyanarayana *, M. Venkateswarlu
a
Department of Physics, Pondicherry University, Pondicherry 605 014, India
b
R & D, Amara Raja Batteries Ltd, Tirupati 517520, AP, India
A R T I C L E I N F O
A B S T R A C T
Different compositions of scheelite type nanocrystalline La doped BaMoO4 [Ba La MoO4+x/2, where
Article history:
1ꢁx
x
Received 3 March 2010
x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5] samples were prepared by acrylamide assisted sol–gel combustion
Received in revised form 3 September 2010
Accepted 1 October 2010
Available online 30 October 2010
process. Dried gels prepared at 60 8C were heated at different temperatures and characterized using TG/
DTA, XRD, FTIR and SEM-EDX techniques. From XRD patterns, crystalline phases for La doped BaMoO
4
samples were confirmed and their average crystallite sizes were calculated using Scherrer’s formula and
it was found to be less than 80 nm. Structure and thermal behavior of scheelite type nanocrystalline La
Keywords:
doped BaMoO
Microstructure and existence of O, La, Ba and Mo elements in the La doped BaMoO
obtained from SEM-EDX and HRTEMtechniques. The ‘d’ spacing values were obtained for different (h k l)
planes and were well matched with the standard BaMoO . (h k l) values for different directions of planes
were assigned for the observed HRTEM images and were matched with standard BaMoO . Grain and
4
samples were identified respectively using FTIR and TG/DTA measurements.
A. Nanostructures
4
samples were
B. Sol–gel chemistry
C. X-ray diffraction
C. Infrared spectroscopy
C. Impedance spectroscopy
4
4
grain boundary conductivities were evaluated by analyzing the impedance data, using the Winfit
software, measured at different temperatures.
ß 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
Generally, oxygen ion conductors are much imperative
conductivity between 500 8C and 900 8C [7]. Nanocrystalline metal
oxide compounds have small grain size, which increases ionic
conductivity of material and also stabilize crystal structure at high
temperatures. In recent years, nanostructured ceramic materials
have been extensively investigated due to the presence of a large
fraction of grain boundaries that can lead to enhanced electrical,
magnetic, mechanical, optical, sensing, and biomedical properties
compared with their respective microstructured materials [8].
Nanocrystalline scheelite type metal oxide can be prepared using
different techniques. Preparation using conventional solid state
reaction method involves high temperature heating of oxide and/
or carbonate precursors, which exhibit more disadvantages like
inhomogeneity, abnormal grain growth, poor stoichiometry, etc.
[9]. These difficulties could be avoided using wet chemical
methods, such as sol–gel process [10,11], sol–gel combustion
process [12], co-precipitation, hydrothermal reaction, etc. Sol–gel
combustion is very simple, cost effective and versatile process for
the preparation of muticomponent nanocrystalline metal oxides,
in which citric acid is used as a fuel as well as chelating agent [12–
15]. However, the nanocrystalline metal oxides prepared using
citric acid assisted gel combustion process has some drawbacks
such as presence of impurity phase as well as residual organics due
to the prolonged multi step combustion mechanism [16]. The
acrylamide (dual fuel) assisted combustion process, in which
acrylamide acts as a fuel along with citric acid, yields phase pure
materials and can be used intensively in various technological
ionic devices such as solid oxide fuel cells, oxygen sensors,
electrochemical oxygen pumps, etc. [1,2]. Several families of
oxygen ion conductors are being investigated for intermediate
temperature solid oxide fuel cells (ITSOFCs) like fluorite type
(
stabilized ZrO
BaCeO and SrCeO
type (BIMEVOX), pyrochlore type (Gd
PbWO ) oxides [3–5]. Among these, scheelite type oxide materials
2
, CeO
), brownmillerite type (Ba
Zr ) and scheelite type
2
and
d
-Bi
2
O
3
), pervoksite type (LaGaO
3
,
3
3
2
2 3
In O ), Aurivillius
2
2 7
O
(
4
exhibit high ion conductivity, which are comparable with
conventional oxide ion conductors like yttria stabilized zirconia.
Esaka et al. systematically investigated composition dependence of
electrical conductivity for scheelite type PbWO
4
materials and
ꢁ2
ꢁ1
observed high electrical conductivity 4.2 ꢀ 10 S cm at 800 8C
for Pb0.8La0.2WO4.1 [6]. Thangadurai et al. prepared ABO
Sr, Ba; B = Mo, W) scheelite type materials and noticed that
scheelite type PbWO and SrWO materials showed high electrical
4
(A = Ca,
4
4
*
Corresponding author. Tel.: +91 413 2654404; fax: +91 413 2655260/11.
