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Anisotropic Oxygen Ion Diffusion in Layered PrBaCo2O5+δ Mónica Burriel,†,* Juan Peña-Martínez,†,‡ Richard J. Chater,† Sarah Fearn,† Andrey V. Berenov,† Stephen J. Skinner,† and John A. Kilner† †

Department of Materials, Imperial College, London SW7 2AZ, United Kingdom Inorganic Chemistry Department, Complutense University of Madrid, 28040-Madrid, Spain

‡

ABSTRACT: Oxygen diffusion and surface exchange coefficients have been measured on polycrystalline samples of the double perovskite oxide PrBaCo2O5+δ by the isotope exchange depth profile method, using a time-of-flight SIMS instrument. The measured diffusion coefficients show an activation energy of 1.02 eV, as compared to 0.89 eV for the surface exchange coefficients in the temperature range from 300 to 670 °C. Inhomogeneity was observed in the distribution of the oxygen-18 isotopic fraction from grain to grain in the ceramic samples, which was attributed to anisotropy in the diffusion and exchange of oxygen. By the use of a novel combination of electron back scattered diffraction measurements, time-of-flight, and focused ion beam SIMS, this anisotropy was confirmed by in-depth analysis of single grains of known orientation in a ceramic sample exchanged at 300 °C. Diffusion was shown to be faster in a grain oriented with the surface normal close to 100 and 010 (ab-plane oriented) than a grain with a surface normal close to 001 (c-axis oriented). The magnitude of this anisotropy is estimated to be close to a factor of 4, but this is only a lower bound due to experimental limitations. These findings are consistent with recent molecular dynamic simulations of this material where anisotropy in the oxygen transport was predicted. KEYWORDS: anisotropy, oxygen diffusion, layered cobaltites, double perovskites, mixed conductors (MIEC), cathodes, solid oxide fuel cells (SOFC)

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INTRODUCTION The current interest for economical, efficient, and sustainable energy conversion systems has driven research on materials with fast oxygen kinetics for high temperature electrochemical devices. More recently, it has been recognized that to achieve these goals the temperature of operation of many of these devices must be lowered into the so-called intermediate temperature zone (500−700 °C). The search for materials with fast oxygen kinetics at lower temperatures has covered many mixed oxide materials, and some progress is being made toward viable materials. Most work has been focused on isotropic cubic material, as it was believed that high mass transport rates were mainly to be found in such materials. Recent experimental and theoretical studies have highlighted the potential of layered oxides, such as those of the Ruddlesden−Popper series or the A-site layered perovskite oxides LnBaCo2O5+δ (Ln = lanthanide), as candidate materials for intermediate temperature solid oxide fuel cell (IT-SOFC) cathodes and oxygen separation membranes.1−3 The exact crystal structure of each of the LnBaCo2O5+δ series of oxides is a complex function of many variables, but they © 2012 American Chemical Society

are generally classified as a double perovskite structure AA′B2O5+δ (where A = Ln, A′ = Ba and B = Co). The size difference between the large Ba cation and the smaller Ln cation results in the formation of the layered orthorhombic crystal structure (space group Pmmm), in ambient atmospheres; however, as mentioned above, the exact structure can vary with Ln cation size, atmosphere, and temperature, because the highly mobile oxygen sublattice can easily change the degree of occupancy (δ) and oxygen vacancy ordering. In the orthorhombic structure (see Figure 1A) the Ba and Ln cations are ordered in alternating BaO, CoO2, and LnO layers (..., BaO, CoO2, LnO, CoO2, .....) along the c-axis, and the oxygen nonstoichiometry is limited to the LnO layers. These materials can display significant mixed conductivity with substantial electronic and oxygen ion conductivity. In the main, the range of electronic conductivity is sufficient for IT-SOFC applications (≈ 50−300 S cm−1 4), but it is not clear Received: November 22, 2011 Revised: January 19, 2012 Published: January 19, 2012 613

dx.doi.org/10.1021/cm203502s | Chem. Mater. 2012, 24, 613−621

Chemistry of Materials

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Figure 1. (A) Polyhedral view of the orthorhombic perovskite PrBaCo2O5+δ structure with lattice parameters a ∼ b ∼ ap and c ∼ 2ap Pr3+ ions are shown in blue, Ba2+ are shown in green, and Co3+ are shown in yellow. (B) Oxygen density plot for PrBaCo2O5.5 lattice at 927 °C (reprinted from ref 9 with permission of Elsevier).

