Microstructure Related Conductivity in La2Mo2O9 Ceramics - The

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ARTICLES Microstructure Related Conductivity in La2Mo2O9 Ceramics Samuel Georges,* Renata Ayres Rocha, and Elisabeth Djurado Laboratoire d’Electrochimie et de Physico-chimie des Mate´ riaux et des Interfaces, LEPMI, UMR 5631 CNRS-INPG-UJF, BP.75. 38402 Saint Martin d’He` res Cedex, France ReceiVed: September 19, 2007; In Final Form: NoVember 21, 2007

La2Mo2O9 oxides were synthesized for the first time by ultrasonic assisted spray-pyrolysis process (SP) and conventional solid-state reaction (SSR). A comparative investigation of the morphology, microstructure, and structure was undertaken on the powders and the ceramics. Poorly crystalline powders containing organic residues were obtained by SP at 400 °C. On the contrary, high purity polycrystalline spherical particles ranging from 50 nm to 3 µm and homogeneous in composition were obtained at 600 °C and consisted of crystallites of 30 nm. The ceramics prepared from these powders obtained by spray pyrolysis at 600 °C present small particle size (2-3 µm). The sintering temperature was significantly reduced by 300 °C with comparison to the one obtained for pellets starting from SSR powders. A careful line broadening analysis of the X-ray diffraction patterns demonstrated a crystalline growth from 30 nm to only 150 nm after sintering process. Electrical properties of fine ceramics starting from SP powders were found fairly similar to conventional LAMOX micronic ceramics from room temperature to 850 °C.

1. Introduction Oxygen ion conductors are at present of great interest for promising technological applications as oxygen sensors, ceramic membranes for oxygen separation, solid oxide fuel cells (SOFC) electrolytes, or oxygen pumping devices. However, in SOFC applications, the most important limitation is related to the high operating temperature around 800 °C of conventional electrolytes such as stabilized zirconia (YSZ). In order to improve the performance of these devices, a lower operation temperature is desired. One approach to reduce the operating temperature is to increase the ionic conductivity of YSZ electrolyte by decreasing its thickness as much as possible.1,2 Ionic conductivity of YSZ can be also enhanced by either preparing cubic or tetragonal polycrystalline films with grain sizes in the nanometer range3,4 or by selecting another electrolyte which presents a larger ionic conductivity than YSZ at lower temperature. The main oxygen ion conductors known at the moment crystallize in the following structural types: fluorite, perovskites, Aurivillius, brownmillerites, and pyrochlores. For these materials, the ionic mobility occurs by vacancy migration and is vacancy concentration-dependent.5-7 Among this class of materials, a new family of oxides based on La2Mo2O9 was recently discovered. This compound was first synthesized in 19698 by conventional solid-state reaction and its high ionic conductivity was reported for the first time in 2000.9 La2Mo2O9 presents a phase transition at about 580 °C which results in an increase of ionic conductivity by almost 2 orders of magnitude,9,10 leading to a higher conductivity than stabilized zirconia. Therefore, La2Mo2O9 has been reported to be another potential candidate to be used as an electrolyte for SOFC application. * Corresponding author. E-mail: [email protected].

Several synthesis, deposition, or sintering techniques have been used to prepare La2Mo2O9 ceramics, such as high-energy ball milling,11 sol-gel,12-15 polymeric precursors,16-18 freezedrying,19-21 crystallization,22,23 spark plasma sintering,12 thin film deposition,24 from different precursors methods as citrate, precipitation of acetylacetonate,21 or citrate-nitrate auto-ignition process.14 Wang et al. obtained a conductivity increase in La2Mo2O9 fabricated by a novel three-stage thermal processing method.25 Besides, only a few detailed microstructural studies of La2Mo2O9 are available in the litterature. Marrero-Lopez and coworkers reported average crystallite size determined by the Scherrer approximation of 15 nm (powder) and 30 nm (sintered pellet) for monoclinic La1.95Eu0.05Mo2O9,21 20 nm for monoclinic La2Mo2O9,22 and 25 nm for cubic La1.9Sr0.1Mo2O9 powders,20 all prepared by modified freeze-dried precursors method. Yang and co-workers reported crystallites of about 20-50 µm for La2Mo2O9 prepared by sol-gel, leading after spark plasma sintering to 300-500 nm grains12 (as observed by TEM and SEM respectively). In this context, spray pyrolysis technique has been used for the first time in order to prepare La2Mo2O9 nanometric powders. Spray pyrolysis is efficient to obtain chemical homogeneous spherical nanoscaled particles.26 The solution-based aerosol process integrates the precipitation and thermolysis (or calcination) stages of powder synthesis into a single continuous process.27 Furthermore, powders are calcined in a very short time, 10 s maximum. The oxides produced by this method are composed of high purity polycrystalline particles with submicronic size. One of the most important advantage of this method is its ability to controlling particle size as a function of process parameters.28,29