E-mail addresses: nallanis2010@yahoo.com, ionics2003@yahoo.co.in
N. Satyanarayana).
(
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.10.001

N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
33
organic free nanocrystalline metal oxides at lower temperature. In
the present study, different compositions of La doped BaMoO
composition of La doped BaMoO
ray fluorescence spectrometry (Bruker S4-Pioneer model). The
synthesized La doped nanocrystalline BaMoO powder was
4
samples are observed using X-
4
[
Ba1ꢁxLa
x
MoO4+x/2 (x = 0 (BaMoO
4
), 0.1 (BLM1), 0.2 (BLM2), 0.3
4
(
BLM3), 0.4 (BLM4) and 0.5 (BLM5))] samples are prepared using
pressed into 10 mm diameter and 2–3 mm thickness pellet at
5000 kg/cm using KBr press. Silver paste is painted on both side of
2
acrylamide assisted sol–gel combustion process and are charac-
terized using TG/DTA, XRD, FTIR and SEM-EDX techniques. Ion
transport is studied by calculating the electrical conductivity from
measured impedance data at different temperatures for the
4
sintered La doped BaMoO pellet sample as electrodes and heated
at 200 8C for half an hour to ensure maximum contact and
0
00
adherence. Real (Z ) and imaginary (Z ) parts of the impedance data
are measured for pellets using Novocontrol Alpha High
sintered La doped BaMoO
4
pellets.
A
performance frequency analyzer in the frequency range 0.1 Hz
to 10 MHz at different temperatures. The impedance data are
analyzed using Win fit software to obtain the bulk resistance for
calculating the conductivity and activation energy of the nano-
2
. Experimental method
2.1. Sol–gel combustion process
crystalline La doped BaMoO
3. Results and discussion
3.1. TG/DTA
4
samples.
Different compositions of La doped nanocrystalline BaMoO
4
[
x
Ba1ꢁxLa MoO4+x/2 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5)] samples were
synthesized using acrylamide assisted sol–gel combustion process.
Analar grade precursor chemicals such as barium nitrate (Quali-
gens), lanthanum nitrate (SRL), ammonium molybdate (loba
chemi) with acrylamide [Ac] (Qualigens) and citric acid [CA]
Fig. 1a–f shows the TG/DTA thermograms for the different
compositions of La doped BaMoO gel samples. The observed two
(
Qualigens) as fuels were used in sol–gel process. The precursor
4
chemicals (barium nitrate, lanthanum nitrate and ammonium
molybdate) are taken according to their respective molecular
weight percentages and the fuels (acrylamide and citric acid) are
also varied according to molar ratio of metal ions [M] to fuel ratio
wide endothermic peaks between 40 8C and ꢂ150 8C and the
observed weight losses ꢂ5% for the dried gel samples are due to the
evaporation of water molecules and other organic residues existing
in the sample. In Fig. 1a, one exothermic peak is observed at 178 8C
and the corresponding weight loss of ꢂ35% is due to the removal of
remaining organic residues along with the evaporation of nitrate
groups, which is also confirmed from the FTIR results. The narrow
exothermic peak at ꢂ575 8C and the corresponding weight loss
ꢂ2% is due to the combustion reaction as shown in the TG/DTA
curves. From Fig. 1, in all TG/DTA curves, the observed exothermic
curve at 178 8C is due to the decomposition of citric acid and
decomposition of nitrates from barium nitrate as well as from
lanthanum nitrate. The observed exothermic DTA curve is
intensified and split into two curves and also shift towards higher
temperature with the increase of lanthanum content in the
(M:Ac:CA = 1:1:1). Required quantity of barium nitrate is dissolved
in distilled water and mixed with citric acid and acrylamide
solutions. Lanthanum nitrate solution is prepared by dissolving the
required amount of lanthanum nitrate in the distilled water.
Ammonium molybdate is added to distilled water and stirred to
form the transparent solution. Lanthanum nitrate solution is
mixed with the previous barium nitrate solution under constant
stirring. Half an hour later, ammonium molybdate solution is also
added to the previous mixture solution and stirred continuously at
8
0 8C, till the formation of the gel. Prepared gels are dried and
characterized using TG/DTA. Dried gels are heated at various
temperatures 60 8C, 150 8C, 250 8C, 350 8C, 500 8C, 600 8C, 700 8C,
BaMoO
of barium nitrate and lanthanum nitrate and hence, decomposed at
two different temperatures. Doping of lanthanum in the BaMoO
4
sample. This may be due to the different binding energies
8
00 8C and 900 8C and characterized using FTIR, XRD and SEM-EDX
techniques.
4
increases, the total quantity of nitrates in the precursor, which
increases the oxidant ratio and helpful for combustion reaction.
The exothermic curve observed at ꢂ573 8C may be due to the
decomposition of metal acrylate complex, which might have
formed from acrylamide and metal nitrates during calcinations
and it is also confirmed from the observed FTIR bands, and
unreacted organic residuals. The heat generated from the
exothermic reaction and the corresponding weight loss are not
vary with increase of lanthanum content and may depend upon the
nature of the combustion, which is influenced by other factors such
as porous nature, volume expansion, and the weight of the
polymeric intermediate. However, after 600 8C, there is no weight
loss is observed in the TG curve, which indicates that there is
complete decomposition of organic derivatives from the sample
and the same is also confirmed from FTIR and XRD results.