So far experimental studies of the oxygen transport kinetics have been less extensive. The oxide-ion diffusion and surface exchange kinetics of LnBaCo2O5+δ (Ln = Pr and Gd) have been measured using an 18O/16O isotope exchange depth profile1,10−12 and electrical conductivity relaxation,1 and both PrBaCo2O5+δ (PBCO) and GdBaCo2O5+δ (GBCO) have been shown to exhibit fast oxygen diffusion and surface kinetics in the IT-SOFC range (500−700 °C). The oxygen surface exchange and oxygen tracer diffusion coefficients for GBCO measured by the 18O/16O isotope exchange depth profile (IEDP) method are k* = 1.6 × 10−8 cm/s and D* = 7.6 × 10−11 cm2/s at 396 °C.10 For PBCO much higher k* and D* values, ∼10−6 cm/s and ∼10−7 cm2/s at 400 °C, respectively, have been reported by Kim et al.,1 in apparent confirmation of the predictions of Seymour et al.,6 although there is some doubt about these measurements because of problems with sample density.13 In addition, very recently Frison et al.12 have reported for PBCO a limited set of k* and D* data at two temperatures: 350 and 450 °C. The values obtained are slightly higher than those reported for GBCO10 but much lower than the ones reported for PBCO by Kim et al.1 However, the authors comment on the differences in the shape of the diffusion profiles which could be explained by the presence of residual porosity or extended defects such as dislocations and grain boundaries. Finally Zhang et al.4 have reported that the performance for LnBaCo2O5+δ (Ln = Pr, Gd, Sm) oxides as oxygen permeation membranes follow the order Pr3+ > Gd3+ > Sm3+, indicating that PBCO may well be the material with optimized oxygen transport in this oxide series. The interesting predictions from the theoretical studies, mentioned above, of anisotropic oxygen transport in the LnBaCo2O5+δ series and the optimum nature of PBCO clearly need

what composition is the optimum for the oxygen transport kinetics. A number of recent theoretical studies have shown that there is considerable interaction between the cation and the anion sublattices.5−9 It has been suggested, citing results from molecular dynamics simulations of both PrBaCo2O5+δ and GdBaCo2O5+δ, that the oxygen transport for the LnBaCo2O5+δ series will be anisotropic (see Figure 1B) and that the oxygen ions will move easily in the LnO and CoO2 layers but not in the BaO layers.5,8,9 It was also shown that the presence of an A-site cation ordered structure will improve the oxygen transport properties compared to the Ln0.5Ba0.5CoO3−δ materials with the same nominal composition but with a cation disordered perovskite structure.5 Lattice static calculations for the fixed stoichiometry orthorhombic structured LnBaCo2O5.5 series have shown that, given the ordered structure, the defect energies are dependent upon the size of the Ln cation.6 Perhaps the most interesting result of these simulations is the finding that the order on the oxygen sublattice, as defined by the oxygen Frenkel energy, is lowered as the Ln cation size approaches that of the largest cation (La). Seymour et al.6 also found that the A-cation ordering is dependent upon cation size, and the same tendency is found; that is, the A-cation sublattice will tend to become more disordered, for the larger lanthanide ions. They speculate that PrBaCo2O5+δ is the composition most optimized for oxygen transport, because the cation lattice should be well ordered but the anion sublattice should become disordered at high temperatures. These simulations, both the lattice static and the molecular dynamic, are of course simplifications of the behavior of real materials where, for example, it is well-known that the oxygen stoichiometry of the materials varies with temperature and with composition.4 Hence there is considerable interest in experimental verification of these simulations. 614



