006) 3267–3273www.elsevier.com/locate/ssi

Solid State Ionics 177 (2

Synthesis, structural characterization and electrical property of newoxide ion conductors: La3MMo2O12 (M=In, Ga and Al)

Tian Xia a,b, Jiayan Li b,c, Qin Li b,c, Xiangdong Liu b,c, Jian Meng b, Xueqiang Cao b,⁎

a College of Chemistry and Chemical Technology, Heilongjiang University, Harbin 150080, Heilongjiang, PR Chinab Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,

Changchun 130022, Jilin, PR Chinac Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China

Received 19 November 2005; received in revised form 20 July 2006; accepted 12 September 2006

Abstract

New series of oxides, La3MMo2O12 (M=In, Ga and Al), have been prepared by the solid-state reaction. The composition and elementaldistribution were analyzed by the energy-dispersive X-ray (EDX) analysis. As determined by the X-ray diffraction (XRD), these compounds havesimilar crystal structures that can be indexed on a monoclinic cell at room temperature. AC impedance spectra and the DC electrical conductivitymeasurements in various atmospheres indicate that they are oxide ion conductors with ionic conductivities between 10−2 and 10−3 S/cm at 800 °C.The conductivity decreases in the order of La3GaMo2O12NLa3AlMo2O12NLa3InMo2O12, implying that the effect of cell volume and polarizationassociated with In3+, Ga3+ and Al3+ play an important role in the anion transport of these materials. The reversible phase transition was observedin all these compounds as confirmed by the differential thermal analysis (DTA) and dilatometric measurements.© 2006 Elsevier B.V. All rights reserved.

Keywords: Oxide ion conductors; AC impedance spectra; Ionic conductivity; Phase transition

1. Introduction

Oxide ion conductors are an important class of functionalmaterials due to their potential applications in many fields, such asoxygen separation membrane, oxygen sensor and solid oxideelectrolyte in solid oxide fuel cells (SOFCs) [1–4].Yttria-stabilizedzirconia (YSZ) is commonly employed as an oxide ion conductorin technological application. Although YSZ is an excellentelectrolyte there remains much interest in trying to develop newoxide ion conductors that have higher ionic conductivity comparedwith YSZ at intermediate temperature. This could be achievedeither by improving the electrical properties of compounds throughproper chemical doping or by designing a newoxide ion conductor.The known oxide ion conductors belong to several structural typesas follows: fluorite, perovskite, pyrochlore, apatite and brownmil-lerite structures et al. [5–10]. Recently, a new family of oxide ionconductors based on La2Mo2O9 has been reported by Lacorre et al.

⁎ Corresponding author. Tel.: +86 431 5262285; fax: +86 431 5262285.E-mail address: xcao@ciac.jl.cn (X. Cao).

0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2006.09.011

[11,12], whose conductivity is slightly higher than that of YSZ atintermediate temperature [11]. It has amonoclinic structure at roomtemperature and shows a first-order structural transition at 580 °Cto a cubic structure [12], which is different from the conventionalfluorite and perovskite type compounds. The ionic conductivityhas been confirmed by the measurements performed in variousflowing atmospheres. Marrero-López et al. reported that theconductivity of La2Mo2O9wasmainly ionicwith an ionic transportnumber of about 0.98 in a moderate reducing atmosphere, and thephase stability was limited to PO2

=10−8 Pa at 800 °C [13].Therefore, the rare earth–molybdenum oxides have attractedmuchinterest. La1.94Ba0.06Mo2O9 displays cubic symmetry at roomtemperature with a high conductivity of 8×10−2 S/cm at 800 °C inair atmosphere [14]. A study is thus of interest in the research fornew oxide ion conductors.

In this paper, the synthesis and electrical properties of thenew oxide ion conductors La3MMo2O12 are reported. Inaddition, the indexed room temperature XRD patterns basedon monoclinic cells, differential thermal analysis and thermalexpansion behaviors are also studied.

Fig. 2. Observed (full line), calculated (circles), peak positions and difference(bottom) RT X-ray powder diffraction patterns for La3GaMo2O12.