2
.2. Measurement techniques
TG/DTA curves of the dried gel samples are recorded at the
heating rate of 10 8C/min between 30 8C and 900 8C in nitrogen
atmosphere using TA instruments SDT Q600 V20.5. Thin transpar-
ent pellet samples were prepared using dried gel samples calcined
at 60 8C, 150 8C, 250 8C, 350 8C, 500 8C, 600 8C, 700 8C, 800 8C and
9
1
00 8C, and grounded well with spectra pure KBr powder taken in
:20 ratio. Fourier Transform Infrared (FTIR) Spectra are recorded
ꢁ
1
using Thermo Nicolet FTIR-6700 spectrometer from 4000 cm to
ꢁ1
4
00 cm for 32 scans. Powder XRD (X-ray diffraction) patterns are
recorded using X’Pert PRO MPD, PANalytical (Philips) X-ray
powder diffractometer employing Cu K radiation. The average
crystallite sizes of different compositions of La doped BaMoO
samples are calculated using Scherrer’s formula. Microstructures
of La doped BaMoO powder samples are obtained using scanning
electron microscopy (SEM) [Hitachi—450 model]. Fine powdered
La doped BaMoO samples and also the broken sintered La doped
BaMoO sample pellets are spread separately on conducting
a
4
4
3.2. FTIR
4
4
Fig. 2 shows the FTIR spectra recorded for the BaMoO dried gel
4
samples calcined at 60 8C, 150 8C, 250 8C, 350 8C, 500 8C, 600 8C,
700 8C, 800 8C and 900 8C. In Fig. 2, the observed major IR bands
carbon tape pasted over the aluminium stub. Later, they are coated
with a thin layer of Gold using sputter coater for taking SEM
images. HRTEM images were taken for BaMoO
Transmission Electron Microscope of JOEL 2010F, HRTEM, Japan,
with 200 kV as operating voltage. Barium molybdate powder,
obtained at 900 8C, is mixed with acetone and spread over a copper
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
are 3440 cm , 3210 cm , 1650 cm , 1390 cm , 1220 cm ,
ꢁ1
ꢁ1
ꢁ1
4
sample using
1050 cm , 903 cm and 850 cm . The IR bands at 3440 cm
ꢁ
1
and 1650 cm are respectively, attributed to the stretching and
bending vibrational modes of O–H of molecular water and
ꢁ
1
3210 cm
is due to the asymmetric deformation vibration of
CH groups and NH
ꢁ
1
grid. The presence of La
2
O
3
, BaO and MoO
3
for the various
3
3
groups [17]. The band at 1390 cm
is

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0
.0
110
3
2
1
0
1
2
3
1
9
00
0
BLM1
1
9
8
00
0
-
-
-
-
-
-
0.2
0.4
0.6
0.8
1.0
1.2
BaMoO4
80
0
7
6
5
0
0
0
70
-
-
-
6
5
4
3
0
0
0
0
40
30
-4
2
00
400
600
o
800
200
400
600
o
800
Temperature ( C)
Temperature ( C)
2
1
1
00
1
8
6
4
00
0
-
-
-
-
-
-
2
3
4
5
6
7
BLM2
BLM3
0
80
-1
-2
6
0
0
-
-
-
3
4
5
40
20
0
2
00
400
600
o
800
200
400
600
o
800
Temperature ( C)
Temperature ( C)
1
4
100
BLM4
100
90
3
2
1
0
1
2
3
4
9
8
7
0
0
0
0
1
2
80
-
BLM5
7
6
5
4
0
0
0
0
-
-
-
-
-
60
50
-3
4
3
0
0
-4
30
2
00
400
600
o
800
200
400
600
o
800
Temperature ( C)
Temperature ( C)
Fig. 1. TG/DTA curves for the dried gels of various compositions of La doped BaMoO
BLM4) and 0.5 (BLM5)] samples.
4
[Ba1ꢁxLa
x
MoO4+(x/2), where x = 0 (BaMoO
4
), 0.1 (BLM1), 0.2 (BLM2), 0.3 (BLM3), 0.4
(
ꢁ
ꢁ1
ꢁ1
corresponds to the asymmetric stretching vibration of COO
groups and also the presence of nitrate groups in the dried gel
showed a decrease in the intensity of the bands at 3440 cm
,
,
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
1620 cm , 1430 cm , 935 cm , 685 cm , 612 cm , 519 cm
ꢁ
1
ꢁ1
ꢁ1
sample. The IR bands at 1220 cm and 1050 cm is formed due to
450 cm , which are due to the removal of molecular water,
2
ꢁ
the CO
17]. The observed bands at 903 cm and 850 cm are due to the
Mo–O stretching vibration and are attributed to the formation of
MoO in BaMoO sample.
At 150 8C and 250 8C, FTIR spectra showed the disappearance of
3
functional groups indicate the formation of carboxylate
nitrate groups and also some organic residues existing in the
ꢁ1
ꢁ1
ꢁ1
[
sample. The appearance of new IR bands at 903 cm
and
ꢁ
1
850 cm are corresponding to the formation of MoO
sample. Fig. 3 shows the FTIR spectra for various compositions of
La doped BaMoO dried gel samples, calcined at 900 8C. The
appearance of the bands at 903 cm and 850 cm are attributed
3 4
in BaMoO
3
4
4
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
the IR bands at 3210 cm , 1220 cm , and 1050 cm
which
indicate the removal of the organic residues. The IR band at
to the formation of MoO
640 cm is assigned to the La–O network formation. In Fig. 3, for
all the compositions of lanthanum doped barium molybdate
3
and also the observed IR band at
ꢁ
1
ꢁ1
1
390 cm is attributed to the presence of nitrate groups. The
ꢁ1
ꢁ1
observed bands at 903 cm and 850 cm are due to the Mo–O
stretching vibration. At 500 8C, the IR bands at 3440 cm and
1
ꢁ1
ꢁ1
samples, the appearance of the IR peak at 850 cm are attributed
ꢁ
1
650 cm are due to the presence of water molecules and the
to the Mo–O stretching vibration and also the showed IR bands at
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
additional IR bands are also appeared at 1440 cm , 935 cm
,
940 cm
and 640 cm
are assigned to the La–O network
ꢁ ꢁ1 ꢁ1 ꢁ1
1
ꢁ1.