10.1021/jp0775552 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/13/2008

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Figure 1. Sketch of the spray pyrolysis experimental setup.

In this paper, two kinds of ultrafine La2Mo2O9 powders were prepared by pyrolysis of an aerosol produced by ultrahigh frequency at 400 and 600 °C, and one batch was obtained by conventional solid-state reaction route (SSR). Sinterability was investigated by dilatometry. The microstructural characteristics of La2Mo2O9 powders and ceramics were studied by using appropriate characterization techniques such as thermal analysis, X-ray diffraction, SEM, and TEM. The aim of this work was to analyze the influence of the microstructure of La2Mo2O9 ceramics on their electrical properties by AC impedance spectroscopy starting from these three batches of powders. 2. Experimental Section 2.1. Synthesis. Two batches of La2Mo2O9 nanostructured powders were prepared by the ultrasonic spray pyrolysis route, from an aqueous precursor solution. The precursor solution was obtained from a stoichiometric mixture of decarbonated La2O3 (99.9%, Alfa Aesar) and MoO3 (99.95%, Alfa Aesar), both dissolved in a mixture of nitric acid and distilled water. For the complete MoO3 dissolution, droplets of H2O2 were added to the solution heated at about 70 °C during the process. The concentration of the final solution was fixed to 2.5 × 10-2 mol‚L-1. This solution was atomized by a high-frequency ultrasonic generator with three ceramic transducers. The frequency vibration of the piezoeletric ceramics was 1.7 MHz. The produced aerosol was carried by an N2 and O2 mixture, with a flow rate of 6 L‚min-1 through a three-zone tubular furnace (Carbolite TZF, 60 mm in diameter and 1090 mm in heated length) heated at two different temperatures, 400 °C and 600 °C. Then the powders were collected in an electrostatic receptor at the furnace exit. A very short pyrolysis time of about 8s was estimated for a 6 L‚min-1 flow rate. These conditions were based on a preliminary optimization work on the synthesis of ZrO2 based powders with controlled microstructure.28 The experimental setup of the synthesis process is sketched in Figure 1. Another La2Mo2O9 powdered batch was prepared from a conventional solid-state synthesis from oxides at 1000 °C. Calculated amounts of decarbonated La2O3 (99.9%, Alfa Aesar) and MoO3 (99.95%, Alfa Aesar) were ground in ethanol and, after evaporation, calcined at 1000 °C. Several annealings of 24 h and wet grindings were successively applied in order to reach a full solid-state reaction. The high purity powder was then ball milled in a planetary micromill pulverizette apparatus according to ref 30. For both synthesis methods, three batches of powders were pellet-shaped (typically, 5 mm in diameter and 1.5 mm thick), by uniaxial and isostatic pressing (250 MPa) at room temper-