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2. Experimental

La3MMo2O12 (M=In, Ga and Al) were synthesized by theconventional solid-state reactions using La2O3 (99.99%, Chen-ghai Chemicals of Guangdong, China), Ga2O3, In2O3 (99.9%,The First Fine Chemicals of Shanghai, China), and Al2O3, MoO3

(99.5%, The First Fine Chemicals of Shanghai, China) as startingmaterials. All the oxides besides MoO3 were dried at 1000 °Cprior to weighting. Stoichiometric mixtures were ground in anagate mortar for 30 min and first heated at 500 °C for 12 h, thenfired between 1150 °C and 1250 °C for 24 hwith one intermediateregrinding, followed by cooling slowly to room temperature. Thecrystal structures of these specimens were analyzed by roomtemperature X-ray diffraction (XRD) (Rigaku D/Max 2500diffractometer with CuKα1 radiation λ=1.5406 Å). The 2θranged from 10° to 100° with increments of 0.02° and a countingtime of 1 s. Material analysis software MDI jade 5.0 andDICVOL91 in Material Studio were used to index the XRDpattern and refine the precise lattice parameters [15]. Structurerefinement was performed using the program GSAS [16]. Field-emission scanning electron microscopy (FE-SEM, XL-30,Philips) equipped with energy-dispersive X-ray fluorescence(EDX) analysis was used to investigate the morphology andcomposites of samples. For SEM observation, the product waspasted on the silicon substrate. The differential thermal analysis(DTA) and thermogravimetric analysis (TGA)were collected on aTA SDT2960 instrument in air with a heating and cooling rate of10 °C min−1. A sample of ∼ 50 mg was used for this analysis,with alumina powder as a reference.

The powder was pressed under a pressure of 175 MPa andthen sintered at 1150 °C or 1300 °C for 12 h in air atmosphere.The pellets have about 85–90% of relative density. Bar-shapesamples with a dimension of 2×2×25 mm3 were cut from thesintered bodies for the dc conductivity and dilatometric (NetzschDIL-402 C) measurements, and the cylindrical pellets (∼ 2 mm

Fig. 1. XRD patterns of La3MMo2O12 (M=In, Ga and Al) at room temperature.

thickness and ∼ 10 mm diameter) were used for ac impedancemeasurements.

AC impedance techniques were performed in air which wascooled from 800 °C to 300 °C at 5 °C min−1 (accuracy±1 °C)with a dwell time of 60 min between measurements. Twoelectrodes of platinum paste were painted on either side of thesample, and fired at 800 °C during 60min to ensure adhesion. TheSolartron 1255 and 1287 impedance analyzers were used forelectrical characterization. Spectra were obtained in the frequencyrange from 0.1 Hz to 1 MHz with an applied voltage of 10 mV,which was controlled by the computer program Zplot. Dataanalysis was made by equivalent circuits using the programZview allowing us to estimate the different contributions of theconductivity. The total electrical conductivity was measured bythe DC four-probe technique with an automatic measuring system(programmable current source Model 2400, multimeter Model2000, switch system Model 7001, Keithley Instruments). Theoxygen partial pressure (PO2

, with unit “Pa” in this work) in theranges of log(PO2

)=5 to 1 and log(PO2)=1 to −8 was controlled

by the mixture of O2+N2 and CO+CO2, respectively. PO2was

precisely measured with a YSZ oxygen sensor.

3. Results and discussion

3.1. Structural characterization

XRD patterns of La3InMo2O12, La3GaMo2O12 and La3AlMo2-O12 are slightly similar between 2θ=20° and 30° (Fig. 1),suggesting these materials might have similar crystal structure.However, there are clear differences between the patterns between2θ=45–50° and 30–40° where neither relative intensity, positionor number of peaks match, which may be attributed to the latticedistortion. RT refinements of XRPD data for La3GaMo2O12 werecarried out with space groupPc7 by using the monoclinic structureas a starting model. The refined patterns converged toRwp=9.73%,Rp=6.95% and χ2=3.9%. Fig. 2 shows the observed, calculatedand difference profiles. The XRD patterns of all these compoundscould be fully indexed on the monoclinic cells using the MDI jade

Fig. 3. X-ray images, SEM photographs and EDX spectrum of La3GaMo2O12: (a) SEM photograph, (b) EDX spectrum and (c) EDX analysis area image (A), X-rayimages of the (B) La Lα line, (C) Ga Kα line, and (D) Mo Lα line.