8
1
27 cm , 685 cm , 612 cm , 519 cm , 450 cm The bands at
formation [19]. For Ba0.5La0.5MoO4.25 (BLM5) sample, the ob-
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
440 cm , 670 cm and 620 cm are due to 55 CH
2
deformation
served additional IR peaks at 750 cm and 450 cm are observed
stronger than the other compositions, which may due to the
formation of the La–O–Ba bond [20]. When the dopant quantity
increases, the defects concentrations increases and hence, the
observed shift in the IR bands at 750 cm and 450 cm may due
to the deformed Mo–O and La–O network. Further, the formation
of phase and microstructure are conformed from XRD, SEM-EDX
and HRTEM results.
ꢁ
1
vibration at acrylamide groups [18]. The IR band at 519 cm and
4
ꢁ1
50 cm are due to the wagging and stretching vibrations of M–
OH and M–O bonds. The IR band at 935 is attributed to the NH
2
2
ꢁ1
ꢁ1
wagging vibrations. The above IR bands are due to the presence of
organic residues, which are formed due to the participation of
ꢁ1
acrylamide in the reaction. The IR peak 827 cm is attributed to
Mo–O stretching vibration. At 600 8C and above, the FTIR spectra

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o
9
00 C
o
9
00 C
o
o
8
00 C
800 C
o
7
00 C
o
7
00 C
o
o
600 C
6
00 C
BaMoO ICDD (00-029-0193)
4
o
500 C
o
5
00 C
o
3
50 C
o
3
50 C
o
o
2
50 C
2
50 C
o
Barium nitrate ICDD (00-001-1144)
1
50 C
1
0
20
30
40
50
60
70
80
o
6
0 C
2
θ (degree)
4
Fig. 4. XRD patterns for the BaMoO gel sample, calcined at various temperatures.
4
000
3000
2000
1000
-1
Wave number (cm )
800 8C and 900 8C. At 250 8C, the observed crystalline peaks in the
Fig. 2. FTIR spectra for the BaMoO
4
dried gel sample, calcined at various
XRD pattern are related to the barium nitrate crystalline phase and
it is confirmed from ICDD data (00-011-1144). At 350 8C, the
obtained XRD peaks are matched with the mixed phases of barium
nitrate and also the barium molybdate, which are also confirmed
from the ICDD data (00-011-1144 for barium nitrate and 00-029-
0193 for barium molybdate). At higher calcined temperatures from
temperatures.
3.3. XRD
Fig. 4 shows the XRD patterns for the BaMoO
4
sample calcined
at different temperatures 250 8C, 350 8C, 500 8C, 600 8C, 700 8C,
[()TD$FIG]
6
00 8C to 900 8C, scheelite type pure crystalline BaMoO
formed. The crystalline size of the BaMoO sample is calculated
using Scherrer’s formula: D = 0.9 /( cos ), where is the X-ray
wave length (0.15418 nm), is full width half maximum (FWHM)
of the peak. At 900 8C, the average crystalline size of BaMoO is
found to be ꢂ60 nm. Fig. 5 shows the XRD patterns for the various
compositions of La doped BaMoO samples obtained at 900 8C.
From Fig. 5, all the observed peaks are analyzed by comparing with
the ICDD data (00-029-0193 for BaMoO ). In Fig. 5, the additional
4
phase is
4
l
b
u
l
b
4
4
4
[)(TD$FIG]
-
2 2 9
La Mo O (00-023-1145)
BLM5
BLM4
BLM3
BLM2
BLM1
BaMoO4 ICDD 00-029-0193
2
0
40 60 80
2
theta (degree)
Fig. 3. FTIR spectra for various compositions of La doped BaMoO
at 900 8C.
4
samples, obtained
Fig. 5. XRD patterns for various compositions of La doped BaMoO
obtained at 900 8C.
4
samples,

3
6
N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
observed crystalline peaks in the XRD patterns for the higher
4
relative density of BaMoO sintered pellet at 900 8C is found to be
3+
concentrations of La (starts from Ba0.8La0.2MoO4.1) are matched
with La Mo (ICDD Ref no: 00-023-1145). The XRD patterns for
the higher compositions (BLM3, BLM4 and BLM5) of La doped
BaMoO sample confirmed the formation of mixed BaMoO and
La Mo crystalline phases The average crystallite sizes of all
compositions of La doped BaMoO samples at 900 8C are found to
be <100 nm. Furthermore, conductivity studies are made on the La
doped BaMoO sample pellets, sintered at 900 8C.
91.5% of theoretical density. The sintering behavior of the sample is
alsoinvestigatedbySEMatdifferenttemperatures(600–900 8C).The
grain size is increased and the pore sizes are decreased when
increasing the sintering temperature. SEM-EDX spectra are con-
2
2 9
O
4
4
2
2
O
9
.
firmedtheexistenceofO,MoandBaelementsintheBaMoO
Fig. 8a–e shows the EDX spectra for the different compositions of La
doped BaMoO powder samples obtained at 900 8C. The EDX spectra
4
sample.