ature and then sintered in air for 1 min at 900 °C (spray pyrolysis) and for 2 h at 1050 °C (solid-state reaction). The applied heating and cooling rates were 5 K‚min-1. In the following, the powdered and ceramic samples will be referred to as SP400, SP600 and SSR, for the samples prepared by spray pyrolysis at 400 °C, 600 °C, and by solid-state reaction respectively. 2.2. Characterization. X-ray diffraction was carried out at room temperature using a PANalytical Bragg-Brentano X’Pert PRO MPD diffractometer equipped with the X’celerator detector, using λCu KLIII-KLII radiations in the 10° e 2θ e 90° range with a 2θ step of 0.033° and 10 s counting time. The analysis of the powder XRD patterns was performed using the Rietveld31 whole pattern profile refinement with constant scale factors as implemented in the FULLPROF package.32 The profile of the diffraction line was modeled using a pseudo-Voigt function. The average crystallite size, defined as coherently diffracting domains, was determined by two methods: (i) applying the Scherrer formula assuming pure crystallite size broadening effects and (ii) the integral breadth method. The Scherrer formula (1) was corrected from instrumental resolution with a fully crystallized defect-free silicon sample (Si, SG. Fd3m; a ) 5.4306(3) Å).

L)

0.9λ cos θ xFobs2 - FInstr2

(1)

where λ is the radiation wavelength, θ is the Bragg angle, and Fobs and Finstr are the full width at half-maximum (fwhm) of sample and instrumental reflections at 2θ. Finstr was calculated using a Cagliotti equation, as the following:

Finstr ) xU tan2 θ + V tan θ + W

(2)

The patterns were indexed in the monoclinic cell, with P21 space group and a ) 7.143 Å, b ) 7.155 Å, c ) 7.162 Å, and β ) 89.54° as starting cell parameters, according to ref 10. The crystallite size and dispersion were obtained as the average value and standard deviation of L (eq 1) over the 283 calculated reflections from 15 to 85° in 2θ range. In a second step, the isotropic crystallite size and lattice microstrain were evaluated by a line profile analysis based on the integral breadth method. Given the complexity of the lowtemperature polymorph structure (312 crystallographically independent atoms33) and the large peak overlap due to slight monoclinic distortion (283 reflections in the 15 e 2θ(°) e 85 range), this study was carried out without structural constraints.

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Figure 2. (a) SEM micrographs of La2Mo2O9 powders prepared by solid-state reaction; (b) Left, SEM micrographs of La2Mo2O9 powders (SP400 and SP600); right, evaluation of particle grain size and volumic distributions.

A modified Thompson-Cox-Hastings pseudo-Voigt profile function was used. The angular dependence of the Gaussian and Lorentzian observed line widths are given by:

H2G ) U tan2 θ + V tan θ + W + HL ) X tan θ +

Y cos θ

IG cos2 θ

(3) (4)

HG and HL are the full width at half-maximum for Gaussian and Lorentzian components, and U, V, W, IG, X, and Y are refinable parameters. A different angular dependence allows the separation of size broadening (Y and IG) on the one hand and strain broadening (U and X) on the other hand.34-36 According to the Williamson and Hall method,37 the size and strain components to the line broadening can be obtained from the angular dependence of the integral breadths of the Voigt function from size and strain contributions respectively:

βS )

λ DV cos θ

βD ) e × 4 tan θ

(5) (6)

λ is the wavelength radiation, DV and e are respectively the volume-weighted crystallite size, and the upper limit of lattice microstrain.

Powders and ceramic samples were observed by highresolution scanning electron microscopy (SEM, ZEISS Ultra 55) and transmission electron microscopy (TEM, Jeol 2010). The pellets were optically polished (90%). The SP powders prepared at 400 °C and 600 °C present similar shrinkages except at low temperature (T < 600 °C) where an additional shrinkage was detected in SP400 probably due to the thermal decomposition of organic residues in good agreement with TG/ DTA measurements. After 600 °C, the similar shrinkages in SP400 and SP600 is certainly due to starting powders of similar morphology as shown previously by SEM observations. According to these data, we have selected the optimal sintering conditions in order to limit crystallite growth during the sintering process of La2Mo2O9 ceramics. Then, a short sintering duration (1 min) and the lowest sintering temperature (900 °C) were applied to pellet-shaped SP400 and SP600. Ball-milled SSR powders were sintered at 1050 °C for 2 h. The microstructure of SP400, SP600, and SSR La2Mo2O9 ceramics were observed by scanning electron microscopy on polished, thermally etched, and fractured ceramic samples. The SEM micrographs are presented in Figure 6. The mean grain size, statistically evaluated over 250-300 grains is given in Table 3. The microstructure of the ceramics was found strongly dependent on the elaboration method. For SSR, the micrographs show a very low porosity fraction. The estimated relative density was found 96% of the theoretical one (dth). The ceramic presents large particle size of about 25 µm in average. For SP400 and SP600 respectively, the porosity fraction observed was in agreement with the calculated relative density (∼89 and 92% of dth; see Table 3). The dimensions of the particles were significantly smaller than SSR (about 3.4 and 2.1 µm). On part d of Figure 6, a large scale micrograph of SP400 shows the