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within grains of a similar orientation chosen from the EBSD map (Figure 9B). All the exchanged samples were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS) on a Ion ToF-SIMS 5 machine (ION-TOF GmbH, Münster, Germany) equipped with bismuth LMIS pulsed gun incident at 45°. A 25 kV Bi+ primary ion beam of 1 pA current was used to generate the secondary ions using the low mass resolution or burst alignment mode (eight pulses) for analysis and a Cs+ beam (2 kV) incident at 45° for sputtering. Tracer oxygen depth profiles were measured from the exchanged surface penetrating through the sample for the samples with short diffusion profiles (exchanged at low temperatures) by sputter depth profiling. After SIMS analysis the depth of the sputter etched craters was measured using an optical microscope based interferometer, ZYGO Corp. NewView 200. In the case of the samples with long diffusion profiles (exchanged at higher temperatures) the samples were cut perpendicular to the exchanged surface and the interior cross-section was polished prior to the measurement. SIMS tracer oxygen and oxygen-16 images of the cut cross-section were acquired for selected areas including the edge of the sample. SIMS image acquisition was interspersed with Cs+ sputtering of the surface to increase ion yields and to minimize the interferences due to residual gas adsorption from the vacuum chamber, base pressure 5 × 10−10 mbar. More details of this method have been described elsewhere.16

experimental verification. In addition, as there is some doubt as to the relative level of the oxygen transport behavior of the GBCO and PBCO, it was thought necessary to remeasure the oxygen exchange kinetics of dense high quality PBCO samples for comparison with the results of simulation and previous experiments.

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EXPERIMENTAL SECTION

PrBaCo2O5+δ (PBCO) powder was synthesized by conventional solid state reaction using high purity commercial (Alfa Aesar) oxides and carbonates as starting materials, that is, Co3O4 (99.7%), Pr6O11 (99.99%), and BaCO3 (99.95%). After calcination at 1000 °C for 10 h, single phase was confirmed by powder X-ray diffraction (Panalytical X’Pert Pro diffractometer, Cu Kα radiation, and X’Celerator detector). In order to study the densification process, as-prepared powders were ball milled, pressed uniaxially (75 MPa) into pellets, and fired at 1050, 1100, and 1150 °C for 6−48 h. Density was calculated using the Archimedes method, and the sample morphology was investigated by scanning electron microscopy (SEM) (microscope Jeol JSM-6400) combined with energy dispersive spectroscopy analysis (EDS) to detect possible phase segregation. The average grain sizes of the sintered pellets were estimated from the SEM micrographs using the linear intercept method. Conductivity in air was measured by a DC 4-probe method on PBCO bars (20.0 × 6.5 × 2.0 mm) which had been pressed first uniaxially (75 MPa) and then isostatically (300 MPa) and sintered at 1150 °C for 48 h. The oxygen isotope exchange depth profile (IEDP) technique was used to measure the oxygen surface exchange and diffusion properties of dense PBCO pellets (relative densities > 95%) polished to 0.25 μm finish. The isotopic exchange was carried out at six different temperatures ranging from 300 to 675 °C. In order to ensure that the material was in equilibrium at the exchange temperature and oxygen partial pressure of the exchange (pO2 = 0.2 bar), the samples were first subjected to a preannealing step in pure oxygen (research grade 99.9995%) of natural isotopic abundance for a period of time approximately 1 order of magnitude greater than the 18O tracer annealing time and at the same temperature as the subsequent exchange. The samples were then cooled to room temperature, the natural oxygen gas was removed, and an 18O enriched gas (26.0%) was introduced. The samples were rapidly heated to the annealing temperature by rolling the preheated furnace over the sample holder and then maintained at constant temperature for a selected period of time and subsequently quenched to room temperature by rolling the furnace off the sample holder. The corrected effective 18O exchange time was calculated taking into account the noninstantaneous increase/decrease of the sample temperature.14 The 18 O exposure time of the exchange experiments was chosen to obtain short diffusion lengths on the order of a few micrometers ( 95% were achieved for PBCO after sintering at 1100 °C for 24 h. The grain size distribution and the grain growth for the aforementioned 615