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5.0 and DICVOL91 software with parameters of a=8.614(1) Å,b=5.006(3) Å, c=7.454(1) Å, and β=108.15°, V=305.4 Å3 forLa3InMo2O12, a=11.204(2) Å, b=3.224(1) Å, c=15.741(1) Å,and β=102.55°, V=554.96 Å3 for La3GaMo2O12, and a=11.546(3) Å, b= 5.015(3) Å, c= 7.151(5) Å, and β=95.93°,V=408.28 Å3 for La3AlMo2O12, respectively.

The morphology, compositions and elemental distributions ofthe final products were analyzed by FE-SEM and EDX. SEMobservation for La3GaMo2O12 suggests that the spherical andirregular morphologies with an even size distribution of about 2–10 μm (Fig. 3a). Fig. 3b displays the energy-dispersive X-ray(EDX) spectrum, which confirms the presence of La, Ga and Moin the sample. The EDX analysis area and X-ray images of La, Gaand Mo in La3GaMo2O12 are shown in Fig. 3c. The results provethat the distribution of all elements in products was completelyuniform and no segregation La, Ga and Mo are observed.Therefore, it is obvious that the formation of segregation of thecomponent element is negligible in the resultant specimens.Quantitative EDX analysis on La3GaMo2O12 gives a compositionof La:Ga:Mo≈3:1:2, which is in good agreement with thestarting materials of La3GaMo2O12.

3.2. DTA and dilatometric measurements

These materials showed no significant weight loss whenheated 1000 °C in air from TGA experiments, indicating that

they were thermally stable. The DTA curves of La3MMo2O12 inair atmosphere are shown in Fig. 4. DTA of La3GaMo2O12

exhibits the strong endothermic and exothermic peaks at 570 °Cand 539 °C upon heating and cooling, respectively. Similarly forLa3AlMo2O12, there is an endothermic peak at 566 °C in theheating cycle and an exothermic peak at 519 °C in the coolingcycle. For La3InMo2O12, the weak endothermic and exothermicpeaks are observed at 562 °C and 516 °C, respectively. Thesethermal events are probably related to an increase in crystalsymmetry which may be related to a reduction in crystal struc-ture distortions. The results suggest that all these compoundsprobably undergo a reversible phase transition at thesetemperatures. At the same temperatures, the anomalies wereobserved in their thermal expansion curves as shown in Fig. 5.This further proves the presence of phase transition at elevatedtemperatures. The thermal expansion coefficients (TECs) ofLa3MMo2O12 are very similar and a sharp increase of for thesematerials was observed near the phase transition temperatures.The TECs of these materials for different temperature regimesare listed in Table 1. Among all these composites, La3InMo2O12

has the lowest TECs of (14.7±0.2) and (12.7±0.6)×10−6/K forthe high and low temperature phases, respectively. Thesevalues are slightly lower than those of La2Mo2O9 (16.8 and13.5×10−6/K) [17], and higher than those of 8 mol% YSZ(10.5 × 10− 6/K, 25–1000 °C) [18] and Ce1.8Sm0.2O1.9

(11.4×10−6/K, 25–900 °C) [19]. A suitable chemical doping

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might suppress the thermal expansion behavior as the case ofLa2Mo2O9.

3.3. Electrical conductivity measurements

Fig. 6 shows the impedance spectra for La3AlMo2O12

sample sintered at 1250 °C and the other samples sintered at1150 °C. The impedance spectra of La3MMo2O12 show three

Fig. 5. The thermal expansion behaviors of La3MMo2O12 near phase transitiontemperatures in air atmosphere. Insert: the thermal expansion coefficients(TECs) of La3MMo2O12 (M=In, Ga and Al) as a function of temperature in airatmosphere.