Figure 6. SEM micrographs of polished and thermally etched cross sections of La2Mo2O9 ceramics sintered from different powders: (a) SSR and ball milling sintered at 1050 °C; (b) SP400 sintered at 900 °C; (c) SP600 sintered at 900 °C; (d) large scale micrograph of SP400 with cracks; (e) fracture of SP600.

appearance of large cracks. During the shrinkage, the organic matter decomposition, detected by thermal analysis and dilatometry, occurs (Figure 4 and Figure 5) developing stresses and leading to large scale crack defaults within the pellet. An annealing of SP400 powder at low temperature (400 °C) should be attempted to complete the decomposition of organic matter before sintering. On the right part of Figure 6, one can observe the irregular shape of the grains surface for SP400 and SP600 which is confirmed by the fractography of SP600 ceramic (Figure 6e). This micrography suggests a submicrometric morphology of the grains in the ceramic, which does not appear

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TABLE 1: Structural and Microstructural Characteristics Obtained from X-ray Diffraction Data: Crystallite Size, Lattice Microstrain, Cell Parameters, and Fits Reliability sample SSR SP400 SP600 a

powder

unmilled milled

pellet powder pellet powder pellet

dmean (nm) ( 1

anisotropy (%)

max. strain (%)

anisotropy (%)

a

a

a

a

R.L. 2.2 R.L.a 0.1 3.2 0.5 4.2

R.L. 109.5 R.L.a 1.7 117.4 25.5 158.9

R.L. 0.054 R.L.a N. R.b 0.09 0.04 0.1

RBragg

R-F

7.1448 7.1553 7.1646 89.539 7.1446 7.1575 7.1639 89.565 7.1371 7.1531 7.1623 89.499 set to the values of SSR powder unmilled 7.1395 7.1527 7.1613 89.514 7.1211 7.1509 7.1622 89.527 7.1316 7.1517 7.1591 89.585

1.84 1.36 1.08 0.192 1.56 0.401 1.01

3.08 2.29 1.78 0.157 2.02 0.556 2.26

b (Å) ( 0.0005

c (Å) ( 0.0005

R.L. ) resolution limited (no peak broadening). b N.R. ) nonrefined.

TABLE 2: Comparison of the Average Crystallite Size Obtained by the Scherrer Formula and by Integral Breadth Method crystallite size (nm) Scherrer formula SSR powder unmilled SSR powder milled SSR pellet SP400 powder SP400 pellet SP600 powder SP600 pellet a

R.L. 0.0001 R.L.a N. R.b 0.0001 0.00003 0.0002

β (°) ( 0.001

a (Å) ( 0.0005

integral breadth

a

R.L. 76 (( 3) R.L.a 1.85 (( 0.01) 93.5 (( 23) 30.7 (( 2.6) 105 (( 7)

R.L.a 109.5 R.L.a 1.7 117.4 25.5 158.9

R.L. ) resolution limited (no peak broadening).