4
4
4
showedtheexistenceofO,La,MoandBaelements.Thepercentageof
eachelement(O, Ba, Moand La)inall compositionsofthe lanthanum
doped barium molybdate samples obtained from energy dispersive
X-ray (EDX) spectra results are given in Tables 1–5.
3.4. SEM-EDX measurements
SEM images along with EDX spectra and relative density of the
BaMoO pellets,sinteredatvarioustemperaturesareshowninFigs.6
and 7 respectively. The relative density is calculated using the mass
andthicknessoftheBaMoO pellets,sinteredat600 8C,700 8C,800 8C
and900 8C. Therelativedensityof the BaMoO pelletsincreaseswith
increasing the sintered temperature from 600 8C to 900 8C. The
4
3.5. HRTEM measurements
4
4
Fig. 9a and b showed the HRTEM images of the BaMoO powder
4
sample, including their measured ‘d’ spacing for the corresponding
h k l planes. HRTEM images of barium molybdate sample, obtained
[()TD$FIG]
Fig. 6. SEM images and EDX spectra of BaMoO
4
pellets, sintered at different temperatures.

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92
90
88
86
84
5
00
600
700
800
900
Temperature (ºC)
4
Fig. 7. Relative density of BaMoO pellets, sintered at different temperatures.
at 900 8C are having the agglomerated cluster particles. The
measured average particle sizes of the barium molybdate are in the
˚
˚
range of <100 nm and the ‘d’ spacing (3.35 A and 3.2 A) measured
from the HRTEM image is matched with standard barium
molybdate with h k l planes of (1 1 2) and (0 0 4), which are also
confirmed from XRD results.
3.6. X-ray fluorescence analysis
The presence of the La
X-ray fluorescence spectra results of the various composition of La
doped BaMoO samples. Fig. 10 shows the XRF spectra for the
various compositions of lanthanum doped BaMoO samples. The
2 3 3
O , BaO and MoO are observed from the
4
4
presence of the elements, La, Ba, Mo and O are identified from the
formation of their respective characteristic peaks shown in Fig. 10.
The percentages of the existing La
2
O
3
3
, BaO and MoO for all the
composition of the La doped BaMoO
4
samples are in Table 6. The
percentage of the La
of La doping in BaMoO
2
O
3
increases and BaO decreases with increase
samples, which are also in accordance with
4
the XRD, FTIR, SEM-EDX and HRTEM results.
3.7. Electrical impedance and conductivity studies
0
0
Fig. 11a–d shows the complex impedance plots (imaginary ꢁZ
0
vs. real Z ) obtained at different temperatures for nanocrystalline
4
BaMoO sample. In the impedance plots, the observed two
overlapped semicircles analyzed by win fit software, where large
semicircle in the lower frequency region indicates the grain
boundary response and the lower semicircle in the higher
frequency region indicates the grain interior response of the
Fig. 8. (a–e) EDX spectra for various compositions of La doped BaMoO
obtained at 900 8C.
4
samples,
nanocrystalline BaMoO
4
sample. The grain interior resistance (Rgi
)
and the grain boundary resistance (Rgb) of nanocrystalline BaMoO
4
sample are obtained respectively from the intercept of the
depressed first and second semicircles with the real axis. In the
complex impedance plots, the depressed two semicircles are
represented by its equivalent circuit, which is due to the parallel
combinations of two R and C elements in series. The observed
frequency (fmax) at the maximum of the two semicircles and the
resistances of the grain and grain boundary effects are used to
calculate the grain and grain boundary capacitances for the
Table 1
Percentage of each element (O, Mo, Ba and La) for lanthanum doped barium
molybdate (Ba0.9La0.1MoO4.05) obtained from SEM-EDX.
Element
line
Net
Net counts
error
Weight %
Atom %
Formula
counts
nanocrystalline BaMoO
gi) and grain boundary conductivity (
the nanocrystalline La doped BaMoO
using the following equations obtained based on the brick layer
model [21,22]
4
sample. The grain interior conductivity
O K
72
23
72
23
36
19
46
14
27
11
7.76
–
39.10
–
O
Mo K
Mo L
Mo M
Ba L
(
s
sgb) for all compositions of
1079
19
ꢃ
26.76
–
22.50
–
Mo
Ba
La
4
samples were calculated
1515
80
ꢃ
ꢃ
ꢃ
ꢃ
56.97
–
33.46
–
Ba M
La L
216
8
8.51
–
4.94
–
1
ND
A
1 t
¼ R
La M
Total
sgi ¼ R
(1)
100.00
100.00
A
gi
gi

3
8
N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
Table 2
Table 4
Percentage of each element (O, Mo, Ba and La) for lanthanum doped barium
molybdate (Ba0.8La0.2MoO4.10) obtained from SEM-EDX.
Percentage of each element (O, Mo, Ba and La) for lanthanum doped barium
molybdate (Ba0.6La0.4MoO4.2) obtained from SEM-EDX.
