A potential low-temperature oxide-ion conductor: La 2 − x Ba x Mo 2 O 9
S. Basu, P. Sujatha Devi, and H. S. Maiti
Citation: Applied Physics Letters 85, 3486 (2004); doi: 10.1063/1.1808505
View online: http://dx.doi.org/10.1063/1.1808505
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 16
18 OCTOBER 2004
A potential low-temperature oxide-ion conductor: La2−xBaxMo2O9
S. Basu, P. Sujatha Devi,a) and H. S. Maiti
Electroceramics Division, Central Glass and Ceramic Research Institute, Kolkata 700 032, India
(Received 22 December 2003; accepted 23 August 2004)
An oxide ion conducting material, La1.94Ba0.06Mo2O9, with ionic conductivity of the order of
0.084 S / cm at 800 ° C in air was prepared by a citrate-nitrate auto-ignition process. A 3% Ba doping
has suppressed the resistive transition of unsubstituted La2Mo2O9, which in turn stabilized the
high-temperature cubic phase at room temperature as confirmed from x-ray diffraction, differential
thermal analysis, and dilatometric studies. Impedance measurements on sintered La1.94Ba0.06Mo2O9
further lend strong support that a small amount of Ba doping has increased the overall conductivity
of the parent compound La2Mo2O9 to a notable extent both at low and high temperatures. © 2004
American Institute of Physics. [DOI: 10.1063/1.1808505]
The scientific and technological importance of oxide ion
conducting materials has increased considerably and consequently, a major thrust to tailor and develop new materials
that exhibit high ionic conductivity at lower temperatures.
This could be achieved either by improving the properties of
known compounds through proper choice of effective dopants or by designing and developing a new class of materials.
The known oxide ion conducting materials generally belong
to fluorite, perovskite, brownmillerite, aurivillius, and/or pyrochlore structural types.1 Recently, this class was extended
further to a new series, based on the parent compound
La2Mo2O9, that exhibited improved ionic conductivity at
lower temperatures due to the presence of intrinsic oxygen
vacancies.2,3 The increased conductivity of pure La2Mo2O9
is associated with a first-order phase transition from monoclinic ␣-phase to cubic ␤-phase occurring at 580 ° C.2,3 Although various substitutions were effected in both La and
Mo sites to stabilize the high temperature ␤-La2Mo2O9 phase
at room temperature, none of them increased the conductivity of pure La2Mo2O9 to a notable extent.2–9
In this letter, we report the solution phase synthesis,
characterization, ionic conductivity, and thermal expansion
behavior of a potential oxide ion conducting material based
on the parent compound, La2Mo2O9. Ba doping has resulted
in both high temperature phase stabilization at room temperature and enhanced oxide ion conductivity compared to
pure La2Mo2O9 and hence could be a potential material for
future applications.
A simple solution phase synthesis namely, citrate-nitrate
gel combustion process, was followed for the preparation of
Ba-doped La2Mo2O9 compositions.10 Briefly, a mixed solution containing lanthanum nitrate, ammonium molybdate,
barium nitrate, and citric acid in the requisite ratio to give
La2−xBaxMo2O9 with x varying from 0.06 to 0.2 was allowed
to evaporate on a hot plate to form a gel that auto-ignited and
formed a white voluminous powder. This powder was calcined at 600 ° C for further processing and characterization.
The green compacts sintered at 1000 ° C for 10 h had a bulk
density of 95% of theoretical density and their fracture surface exhibited a layered grain growth. ac impedance measurements were conducted on sintered samples in the frequency range 40 Hz– 5 MHz in different atmospheres in the
a)
Electronic mail: [email protected]
temperature range of 300– 1000 ° C using a HIOKI-LCR
meter 共3532-50 LCR Hi TESTER兲. The effect of Ba2+ ions
on the ionic conductivity of La2Mo2O9 was verified directly
through the dc conductivity measurement. Absence of a
phase transition around 580 ° C in the Arrhenius plots of
these materials was clear evidence for the phase stabilization
by Ba doping. Nevertheless, we have observed a decrease in
conductivity with increase in the dopant level and the composition with 3 mol% Ba exhibited the highest conductivity.
At higher Ba doping levels, the extrinsic oxygen vacancies
could form clusters and would lead to a decrease in the overall mobility and conductivity of the heavily doped samples.