in the polished, thermally etched cross sections probably because of the relaxation of local strains during thermal etching. In this work, we have reported an efficient elaboration method to prepare dense La2Mo2O9 ceramics after sintering of SP600 fine powders of good purity at 900 °C for 1 min (total of 40 min between 800 °C and 900 °C considering the heating rate). 3.2. Structural and Microstructural Study from X-ray Diffraction. The average crystallite size and lattice microstrains were evaluated from line broadening by two methods: the Scherrer formula and the integral breadth method, as described in the experimental part. La2Mo2O9 SSR, SP400, and SP600 powders and ceramics crystallize in a monoclinic symmetry (P21 space group). The final observed and calculated X-ray patterns for the powder samples (SSR, SP400, and SP600) and one for SP600 pellet are presented in Figure 7. The inserts present the detail of the ((123) reflections region with observed and calculated profiles. The dotted line represents the best fit to observed points using the instrumental profile function and refining the cell parameters, while the solid line gives the calculated profile taking into account size and microstrain broadening. Preliminary attempts were carried out involving the monoclinic supercell (2 × 3 × 4), leading to numerous superstructure reflections with correct reliability factors (Bragg R ) 2.64; Rf ) 7.25). P21 space group correctly accounted for the additional superstructure peaks and the following refined cell parameters : a ) 14.2919(5) Å; b ) 21.4587(9) Å; c ) 28.66(1) Å, and β ) 89.50(3)° were found very close to that reported by Evans et al., obtained from single crystal.33 However, the number of calculated reflections (6397 reflections in the 15 e 2θ(°) e 85 range) led to a substantial increase of peak overlap which was tricky for the significance of the obtained parameters. All of the patterns were thus indexed in the monoclinic subcell. First, the best fit between calculated and observed profiles was obtained by a least-squares refinement procedure using the instrumental resolution function and refining the detector-zero, the monoclinic cell parameters and the background points. Then, the observed and calculated profiles were adjusted by refining size and microstrain parameters only.

After many attempts, we found that Lorentzian size broadening and Gaussian strain broadening gave the best reliability and calculated to observed profiles agreement. Consequently, only Y and U parameters were refined. Let us mention that, given the important peak overlap and subsequent systematic errors in integral breadth values obtained by profile fitting, we just intended to provide a microstructural comparison between the series of samples under investigation in this study, whereas absolute values of size and strain should not be considered as trustworthy. The results of the integral breadth study are summarized in Table 1. For the unmilled powder prepared by solid-state reaction, no peak broadening was detected, indicating a large crystallite size (>1 µm). In the case of the milled SSR powder, a large crystallite size of 109 nm was found. Whatever the elaboration route, for La2Mo2O9 powders and ceramics, the values of microstrain are very low (Table 1). These data indicate that XRD line broadening was dominated by crystallite size broadening effects. As a consequence, the values of size were close to that found from the Scherrer formula (Table 2). We observed an overestimation of domain size by the integral breadth method with comparison to Scherrer method as previously observed by Balzar et al.34 SP400 powder presents crystallized germs of a few nanometers (∼2 nm) corresponding to an initiation of the crystallization process while SP600 contains grains of about 27 nm. This result can be correlated to the average crystallite size of 15 to 50 nm reported for nanometric La2Mo2O9-type powders.12,19-21 Although SP400 and SP600 present similar morphologies as shown by SEM (Figure 2), their crystallinity degree is dependent on the spray pyrolysis temperature. Crystallites of about 27 nm in average in raw SP 600 powders reached about 130 nm after sintering at 900 °C while SP400 crystallite size increased from 2 to 105 nm as confirmed by microscopic observations (Figure 3, Figure 6). After sintering at 900 °C, we have measured a crystallite growth which was remarkably enhanced by 10 times when starting from SP400 powders with comparison to SP600 powders. Consequently, the nanoscaled crystallites obtained by spray pyrolysis grew during the sintering process but remained significantly smaller than that observed in the conventional solidstate reaction sample. The results of this microstructural analysis appear consistent with the microscopic observations. 3.3. Electrical properties The electrical properties of La2Mo2O9 pellets (SSR from milled powders, SP400, and SP600) were investigated by AC impedance spectroscopy in air from 240 to 850 °C. The impedance spectra of SP400 are reported in Figure 8 from 239 to 482 °C. An equivalent electrical circuit composed of four contributions (see Figure 8, inset) was used to fit the measured impedance by a least-squares procedure to extract the electrical features of the ceramic as explained in the experi-

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Figure 7. X-ray diffraction patterns of La2Mo2O9 prepared by solid-state reaction (a) and spray pyrolysis at 400 °C (b) and 600 °C (c, powder; d, pellet). Results of the whole profile refinement in monoclinic symmetry (SG P21). Observed, calculated profiles, difference, Bragg positions, and Miller indexes (cubic/monoclinic). The insets show the calculated profiles using the instrumental resolution function.