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Figure 3. SEM Images of PrBaCo2O5+δ samples fired at (A−D) 1050 °C, (E−H) 1100 °C, and (I−L) 1150 °C for 6, 12, 24, and 48 h.

samples was analyzed regarding the following kinetics equation20,21

Dn /t = K 0e−Q / RT

The overall electrical conductivity for the PBCO sample sintered at 1150 °C for 48 h was measured in air, and the conductivity values were comparable although slightly higher than those reported by Kim et al.1 The conductivity was around 1600 S cm−1 at 150 °C. However, above 150 °C the conductivity continually decreases down to 600 S cm−1 at 800 °C. This is explained in terms of reduction of Co4+ to Co3+ with a loss of oxygen, namely, an increasing concentration of oxygen vacancies.22 Oxygen Diffusion and Surface Exchange Properties. The PBCO oxygen diffusion and surface exchange coefficients were measured from SIMS depth profiles for samples exchanged at low temperatures and from integrating cross-section images for those exchanged at high temperatures. All the PBCO pellets used for these experiments had been sintered at 1150 °C for 48 h. As an example of a depth profile measurement, for the PBCO pellet exchanged at 300 °C, Figure 4A shows the

(1)

where D is the average grain size, t is the time, K0 is a rate constant, Q is the activation energy of grain growth, and n is an integer ranging from 1 to 4. Given that in a temperature range the activation energy Q is constant, the exponent n can be determined from the slope in the plot of ln(D) versus ln(t). Accordingly, the activation energy Q can be obtained from the slope of the Arrhenius plot of ln(Dn/t) versus 1/T. Thus, with a n value of 3, activation energies around 560 and 530 kJ mol−1 have been calculated for PBCO samples fired up to 1150 °C for 24 and 48 h, respectively. In other words, a treatment at 1150 °C for 48 h has been the most appropriate in order to obtain specimens with relative density > 95%; see Figure 3. 616



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PrO− (141Pr16O−), BaO− (138Ba16O−), and CoO− (59Co16O−) measured as a function of depth. The ions of interest were selected among all secondary ions of negative polarity which were acquired by ToF-SIMS in parallel in a single measurement. The PrO−, BaO−, and CoO− are constant through the depth of the crater (2.8 μm), while the 18O− signal decreases and the 16O− signal increases with depth, following a typical oxygen isotope exchange profile. As explained in ref 16, the 18O fraction c* was calculated as:

c* =

[18O] [18O] + [16O]

=

∑ik Ni18 ∑ik Ni18 + kN116

(2)

where NiM is the number of counts contained within the ith peak of the k MO burst peaks. k, the number of burst peaks, was set to be 8 for all the measurements on the PBCO samples. To obtain the oxygen tracer diffusion (D*) and surface exchange (k*) coefficients, the normalized 18O isotopic concentration c′(x) was calculated by:

c′(x) =

c*(x) − c*bg c*g − c*bg

(3)

where c*bg is the background isotope fraction and c*g is the isotope fraction of the annealing gas. Then the obtained diffusion profiles were fitted by nonlinear least-squares regression to

⎛ k*x ⎛ x ⎞ k*2 t ⎞ ⎟ c′(x) = erfc⎜ + ⎟ − exp⎜ ⎝ 2 D*t ⎠ D* ⎠ ⎝ D* ⎛ x t ⎞ × erfc⎜ + k* ⎟ ⎝ 2 D*t D* ⎠