Fig. 4. DTA thermograms of La3MMo2O12 (M=In, Ga and Al) in airatmosphere.

different processes. The two processes at high frequency areoverlapped and can be ascribed to the bulk (i.e., grain interiors)and internal interfaces (grain boundaries and pores). The thirdcontribution at low frequency is ascribed to the electrodeprocesses. With decreasing temperatures, e.g. 550 °C, lowfrequency response consists of an inclined-spike at an angle of45°, suggesting that ion diffusion between the electrode andelectrolyte interface [20]. From these data, it may be concludedthat the samples show ionic conductivity.

The impedance spectra can be described by a seriesassociation of (R1Q1) and (R2Q2) terms, Ri being a resistancecontribution, and Qi a pseudo-capacitance, which is related tothe angular relaxation frequency ωi, and capacitance Ci [20].The high-frequency contribution processes typical values ofcapacitance in the range 0.1 nF, and was thus contributed to theinternal interfaces (e.g., grain boundary). The capacitance of thelow-frequency contribution (N5 μF) is clearly related to theexternal material/electrode interface.

The total ionic conductivity has been fitted as a function oftemperature (T) following the Arrhenius law

r ¼ ðr0=TÞexpð−Ea=kBTÞ ð1Þwhere Ea is the activation energy for ionic migration, kB is theBoltzmann constant, and σ0, the pre-exponential factor, is a

Table 1The TECs of La3MMo2O12 (M=In, Ga and Al) for different temperatureregimes

Composites TECs (×10−6/K)

Above Tc Below Tc Peak value near the phase transitiontemperature

La3InMo2O12 14.7±0.2 12.7±0.6 28.8La3GaMo2O12 17.6±0.8 13.7±1.4 79.1La3AlMo2O12 14.0±0.7 17.3±1.3 64.2

Tc: Phase transition temperature.

Fig. 6. Impedance spectra of La3MMo2O12 (M=In, Ga and Al) at different temperatures in air atmosphere.

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constant related to the density of charge (in this case, oxidevacancies). Fig. 7 displays the Arrhenius plots of total ionicconductivities for La3MMo2O12 in air atmosphere. Curvatures

at different temperatures can be observed in these curves whichindicates that there is a critical temperature Tc [21], belowwhich the oxygen vacancies are progressively trapped out into

Fig. 7. Arrhenius plots of conductivities for La3MMo2O12 in air atmosphere.Fig. 8. Arrhenius plots of conductivity for La3GaMo2O9 in air atmosphere. Thesimilar data for several oxide ion conductors are present for comparison.

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the clusters with decreasing temperature, and above Tc thevacancies are dissolved into the matrix of oxygen sites. Theabrupt change of conductivity at elevated temperatures isobserved in all these materials. Some compounds undergodisorder-order phase transitions upon cooling, whereby theylose their high ionic conductivities [22]. The activationenergies of La3MMo2O12 for different transport processes arelisted in Table 2. The conductivity decreases in the order ofLa3GaMo2O12NLa3AlMo2O12NLa3InMo2O12, which may beexplained by the decrease of the cell volume as M varies fromGa to In. According to the Giraud's report, the reduction ofcell volume is not favorable to the anion migration of oxideion conductors [23]. This may be correct but may depend moreon the free volume in the unit cell or the cation distances. Suchtheories have been suggested for perovskite-type materials. Thefree volumes of La3GaMo2O12, La3AlMo2O12 and La3InMo2-O12 calculated from cell volumes and ionic radiusare 339.75 Å3, 193.55 Å3+ and 88.43 Å3+, respective-ly. On the other hand, the polarization decreases in the orderof In3+NGa3+NAl3+ accompanied by the decrease of cationradius. Therefore, the chemical affinity with O2− decreases inthe order of Al3+NGa3+N In3+. These two effects will corporateand affect the diffusion of oxygen ions. La3GaMo2O12 has thehighest conductivity among all these materials, and theirconductivities are 3.6×10−2, 2.4×10−2 and 3.6×10−3 S/cmfor La3GaMo2O12, La3AlMo2O12 and La3InMo2O12 at 800 °C,