Figure 8. Experimental and calculated AC impedance spectra of La2Mo2O9 SP400 pellet for different temperatures. The numbers indicate the log of frequency. In inset, the equivalent electrical circuit used for the fits.

TABLE 3: Electrical Characteristics of SSR, SP400, and SP600 Ceramics at about 380 °C high freq. (grains) f cm-1

da

accuracy (0.1 (1 SSR 0.59 96 SP 400d 0.60 89 SP 600d 0.60 91 a

dmean C1f F1 ) R1/f (µm)c (°C) 104 Ω.cm 10-1 2F.cm-1 Ta

(0.2 25.0 3.4 2.1

(2 384 379 383

(0.1 3.3 3.3 3.0

(0.2 4.3 8.28 5.6

grain boundaries ω(1) 0

β1 (°)

106 rad.s-1

(0.1 13.2 3.3 11.5

(0.1 6.9 3.6 5.9

R2 103Ω

β2 (°)

C2 10-8 F

low freq. (electrode) ω(2) 0

R3 T3 104 rad.s-1 105 Ω F.s1-p3

(0.1 (0.2 (0.1 0.302 36.6 24.6 70 5460 20 4.7 1.00 44.2

(0.1 0.9 26.1 2.1

(0.1 0.9 22.8 4.8

p3

(0.2 3.67 0.75 0.13 0.43 7.72 0.61

Relative density (%). b Measurement temperature. c Mean particle size. d Sintered at 900 °C.

mental part. The total impedance and the elementary contributions are shown in bold and thin line, respectively in Figure 8. For each spectrum, a good agreement was obtained between experimental and calculated points. The deconvolution allowed us to identify a contribution of the grains interior (bulk) at high frequency and an important blocking effect of the microstructure at lower frequencies, assumed to be essentially due to interfacial processes. The impedance spectra of SP400, SP600, and SSR were compared at about 380 °C (Figure 9) after a stabilization period of 24 h. For each sample, the same electrical circuit was used to fit the experimental points. For each (R//CPE) contribution, the relaxation frequencies, capacitances and depression angles (β) were deduced from the refined parameters according

to:

ω0 ) (RA)-1/p ) (RC)-1

β ) (1 - p)

π 2

(9)

For the grains contributions only (high-frequency relaxation), the resistances and capacitances were normalized according to geometrical factors:

Fi )

Ri f

oR(i) ) Cif

(10)

All electrical parameters in high, intermediate, and low frequency domains are summarized in Table 3 for SSR, SP400,

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Figure 9. Comparison of experimental and calculated AC impedance spectra at about 380 °C of SSR, SP400 and SP600 La2Mo2O9 pellets. G and GB indicate grain and grain boundaries contributions respectively. Nyquist and Bode representations for left and right parts respectively.

and SP600 ceramics characterized by average grain size of larger than 1 µm, 105 and 130 nm respectively. The first main result is that the grains resistivity as extracted from AC spectra (Figure 9) has the same value whatever the elaboration method in the given temperature accuracy. Consequently, the particle and crystallite size reduction obtained in spray pyrolysis ceramics has no significant effect on the intrinsic conductivity of La2Mo2O9. The main significant effect of grain size was detected on the blocking phenomena at intermediate frequency. Indeed, as far as grain size is increased from spray-pyrolysis to SSR pellets, blocking contribution is decreased (Figure 9). In the case of spray pyrolysis ceramics, where grain size and porosity were of the same order of magnitude, a larger blocking effect was detected in SP400 than in SP600 and could be attributed to the large scale fractures observed by SEM (Figure 6d) and due to partially decomposed organic precursors in the starting powder rather than specific microstructural features. On the right-hand part of Figure 9, the same data are presented in the Bode plane (F as a function of log f). The calculated bulk responses are very similar for the three samples (gray areas) with, however, a slight shift of the relaxation frequency from 5.9 × 106 to 6.9 × 106 and 3.6 × 106 ( 0.1 × 106 rad‚s-1 for SSR, SP400, and SP600 respectively, that could be partially explained by the slight temperature difference between the samples. For SP400, the complete deconvolution is shown in Figure 9, the bold line being the sum of the three thin lines. This figure demonstrates the necessity of taking into account the electrode process in the fit, because of the important convolution between the different processes. The dotted line represents the total calculated imaginary resistivity from which the electrode contribution was mathematically subtracted. Figure 10 reports an Arrhenius plot of the bulk conductivities for SSR (solid gray line) and SP600 (triangles), the bulk and grain boundaries conductivities for SP400 (open and closed squares). The grain boundary conductivity of SP400 which was arbitrarily corrected from the geometrical factor of the pellet to be compared with the bulk conductivity was found smaller than the grain contribution from