(4)

which is the solution of Fick’s law for one-dimensional diffusion with surface limitation into a semi-infinite medium, as given by Crank.23 Figure 4C shows the values of the normalized oxygen-18 concentration extracted from the depth profile (Figure 4B) and their fit to Crank’s solution to Fick’s second law. It can be seen that the obtained experimental and fitted curves are in very good agreement, from which the fitted values of the oxygen surface exchange and diffusion coefficients (3.1 × 10−10 cm/s and 4.6 × 10−12 cm2/s, respectively) can be extracted. As mentioned previously, the depth profiling method was used for temperatures below 400 °C. Above this temperature the imaging (line-scan) method was used, as shown in Figure 5. From all negative secondary ions, those of interest (mainly 18 − 16 − O , O , PrO−, BaO−, and CoO−) were selected and imaged for the cut cross sections of the exchanged pellets (including the edge surface where the exchange took place). As an example, Figure 5A shows the 18O− image of the cross section of a PBCO bulk sample exchanged at 675 °C. It can be clearly seen how the 18O ions diffuse from the surface of the sample into the bulk. By integrating the area of the image along the x direction, the normalized values of the 18O concentration along y for that region can be calculated (Figure 5B) and fitted to Crank’s solution, obtaining the D* and k* fitted values at the exchanged temperature (1.7 × 10−8 cm2/s and 4.6 × 10−7 cm/s, respectively). It should be noted that for all the temperatures measured, the 18O− concentration was not completely homogeneous for the whole surface areas measured. In the surface images of the depth profiles, areas of higher and lower 18O concentration can

Figure 4. (A) Surface image of the normalized 18O concentration of the analyzed area in the center of a crater for the PBCO pellet exchanged at 300 °C, (B) SIMS signals obtained for the ions of interest as a function of depth for the analyzed area, and (C) extracted normalized 18O concentration depth profile and fit to Crank’s solution to Fick’s second law.

normalized 18O concentration of the analyzed area (60.3 × 60.3 μm2) in the center of a 300 × 300 μm2 size crater while Figure 4B shows the secondary ion signals for 18O−, 16O−, 617



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that there are areas of the pellet with different surface exchange and oxygen diffusion properties, which in the case of PBCO could be related to regions with different surface terminations and different grain orientation. For high temperatures the oxygen penetration profile (tens to hundreds of micrometers) goes through a large number of grains. Accordingly, the extracted D* and k* measured values at each temperature correspond to average coefficients for all grain orientations. The average oxygen tracer diffusion coefficients obtained for the whole temperature range are represented in an Arrhenius plot in Figure 6 and compared with the literature data published

Figure 6. Arrhenius plot of the oxygen tracer diffusion coefficients obtained for PBCO and comparison with literature data.

for ordered cobaltites. The measured diffusion coefficients are quite high, ranging from 5.7 × 10−12 cm2/s for temperatures as low as 300 °C to 2 × 10−8 cm2/s for intermediate temperatures (670 °C). Despite the high values of the diffusion coefficients, they are clearly several orders of magnitude lower than those measured by Kim et al.1 for the same composition. On the other hand, the D* values measured for PBCO at low temperatures are slightly lower than those measured recently by Frison et al.12 for PBCO and very similar to the ones measured by Tarancon et al.10 for the Gd-analogous compound: GBCO. At temperatures higher than 400 °C the Pr compound possesses higher oxygen diffusion values (up to 1 order of magnitude) related to a higher activation energy (1.02 eV for PBCO compared to 0.60 eV for GBCO). In the case of the surface exchange coefficients (Figure 7), again the values obtained for PBCO are significantly lower than those reported by Kim et al.1 for the same composition but similar to those measured by Frison et al.12 and virtually identical to the ones measured by Tarancon et al.10 for GBCO for the whole measured temperature range and with very similar activation energies (0.89 eV for PBCO and 0.81 for GBCO). As mentioned by several authors,12,13 the low density of the samples used in the measurements by Kim et al1 are suspected to be the origin of this difference due to the deep penetration of tracer into the porosity in the sample. In order to evaluate the relationship between the exchange and diffusion properties of PBCO with its structural anisotropy, an in-depth characterization was performed for a PBCO sample exchanged at the lowest temperature (300 °C). In order to