Element l
ine
Net
Net counts
error
Weight %
Atom %
Formula
Element
line
Net
Net counts
error
Weight %
Atom %
Formula
counts
counts
O K
316
36
ꢃ
ꢃ
20
36
61
39
71
22
44
28
13.77
–
54.67
–
O
O K
959
43
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
35
12
73
73
64
52
70
49
23.45
–
69.70
–
O
Mo K
Mo L
Mo M
Ba L
Mo K
Mo L
Mo M
Ba L
2836
39
27.57
–
18.26
–
Mo
Ba
La
4516
637
26.12
–
12.95
–
Mo
Ba
La
3221
63
ꢃ
ꢃ
ꢃ
ꢃ
48.76
–
22.55
–
2344
847
21.58
–
7.47
–
Ba M
La L
Ba M
La L
624
133
9.90
–
4.53
–
2987
662
28.85
–
9.88
–
La M
Total
La M
Total
100.00
100.00
100.00
100.00
Table 3
Table 5
Percentage of each element (O, Mo, Ba and La) for lanthanum doped barium
molybdate (Ba0.7La0.3MoO4.15) obtained from SEM-EDX.
Percentage of each element (O, Mo, Ba and La) for lanthanum doped barium
molybdate (Ba0.5La0.5MoO4.25) obtained from SEM-EDX.
Element
line
Net
Net counts
error
Weight %
Atom %
Formula
Element
line
Net
Net counts
error
Weight %
Atom %
Formula
counts
counts
O K
380
43
ꢃ
ꢃ
23
43
64
49
77
25
54
28
13.08
–
53.45
–
O
O K
168
25
ꢃ
ꢃ
15
25
62
16
78
20
51
18
6.30
–
33.96
–
O
Mo K
Mo L
Mo M
Ba L
Mo K
Mo L
Mo M
Ba L
3118
49
25.46
–
17.35
–
Mo
Ba
La
3064
16
26.78
–
24.08
–
Mo
Ba
La
3779
82
ꢃ
ꢃ
ꢃ
ꢃ
47.17
–
22.46
–
4061
78
ꢃ
ꢃ
ꢃ
ꢃ
53.46
–
33.59
–
Ba M
La L
Ba M
La L
1093
177
14.29
–
6.73
–
978
36
13.47
–
8.37
–
La M
Total
La M
Total
100.00
100.00
100.00
100.00
[()TD$FIG]
4
Fig. 9. HRTEM images taken at different magnification for the barium molybdate (BaMoO ) powder sample, obtained at 900 8C.

[
(
)
T
D
$
F
I
G
]
N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
39
Mo KA1
BaMoO
4
Mo KA1
BLM1
10000
1000
5000
500
Ba KA1
30
25
20
15
3
2
1
KeV
KeV
Mo KA1
BLM3
Mo KA1
BLM2
10000
10000
5000
5000
30
20
10
30
20
10
KeV
KeV
Mo KA1
BLM5
Mo KA1
BLM4
10000
10000
5000
5000
30
25
20
15
10
5
35
3
25
20
15
10
5
KeV
KeV
4
Fig. 10. XRF spectra for various compositions of La doped BaMoO samples.
boundary conductivities of nanocrystalline BaMoO
activation energy is calculated from the log
4
sample. The
T vs. 1000/T plot of
1
N
d
A
gb
1
t
d
A D
gb
1
t Cgi
s
sgb ¼ R
¼ R
¼ R
(2)
gb
gb
gb
A Cgb
BaMoO crystalline material and it is found to be 0.649 eV for grain
4
interior conductivity and 0.928 eV for grain boundary conductivi-
ty. Fig. 13 shows the log
conductivities for all compositions of nanocrystalline La doped
BaMoO sample pellets, sintered at 900 8C. The activation energies
of the grain interior effect for all compositions evaluated from the
log T vs. 1000/T plot and are tabulated as shown in Table 7. Fig. 14
shows log vs. 1000/T plots for the nanocrystalline barium
molybdate and lanthanum doped barium molybdate samples,
sT vs. 1000/T plots for the grain interior
where Rgi and Rgb are the resistances of grain interior and grain
boundary effects respectively, Cgi and Cgb are the grain interior and
grain boundary capacitances, t is the thickness of the sintered
pellet and A is the area of the sintered pellet. N is the number of
grain boundary planes parallel to the electrodes. Fig. 12 shows the
4
s
s
log sT vs. 1000/T plots for the grain interior as well as the grain
Table 6
2 3 3 4
The percentages of the La O , BaO and MoO exist in the various compositions of La doped BaMoO samples, obtained from XRF spectra.
BaMoO
4
BLM-1
BLM-2
BLM-3
BLM-4
BLM-5
La
BaO
MoO
2
O
3
–
5.634%
49.65%
43.11%
10.81%
42.49%
45.42%
16.67%
38.87%
42.37%
23.52%
33.10%
41.56%
28.54%
26.75%
43.25%
54.54%
43.83%
3

[
(
)
T
D
$
F
I
G
]
40
N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
4
3
2
1
x108
(a)
7
6
6
6
6
(b)
o
1
8
.0x10
.0x10
325 C
3
o
50 C
x108
o
375 C
o
4
00 C
fit data
x108
o
6.0x10
.0x10
2.0x10
225 C
o
2
50 C
o
4
x108
275 C
o
3
00 C
fit data
0
0
.0
8
9
0
.0
5.0x10
1.0x10
7
7
7
0
1x10
2x10
3x10
Z' ohm
Z' ohm
5
4
8
7
6
5
4
3
2
1
x10
7
6
x10
x10
o
o
(c)
525
550
575
C
4
4
25 C
(d)
5
x10
o
4
4
4
4
4
4
o
50 C
C
5
o
x10
475 C
o
o
C
5
500 C
5x10
x10
3x10
x10
fit data
fit data
5
4
x10
5
x10
5
x10
2
x10
x10
5
x10
1
0
0
5
6
6
6
6
0
.0
5.0x10 1.0x10 1.5x10 2.0x10 2.5x10
Z' ohm
4
5
5
5
5
0
.0
5.0x10
1.0x10
1.5x10
2.0x10
2.5x10
Z' ohm
0
00
4
Fig. 11. (a–d) Impedance (Z and Z ) plots for the nanocrystalline BaMoO sample at various temperatures.