Table 2The activation energies Ea calculated from Arrhenius plots of La3MMo2O12 fordifferent conduction processes

Ea (eV,NTc) Ea (eV, Tc~Tt) Ea (eV,bTt)

La3AlMo2O12 0.68±0.03 1.20±0.06 1.34±0.08La3GaMo2O12 0.54±0.01 1.23±0.05 1.76±0.20La3InMo2O12 0.65±0.01 1.35±0.04 1.15±0.20

Tc: critical temperature, Tt: phase transition temperature.

respectively. The value of conductivity for La3GaMo2O12 ishigher than that of the other oxide ion conductor 8 mol% YSZ(2.6×10−2 S/cm at 800 °C) [24].

The comparison of La3GaMo2O12 with high ionic conductiv-ity, fluorite type YSZ [25] and Sm-doped CeO2 [26], La0.9Sr0.1-Ga0.8Mg0.2O2.85 (LSGM) [27], La10Si6O27 [28] and La2Mo2O9

[11] is presented in Fig. 8. At high temperatures, the ionicconductivity of La3GaMo2O12 is lower than the conductivity ofLSGM and La2Mo2O9, and enhances compared with those of theother oxide ion conducting solid electrolytes. La3GaMo2O12

exhibits the phase transition at elevated temperature accompaniedby the abrupt increase of conductivity, however, it is noted that theconductivity of La3GaMo2O12 is much higher than that ofLa2Mo2O9 at 560 °C. It may be possible to enhance the ionic

Fig. 9. The conductivities of La3MMo2O12 as a function of oxygen partialpressure (PO2

) at 800 °C.

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conductivity and stabilize the polymorph with high conductivityby a suitable chemical doping.

In order to prove if these materials are the oxide ionconductors and to check the stability in the reducingatmosphere, the gas was changed sequentially at 800 °C. Theconductivities of all these compounds vs. oxygen partialpressure (PO2

) measured by DC method are shown in Fig. 9.The oxygen partial pressure has a strong effect on the electronicand proton conductors, whose conductivities change withdecreasing or increasing oxygen partial pressure. In themeasured range, the conductivities of all these compoundsare almost constant and independent of PO2

for all thoseisotherms, which indicates that the conductivity is dominatedby the oxygen vacancy rather than electron or proton [29,30].These materials have the different stabilities under variousoxygen partial pressures. In our study, the oxide ionic con-ductions for La3GaMo2O12 and La3AlMo2O12 maintain till log(PO2

)≈−7 and log(PO2)≈−6 at 800 °C, respectively, but

La3InMo2O12 is only stable till log(PO2)≈1. The low stability

of La3InMo2O12 can be explained by the stronger reducibilityof In3+ than those of Ga3+ and Al3+ in the reducingatmosphere. The stability of La3GaMo2O12 and in reducingatmospheres reaches the level of La2Mo2O9 at 800 °C (phasestability limits to PO2

=10−8 Pa).

4. Conclusion

The new oxide ion conductors La3AlMo2O12, La3GaMo2-O12 and La3InMo2O12 crystallize in the monoclinic symme-try, and their conductivities decrease in the order ofLa3GaMo2O12NLa3AlMo2O12NLa3InMo2O12, implying thatthe cell volume plays a key role in anion migration. La3GaMo2-O12 has the highest ionic conductivity among all these materialswith a value of 3.6×10−2 S/cm at 800 °C. The phase transitionhas been observed in all these materials at elevated tempera-tures. It may be possible to enhance the ionic conductivity andstabilize the polymorph with high conductivity by a suitablechemical doping as the cases of ZrO2 and CeO2-basedelectrolytes [31].

Acknowledgement

This work is financially supported by the National NaturalScience Foundation of China (20471058, 20331030).

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