Figure 5. (A) Surface image of the normalized 18O concentration of the analyzed cross-section for the PBCO pellet exchanged at 675 °C, (B) SIMS signals obtained for the ions of interest as a function of distance from the edge of the sample, and (C) extracted normalized 18O concentration profile and fit to Crank’s solution to Fick’s second law.

be distinguished (see Figure 4A). The same observation can be made for the cross-sectional areas (Figure 5A) with slight differences in the 18O concentration profile from one region to the other. The nonuniformity of the 18O distribution implies 618



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with high and low 18O surface concentration, respectively. There normalized 18O surface concentration for subregion 2.1 (0.015) is more than 3 times higher than that of subregion 2.2 (0.004), while the average surface value for the whole region 2 has an intermediate value of 0.010, as expected. For each subregion the normalized 18O concentration was reconstructed in depth and fitted to Crank’s solution (Figure 8). The obtained coefficients are summarized in Table 1, which Table 1. Oxygen Tracer Diffusion and Surface Exchange Coefficients at 300 °C for Different Regions of a PBCO Pellet D* (cm2/s) region 1 (average) region 2 (average) subregion 2.1 subregion 2.2 “c-axis” oriented grain “ab-plane” oriented grain

Figure 7. Arrhenius plot of the surface exchange coefficients obtained for PBCO and comparison with literature data.

4.6 6.8 4.7 3.0 3.6 1.5

× × × × × ×

10−12 10−12 10−12 10−11 10−12 10−11

k* (cm/s) 3.1 4.9 5.9 3.6 6.3 3.1

× × × × × ×

10−10 10−10 10−10 10−10 10−10 10−10

demonstrates a remarkable difference in oxygen diffusivity between different regions. The diffusion coefficient along the direction perpendicular to the sample surface for the grains included in subregion 2.1 is about 1 order of magnitude lower than that of the grains included in subregion 2.2. On the other hand, the surface exchange coefficient for the grains included in subregion 2.1 is slightly higher than that of subregion 2.2. In order to try to relate regions with higher and lower measured diffusivity with the orientation of the individual grains in the pellet, electron backscattered diffraction measurements were performed on a selected region of the surface of the PBCO pellet exchanged at 300 °C. Figure 9A shows the secondary electron image of the measured area, in which the size and shape of the individual grains can be clearly distinguished. It should be noted that, due to the fine polish of the surface, the micrograph exaggerates vertical distances. The surface roughness of the sample was measured, and the difference in height between grains is of the order of tenths of nanometers. It can be seen that the individual grains of known orientation from the EBSD color-coded surface orientation image (Figure 9B) are easily identifiable in the SEM image. In this way it was possible to choose, from the measured area, grains of a given orientation and then depth profile these individual grains by FIB-SIMS, measuring the 16O and 18O secondary ions in depth. Two of the measured grains are highlighted in Figure 9B (i.e., grains A and B), and their corresponding pole figures in the surface normal direction are displayed in Figure 9C. These particular grains were selected to probe the diffusion anisotropy due to their orientation: grain B was selected as its surface normal orientation was close to 001, and therefore close to the c-axis orientation of PBCO. This is the direction along which the oxygen diffusion is expected to be lowest. Conversely, grain A was selected due to its orientation close to the 100 and 010 directions, and therefore, close the ab-plane orientation of PBCO, along which the oxygen diffusion is expected to be maximum. Figure 10 shows the 18O normalized concentration profiles for the selected grains: grain A, labeled as “ab-plane oriented” and grain B, labeled as “c-axis oriented”. The shapes of the profiles are clearly very different, with a higher normalized surface concentration (close to 0.015) and a much steeper concentration decay for the “c-axis oriented” grain. It must be

restrict the oxygen diffusion length to individual grains, an exchange time 42 min was chosen. Figure 8 shows the normalized