Here, detailed characterization studies were performed only
on samples with the lowest Ba doping, La1.94Ba0.06Mo2O9.
The differential thermal analysis (DTA) results collected
on a NETZSCH STA 409 C instrument with a heating rate of
10 ° C / min on undoped and Ba-doped La2Mo2O9 samples
are shown in Fig. 1(a). The presence of a strong endothermic
peak around 578 ° C in Fig. 1(a) signature the order–disorder
transition in pure La2Mo2O9. The absence of this structural
transition in the La1.94Ba0.06Mo2O9 sample unequivocally
confirms the suppression of order–disorder transition and
consequent stabilization of the high temperature cubic phase
at room temperature. A comparative linear expansion behav-
FIG. 1. (a) DTA of 600 ° C calcined samples in air: (1) undoped La2Mo2O9,
(2) La1.94Ba0.06Mo2O9. (b) Thermal expansion characteristics of the 1000 ° C
sintered sample: (1) undoped La2Mo2O9, (2) La1.94Ba0.06Mo2O9.
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© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 85, No. 16, 18 October 2004
Basu, Devi, and Maiti
3487
FIG. 2. Room temperature XRD patterns of (a) undoped La2Mo2O9 and (b) Rietveld refinement plot for
La1.94Ba0.06Mo2O9.
single distinct semicircle passing through the origin (Fig. 3)
ior of undoped and Ba-doped samples is shown in Fig. 1(b).
at the high frequency region corresponding to only the bulk
This also clearly indicates a transition around 580 ° C for the
(intragrain) contribution. In Fig. 3 the frequency dependence
undoped material (indicated by an arrow) that is totally abof the imaginary part of the impedance 共z⬙兲 is also shown at
sent for the Ba-doped sample. This corroborates the DTA
result, and thus confirms the absence of a structural transition
two different temperatures. At most of the temperatures
in the present compound around 578 ° C. The thermal expan(500 ° C is shown in Fig. 3 for example) the relaxation dission coefficient of La1.94Ba0.06Mo2O9 has been calculated to
persion of the grain contribution could only be seen while at
be around 1.07⫻ 10−5 / K at 800 ° C.
580 ° C the relaxation dispersions of both the grain contribuThe x-ray powder diffraction (XRD) pattern (collected at
tion and the grain boundary contributions were seen. Hence,
room temperature on a Philips PW 3207 diffractometer with
the contribution from grain boundary if at all present in these
Cu K␣ radiation) of the 600 ° C calcined undoped La2Mo2O9
samples is considered very small and the resistance thus calpowder sample and the Rietveld refinement data using
culated from the complex impedance is considered as the
POWDEREX program of the Ba-doped La2Mo2O9 sample is
bulk resistance.
shown in Figs. 2(a) and 2(b), respectively. The XRD patterns
To investigate the stability of La1.94Ba0.06Mo2O9 in difconfirm the formation of single-phase material in both the
ferent atmospheres, impedance data were collected at various
cases. Though the low temperature ␣ form of La2Mo2O9 is
flowing atmospheres ranging from flowing oxygen 共⬃2.12
reported to be monoclinic, the distortion from cubic symme⫻ 105 Pa兲 to reducing atmosphere, 2 % H2 + Ar atmosphere
try is too small to be observed in a regular diffractogram.
共⬃1 ⫻ 10−18 Pa兲. The bulk conductivity calculated from the
Hence, undoped ␣-La2Mo2O9 sample is considered to have a
impedance data presented as Arrhenius plots is shown in Fig.
pseudocubic symmetry and most of the reflections of the
4. It is interesting to note that the plots for La1.94Ba0.06Mo2O9
room temperature phase could be indexed based on a
at different atmospheres followed a more or less similar path,
pseudocubic symmetry with a lattice parameter of
7.151± 0.003 Å.11 All the diffraction peaks of the Ba-doped
sample [Fig. 2(b)] on the other hand could be well indexed
by a cubic symmetry with space group P213 similar to the
high temperature cubic phase of ␤-La2Mo2O9 confirming the
cubic phase stabilization at room temperature. The refined
lattice constant, 7.162± 0.003 Å, is slightly higher than the
observed value for ␤-La2Mo2O9 共7.151 Å兲 reflecting the incorporation of a larger Ba ion in the lattice.