obtained at 900 8C and comparable with literature results. Our
x
Ba1ꢁxLa MoO4+x/2 samples (x = 0–0.5 insteps of 0.1), the lanthanum
prepared nanocrystalline barium molybdate shows better con-
concentration increases, which increase the total charge carrier
concentration. The lanthanum doping does not alter its parent
structure at x = 0.1 and slight changes in the phase of x = 0.2
composition, which are confirmed from XRD and FTIR results.
Hence, the conductivity increases linearly because of high charge
carrier concentration. Doping of lanthanum content from x = 0.3–
4 4
ductivity than SrWO [23] and PbWO [24] at 450 8C, which are
shown in Fig. 14. The nanocrystalline lanthanum doped barium
molybdate samples of all compositions have better conductivity
than the nanocrystalline barium molybdate sample. In Fig. 14,
BLM5 (Ba0.5La0.5MoO4.25) showed higher conductivity compared to
other samples such as nanocrystalline BaMoO
PbWO
Fig. 15 shows the grain interior and grain boundary conductivi-
ties for various compositions of La doped nanocrystalline BaMoO
4
, SrWO
4
and
4 2 2 9
0.5, forms mixed phases of BaMoO and La Mo O , which decrease
4
.
the conductivity in x = 0.3 and 0.4 compositions. The BLM5
(Ba0.5La0.5MoO4.25) composition sample showed the highest bulk
and grain boundary conductivity and also higher activation
4
samples, obtained at 425 8C. For all the compositions of barium
molybdate samples, the grain interior conductivity is nearly two
orders of magnitude higher than the grain boundary conductivity.
energies compared to other compositions of La doped BaMoO
4
samples. The observed higher activation energy may be due to the
formation and also for the migration of oxygen ion charge carriers,
which may also increase the conductivity of the nanocrystalline
3+
2+
Doping of Lanthanum (La ) in the place of Barium (Ba ) sites in
[()TD$FIG]
-
-
-
-
-
-
-
-
2
3
4
5
6
7
8
9
[()TD$FIG]
σbulk
σgb
-
-
-
-
1
2
3
4
BLM1
BLM2
BLM3
BLM4
BLM5
-5
-6
-7
1
.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
1000/T (K-1)
-
1
1
000/T (K )
Fig. 12. log
s
T vs. 1000/T plots for the grain and grain boundary of nanocrystalline
Fig. 13. log
sT vs. 1000/T plots for the grain interior of various compositions
BaMoO sample.
4
4
of nanocrystalline La doped BaMoO samples.

N. Nallamuthu et al. / Materials Research Bulletin 46 (2011) 32–41
41
Table 7
from the TG/DTA results. Formation of MoO
network in the La doped BaMoO
spectral results. The XRD pattern revealed the formation of pure
crystalline scheelite type BaMoO phase for the BaMoO and
BLM1samples. The mixed BaMoO and La Mo crystalline
phases have formed BLM2 to BLM5 samples. The average
crystallite sizes of the La doped BaMoO samples were found to
be <100 nm respectively. The existence of O, Mo and Ba elements
in BaMoO sample are confirmed SEM-EDX and XRF spectra
results. The particle sizes (<100 nm) and the ‘d’ spacing (3.345 A
3
structure and La–O
samples are confirmed from FTIR
Activation energies for the charge carriers in the grain interior (GI) and the grain
boundary (Gb) effects of all the compositions of nanocrystalline La doped BaMoO4
samples.
4
4
4
Region I (GI)
in eV
Region II (GI)
in eV
Region I (Gb)
in eV
Region II (Gb)
in eV
4
2
2 9
O
(
<325 8C)
(>325 8C)
(<325 8C)
(>325 8C)
4
BLM0
BLM1
BLM2
BLM3
BLM4
BLM5
0.625
0.928
0.9447
1.2084
1.036
0.891
0.645
0.9694
1.0632
1.020
0.88
1.3768
1.1305
0.5898
0.8718
0.7673
1.077
0.804
4
˚
1.4274
1.6516
˚
and 3.2 A) of the barium molybdate are obtained from the HRTEM
images. The relative density of the BaMoO pellet sintered at 900 8C
1.39
4
[
(
)
T
D
$
F
I
G
]
was found to be 91.5% of theoretical density. The grain interior and
the grain boundary conductivities were calculated by analyzing
the impedance data using the brick layer model. The Ba0.5La0.5-
MoO4.25 (BLM5) sample showed the highest conductivity at 425 8C
-
3
4
5
6
7
8
9
SrWO [23]
4
PbWO [23]
4
-
-
-
-
-
-
compared to pure and all other compositions of La doped BaMoO
4
PbWO [24]
4
BaMoO4
BLM5
samples. Also, the BLM5 sample exhibits higher activation energy.