Figure 8. Normalized 18O concentration profiles in different regions for a PBCO pellet exchanged at 300 °C for 42 min and fit to Crank’s solution to Fick’s second law.

values of the 18O concentration measured in two different regions (region 1 and region 2) and fits to Crank’s solution to Fick’s second law. The fitted diffusion coefficients (4.6 × 10−12 cm2/s and 6.8 × 10−12 cm2/s) and surface exchange coefficients (3.1 × 10−10 cm/s and 4.9 × 10−10 cm/s) correspond to measured areas of 60 × 60 μm2 and 40 × 40 μm2 for regions 1 and 2, respectively, and constitute therefore an average value over a large number of grains. As previously commented, in each image it is possible to identify areas of a few squared micrometers (which could correspond to the size of individual grains) with higher and lower 18O concentration values. One of the advantages of ToF-SIMS is that, due to its high lateral and in-depth resolution, it is possible to select subregions of interest and to reconstruct all the secondary ion information for each area. From the 18O surface image of the analyzed area, subregions 2.1 and 2.2 were selected, which correspond to areas 619



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those for the “c-axis oriented” grain, while the surface concentration (close to 0.004) and curve shape for subregion 2.2 (Figure 8) are very similar to those for the “ab-plane oriented” grain. When fitted to Crank’s solution to Fick’s second law, the diffusion coefficients obtained for the individual grains with different orientations are over a magnitude different: 3.6 × 10−12 cm2/s for the “c-axis” and 1.5 × 10−11 cm2/s for the “ab-plane”. The obtained surface exchange coefficients are 6.3 × 10−10 cm/s and 3.1 × 10−10 cm/s for the “c-axis” and “ab-plane” oriented grains, respectively. From these results we observe that these values match very well those obtained for subregions 2.1 and 2.2 (see Table 1). Therefore the relationship between areas with different oxygen concentrations being associated with different orientations has been confirmed. In addition, the anisotropy in the oxygen diffusion, which had been predicted by simulations,7 has been experimentally proven for the first time. These experiments must be viewed in the light of our previous experiments using the FIB SIMS to depth profile individual grains in ceramic samples of the isotropic material La0.8Sr0.2MnO3.24 In this previous experiment we depth profiled a series of grains in a sample that had been exchanged with oxygen-18. It was found that, within statistical error limits, the oxygen diffusion and surface exchange coefficients were identical in each of the sampled grains. The diffusion coefficients were expected to be identical, as would be the case for an isotropic material, but it was not known if different surfaces had different activities to oxygen exchange. The results for this experiment show that the differences seen in the FIB SIMS depth profiles are indeed significant and that the differences in diffusivity are valid. Finally the anisotropy of diffusion measured here is only a factor of 4 compared to a factor of close to 10 for the materials La2−xSrxCuO4±δ15 and close to 103 for the materials La2NiO4±δ.3 It must be stressed that the measurement of anisotropy is difficult and that the slower diffusion coefficient (c-axis) measured by these techniques only represents an upper bound due to the possibility of “contamination” by the in-diffusion of tracer atoms from adjacent grains and by the incorporation of oxygen atoms from side surfaces. However, it appears that the intrinsic anisotropy of these materials is not as high as for the lanthanum nickelate (K2NiF4-type structure). It is possible that this anisotropy is limited by the presence of structural disorder on the A cation sublattice. Molecular dynamics simulations have shown that when the A site cations in GBCO become fully disordered, the diffusivity becomes isotropic and drops by a factor of approximately 4.5 Small areas of cation disorder might provide diffusional pathways along the c-direction, but this would imply that the intrinsic anisotropy in fully ordered PBCO is higher than has been reported here. It also has been observed that the surface exchange coefficient (k*) measured is slightly larger for the “c-axis” oriented grain (grain B) compared to the ab-plane oriented one (grain A), which is the opposite of the diffusion coefficient (D*) behavior for these two grains. Interestingly, on other layered materials (e.g., La2NiO4±δ3), both the D* and k* are maximum for the ab-plane orientation. We think that the differences encountered for the two families of materials could have several causes: due to a different termination of the outermost surface layer (as they are structurally different materials: K2NiF4 and AA′B2O5+δ), to a different oxygen incorporation mechanism taking place on the surface, or even to the segregation of different species which could migrate to the surface and control the surface exchange process.