A representative complex impedance plot of the
La1.94Ba0.06Mo2O9 sample in air at 450 ° C is shown in Fig. 3
and the data have been fitted by the ZVIEW software. The
capacitance value obtained from the analysis was 5.62 pF,
which is in the range typically observed for the bulk conductivity of ionic conductors. From the impedance plots, the
effective resistance was calculated by measuring the distance
between the vertical axis and the interception points of the
impedance semicircle with the horizontal axis. From the resistance “R,” thickness “L,” and cross-section “A” of the
FIG. 3. A typical complex impedance plot of the 1000 ° C sintered
sample, the bulk conductivity ␴ = 1 / R 共L / A兲 was calculated
La1.94Ba0.06Mo2O9 sample at 450 ° C in air; solid line is the fitting result
at different temperatures from the impedance plots. At most
obtained
equivalent
circuits (bottom) and the frequency dependence
of to IP:
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of the temperatures studied, the impedance plot exhibited a
the imaginary part 共z⬙兲 of complex impedance in air (top).
128.59.226.54 On: Wed, 10 Dec 2014 13:28:20
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Appl. Phys. Lett., Vol. 85, No. 16, 18 October 2004
FIG. 4. Arrhenius plots of the electrical conductivity of 1000 ° C sintered
La1.94Ba0.06Mo2O9 sample in different atmospheres along with that of the
undoped material in air.
which is distinctly different from the sharp transition of the
undoped material in air (Fig. 4). The slope change exhibited
by the doped material in the low temperature region (below
500 ° C) may reflect some degree of distortion remaining in
the sample. The La1.94Ba0.06Mo2O9 sample exhibited a conductivity of 1.53± 0.01⫻ 10−4 and 8.41± 0.03⫻ 10−2 S / cm at
500 and 800 ° C, respectively, compared to 1.19± 0.01
⫻ 10−4 and 5.28± 0.03⫻ 10−2 S / cm exhibited by the undoped La2Mo2O9 prepared by us. This result confirms that
Ba doping can effectively enhance the oxide ion conductivity
of La2Mo2O9 both at low and high temperatures. It may be
noted that though Ba has stabilized the cubic phase of
La2Mo2O9 at room temperature, only a minor increase in
conductivity was observed below the transition. The conductivity of La1.94Ba0.06Mo2O9 is even higher than the value
(3.0⫻ 10−2 S / cm at 800 ° C) reported for yittria stabilized
zirconia.12
In oxide ion conducting materials, in addition to the oxygen vacancies present, the unit cell free volume and polarizability of the ions also strongly influence the diffusion of the
oxygen ions and hence the oxide ion conductivity.13,14 In the
case of La1.94Ba0.06Mo2O9, substitution of divalent Ba2+ ions
for trivalent La3+ ions in the lattice is expected to introduce
extra oxygen vacancies for charge compensation apart from
the intrinsic vacancies already present. Further, the higher
Basu, Devi, and Maiti
ionic radius 共1.612 Å兲 of the Ba2+ ions gives rise to a minor
increase in the lattice constant 共a = 7.162 Å兲 that increases
the unit cell volume in the lattice and thus expedite the transport of oxygen ions to some degree. The higher polarizability
共1.70⫻ 10−24 cm3兲 of the Ba2+ ion compared to La3+ 共1.30
⫻ 10−24 cm3兲 ion also helps in the easy diffusion of oxygen
ions near its vicinity. Moreover, the high purity and phase
homogeneity of the present sample could have helped effectively in improving both phase stability and conductivity of
La1.94Ba0.06Mo2O9 compared to pure La2Mo2O9 or other
substituted La2Mo2O9 compounds.
To summarize, the results presented show that 3 mol%
Ba doping can effectively suppress the order–disorder transition of pure La2Mo2O9 thereby stabilizing the disordered
high temperature cubic phase at room temperature with a
substantial increase in conductivity.
The authors thank Director, CG&CRI for permission to
publish this work. S.B. is a recipient of the Council of Scientific and Industrial Research (CSIR) Fellowship. This work
was supported through the Task Force Programme on “Custom Tailored Special Materials” of CSIR.
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