The observed higher conductivity and activation energy may be
due to the formation and also the migration of oxygen ion charge
carriers in the nanocrystalline La doped BaMoO
4
sample
(
Ba0.5La0.5MoO4.25).
Acknowledgements
NS gratefully acknowledges DRDO, DST, CSIR, AICTE and UGC,
Government of India, for receiving financial support in the form of
major research projects. NN acknowledges the CSIR [CSIR: 9/559/
0
.9
1.2
1.5
000/T K-1
1.8
2.1
1
(
0054)/2008/EMR-I], Government of India, for the award of senior
Fig. 14. log
reported samples.
[()TD$FIG]
s
vs. 1000/T plots for the nanocrystalline BaMoO
4
, BLM5, and other
research fellowship. Authors would like to thank Central Instru-
mentation Facility (CIF), Pondicherry University for using TG/DTA,
FTIR, SEM and XRF facilities for the present work.
Grain interior Conductivity
Grain boundary Conductivity
-
-
-
-
-
4
5
6
7
8
10
10
10
10
10
References
[
[
1] J.P.P. Huijsmans, Curr. Opin. Solid State Mater. Sci. 5 (2001) 317.
2] P.N. Dyer, R.E. Richards, S.L. Russek, D.M. Taylor, Solid State Ionics 134 (2000) 21.
[3] B. John, Goodenough, Annu. Rev. Mater. Res. 33 (2003) 91.
[
[
4] V.V. Khartona, F.M.B. Marquesa, A. Atkinson, Solid State Ionics 174 (2004) 135.
5] G.G. Zhang, Q.F. Fang, X.P. Wang, Z.G. Yi, J. Phys.: Condens. Matter 15 (2003) 4135.
[6] T. Esaka, Solid State Ionics 136–137 (2000) 1.
[
[
7] V. Thangadurai, S. Knilllmayer, W. Weppner, Mater. Sci. Eng. B 106 (2004) 228.
8] G. Martin, Bellino, G. Diego, Lamas, E. Noemi, Walsoe de Reca, Adv. Funct. Mater.
16 (2006) 107.
[9] H.W. Chan, J.G. Duh, S.R. Sheen, J. Power Sources 115 (2003) 110.
[
[
10] N. Nallamuthu, I. Prakash, M. Venkateswarlu, S. Balasubramaniyam, N. Satyanar-
ayana, Mater. Chem. Phys. 111 (1) (2008) 24.
11] Guangzhi Li, Zhenling Wang, Zewei Quan, Chunxia Li, Jun Lin, Cryst. Growth Des. 7
(9) (2007) 1797.
0
.0
0.1
0.2
0.3
0.4
0.5
Fig. 15. Grain interior and grain boundary conductivities of all the compositions of
the nanocrystalline La doped BaMoO samples obtained at 425 8C.
[12] H.L. Lin, R.K. Chiang, W.T. Li, J. Non-Cryst. Solids 351 (2005) 3044.
4
[13] X. Guo, P. Sujatha Devi, B.G. Ravi, J.B. Parise, S. Sampath, J.C. Hanson, J. Mater.
Chem. 14 (2004) 1288.
(Ba0.5La0.5MoO4.25). The higher charge carriers may easily migrate
[14] W.D. Yang, Y.H. Chang, S.H. Huang, J. Eur. Ceram. Soc. 25 (2005) 3611.
in the grain boundary than in the grain interior and hence,
conductivity increment is more in grain boundary compared to
grain interior for the x = 0.5 composition.
[15] K. Du, H. Zhang, J. Alloys Compd. 352 (2003) 250.
[16] S. Vivekanandhan, M. Venkateswarlu, N. Satyanarayana, J. Alloys Compd. 441
(2007) 284.
[17] S. George, Infra Red and Raman Characteristic Group Frequencies Tables and
Charts, 3rd edition, John Wiley & Sons, England, 2001.
4
. Conclusion
[18] R. Murugan, S. Mohan, A. Bigotto, J. Korean Phys. Soc. 32 (4) (1998) 505.
[19] K. Ouzaouit, S. Villain, A. Essoumhi, A. Benlhachemi, H. Benyaich, J.-R. Gavarri, M. J.
Condensed Mater. 11 (2) (2009) 60.
Different compositions of nanocrystalline La doped BaMoO
4
[20] I. Ardelean, M. Toderas, J. Optoelectron. Adv. Mater. 8 (3) (2006) 111.
[
Ba1ꢁxLa
x
MoO4+x/2 (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5)] samples were
[21] C. Tian, Siu-Wai Chan, Solid State Ionics 134 (2000) 89.
[
[
22] J. Fleig, J. Maier, J. Am. Ceram. Soc. 82 (12) (2004) 3485.
23] V. Thangadurai, C. Knittlmayer, W. Weppner, Mater. Sci. Eng. B 106 (2004) 228–
synthesized using acrylamide assisted sol–gel combustion process.
The removal of nitrates, organic residuals along with adsorbed
233.
4
water from the La doped BaMoO dried gel samples are confirmed
[24] T. Esaka, Solid State Ionics 136–137 (2000).