Figure 9. (A) Secondary electron image of the PBCO sample exchanged at 300 °C with the area measured by EBSD marked, (B) EBSD color-coded surface orientation image with grains A and B (measured by FIB-SIMS) marked, and (C) inverse pole figure showing the orientation to the sample normal of grains A and B.

Figure 10. Normalized 18O concentration profiles in individual grains for a PBCO pellet exchanged at 300 °C for 42 min and fit to Crank’s solution to Fick’s second law.

highlighted that both the surface concentration and the shape of the curve for subregion 2.1 (Figure 8) are very similar to 620



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(17) Rodríguez-Carvajal, J. Roisnel, T. Newletter No. 20; International Union for Crystallography: 1998. (18) Maignan, A.; Martin, C.; Pelloquin, D.; Nguyen, N.; Raveau, B. J. Solid State Chem. 1999, 142 (2), 247−260. (19) Streule, S.; Podlesnyak, A.; Mesot, J.; Medarde, M.; Conder, K.; Pomjakushina, E.; Mitberg, E.; Kozhevnikov, V. J. Phys.: Condens. Matter 2005, 17 (21), 3317−3324. (20) Zhang, T. S.; Hing, P.; Huang, H. T.; Kilner, J. J. Eur. Ceram. Soc. 2002, 22 (1), 27−34. (21) Marrero-Lopez, D.; Carmen Martin-Sedeno, M.; Pena-Martinez, J.; Carlos Ruiz-Morales, J.; Nunez-Coello, P.; Ramon Ramos-Barradoz, J. J. Am. Ceram. Soc. 2011, 94 (4), 1031−1039. (22) Kim, J. H.; Prado, F.; Manthiram, A. J. Electrochem. Soc. 2008, 155 (10), B1023−B1028. (23) Crank, J. The Mathematics of Diffusion; Oxford University Press: Oxford, 1975. (24) Fearn, S.; Rossiny, J.; Kilner, J. Solid State Ionics 2008, 179 (21−26), 811−815.

CONCLUSIONS The LnBaCo2O5+δ series of materials have been proposed as MIECs for application in a number of high temperature electrochemical roles. Previous experimental and theoretical investigations had suggested that the optimized material in this series was PrBaCo2O5+δ and that the oxygen diffusion in this material was anisotropic. By using a broad range of characterization and measurement techniques we have shown that the oxygen exchange and diffusion in PBCO is comparable with that of GBCO at low temperatures and slightly higher than GBCO at temperatures greater than 400 °C. The activation energy for oxygen diffusion in PBCO is higher than our previous measurements of GBCO. The main finding of this investigation is that we have observed differences between grains which can be attributed to anisotropy in the diffusion.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 44 20 7594 6771. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Marie Curie Intra European Fellowship within the seventh European Community Framework Programme (PIEF-GA-2009-252711) and by KAUST (King Abdullah University of Science and Technology) Academic Excellence Alliance (for M.B). J.P.-M. acknowledges financial support from the Spanish Government through the “Juan de la Cierva” and “José Castillejo” fellowship programs.

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REFERENCES

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