Figure 10. Arrhenius plots of conductivity of La2Mo2O9 ceramics prepared from conventional SSR powder (solid gray line), SP600 (triangles), and SP400 (squares). Open squares, grains contribution; closed squares, grain boundaries contribution. Inset: Arrhenius plot of the relaxation frequencies for both contributions.

250 to 500 °C and was accompanied with a higher activation energy (1.08 eV). When temperature was increased, the blocking effect was decreased as expected with the reduction of the blocking factor defined by:

RR )

RGB RG + RGB

(11)

where RR ) 0.87 at 239 °C and RR ) 0.67 at 482 °C. It seems that the elaboration method has no significant incidence on the

3202 J. Phys. Chem. C, Vol. 112, No. 9, 2008 bulk conductivity within the whole thermal range. For SP600, the activation energies of ionic mobility in the grains were found equal to 0.88 eV from 250 to 500 °C and 0.74 eV from 600 to 850 °C for R and β polymorphs, respectively. These bulk activation energies were comparable to those of SSR and SP400. The elaboration method had no more effect on the (R T β) first-order transition temperature nor magnitude. The thermal evolution and in particular the activation energies of the relaxation frequencies (Figure 10, inset) extracted from the impedance spectra of SP400 are in agreement with those of resistivity. 4. Conclusion This study provided a consistent set of data on microstructural influence of La2Mo2O9 ceramics on electrical properties. Fine La2Mo2O9 nanoscaled polycrystalline powders were successfully prepared for the first time by spray pyrolysis in a few seconds at temperatures as low as 400 and 600 °C. The sintering temperature was also reduced by almost 150-200 °C as compared to conventional sintering from solid-state powders. Microstructural analyses from X-ray diffraction data and SEM/ TEM images demonstrated that the coherent scattering domains remained nanoscaled after the sintering process, in spite of significant growing. The ceramic particles size was also much smaller than conventional samples. The influence of these specific morphological and microstructural features on the electrical properties of SSR, SP400, and SP600 La2Mo2O9 ceramics was not found significant. Finally, we have demonstrated in this paper that spray pyrolysis is an interesting synthesis route to assess coherent domains and ceramic grains dimensions in La2Mo2O9 ceramics. Further experiments are in progress with a careful control of the spray pyrolysis process parameters to further lower the particle and distribution sizes of La2Mo2O9 powders and to limit crystal growth during the sintering process. Acknowledgment. The authors are grateful to acknowledge CNPq - Brazil for R.A. Rocha scholarship. References and Notes (1) Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345-352. (2) Goodenough, J. B. Nature 2000, 404, 821-823. (3) Kosacki, I.; Rouleau, C. M.; Becher, P. F.; Bentley, J.; Lowndes, D. H. Solid State Ionics 2005, 176, 1319. (4) Kosacki, I.; Suzuki, T.; Petrovsky, V.; Anderson, H. U. Solid State Ionics 2000, 136-137, 1225. (5) Goodenough, J. B. Ann. ReV. Mater. Res. 2003, 33, 91-128. (6) Florio, D. Z.; Fonseca, F. C.; Muccillo, E. N. S.; Muccillo, R. Ceramica 2004, 50, 275-290. (7) Lacorre, P. Solid State Sci. 2000, 2, 755-758.

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