Effect of cationic order–disorder on the transport properties of LaBaCo2O6–δ and La0.5Ba0.5CoO3–δ perovskites
aCentro Atómico Bariloche, Instituto Balseiro–CNEA, Avenida Bustillo 9500, San Carlos de Bariloche, Rio Negro, 8400, Argentina, bDepartamento de Física, Universidad Nacional del Sur, Instituto de Fisica del Sur, Avenida Alem 1253, Bahia Blanca, Buenos AIres, 8000, Argentina, cCentro Atómico Bariloche, Instituto Balseiro–CNEA–CONICET, Avenida Bustillo 9500, San Carlos de Bariloche, Rio Negro, 8400, Argentina, and dDiffraction Group, Institut Laue–Langevin, 6 rue Jules Horowitz, BP 156, Grenoble, Isère, 38000, France
*Correspondence e-mail: firstname.lastname@example.org
A-site cationic ordered LaBaCo2O6−δ and disordered La0.5Ba0.5CoO3−δ perovskite phases were obtained by solid state reaction. Their structural properties were studied at room temperature and 673 K, by combining powder diffraction techniques, X-ray diffraction and neutron powder diffraction with an independent determination of the oxygen content of the samples by thermogravimetry. La0.5Ba0.5CoO3−δ exhibits cubic symmetry with cations randomly distributed, whereas LaBaCo2O6−δ shows tetragonal symmetry with the La3+ and Ba2+ ions distributed in alternating layers. The diffraction data were analyzed using the Rietveld method and different structural and microstructural models. Bond valence and Fourier methods were used to determine bond distances and neutron/electron density maps. LaBaCo2O6−δ exhibits a higher concentration of oxygen vacancies than La0.5Ba0.5CoO3−δ, because the O atom is weakly bonded to the LaO layers. The anisotropic atomic displacement and the neutron density distribution suggest a two-dimensional O-migration path for LaBaCo2O6−δ and a three-dimensional path for La0.5Ba0.5CoO3−δ.The mechanism of electrical conductivity is via electron holes with high mobilities (μLa0.5Ba0.5CoO3−δ = 2.49 cm2 V−1 s−1 and μLaBaCo2O6−δ = 1.48 cm2 V−1 s−1 at room temperature) and low activation energy (EaLaBaCo2O6−δ = 0.019 eV and EaLa0.5Ba0.5CoO3−δ = 0.030 eV). It has also been found that the higher electronic and ionic conductivities in La0.5Ba0.5CoO3−δ compared to those in LaBaCo2O6−δ are due to the higher dimensionality of transport and to greater overlapping between the Co 3d and O 2p orbitals.
In the past few years, cobaltites with perovskite structure and the related layered perovskites have been proposed as materials for high- and intermediate-temperature electrochemical devices.
This kind of material is attractive because of the presence of mixed ionic and electronic conductivities. The use of cobaltites with mixed conductivity as oxygen electrodes allows the oxygen reaction zone to be increased beyond the triple phase boundary, gas/electronic conductor/ionic conductor, to a gas/mixed conductor interface. Thus, (La,Sr)CoO3−δ (Teraoka et al., 1988; Tai et al., 1995), (Ba,Sr)CoO3−δ (Zhou et al., 2009) and (La,Ba)CoO3−δ (Ishihara et al., 2002; Setevich et al., 2012) perovskites have been proposed for replacing the traditional pure electronic conductor as cathode in solid oxide fuel cells (SOFCs), owing to their low electrode polarization resistance and their high power density. Furthermore, the surface exchange properties together with their high oxygen ionic conductivity make these materials suitable as oxygen exchange membranes (Petric et al., 2000).
In the same way as perovskites are attractive for electrochemical application, the layered perovskites LnBaCo2O6−δ (Ln = lanthanide) are also interesting for this kind of application. These materials also show high rates of oxygen surface exchange (Tarancón et al., 2007) and diffusivity (Kim et al., 2009), which combined with their high electrical conductivity (Zhang et al., 2008; Kim et al., 2008) make them potential materials for oxygen separation membranes (Kim et al., 2009; Zhang et al., 2008) and intermediate-temperature SOFC cathode materials (Zhang et al., 2008; Pang, Jiang, Li, Su et al., 2012). The cationic ordering in LnBaCo2O6−δ layered perovskite oxides takes place because of the large difference between the ionic radii of Ba2+ (rBa2+ = 1.61 Å) and Ln3+ ions (rPr3+ = 1.179 Å, rNd3+ = 1.27 Å, rSm3+ = 1.24 Å, rGd3+ = 1.107 Å). Fig. 1 shows an A-cationic disordered perovskite, where the La3+ and Ba2+ ions are randomly distributed (Fig. 1a), and an A-cationic ordered perovskite, where La3+ and Ba2+ are distributed in layers (Fig. 1b). The cubic and tetragonal unit cells are highlighted in these figures. As a consequence of the cationic ordering, the oxygen bond displays a different strength depending on whether it is located in BaO, CoO2 or LnO layers, giving rise to differentiated oxygen crystallographic sites: O1, O2 and O3, respectively. Thus, for example, Taskin et al. (2005) reported that the cationic ordered GdBaMn2O6−δ perovskite exhibits a higher change of oxygen content and a higher rate of oxygen uptake than the cationic disordered Gd0.5Ba0.5MnO3−x perovskite. They assume that the cationic order in the crystal structure will reduce the oxygen bonding strength and will provide a disorder-free channel for ion motion. The reduction of oxygen bonding strength might also induce a two-dimensional distribution of oxygen vacancies in the (001) LnO layers (tetragonal symmetry, P4mmm) of layered cobaltites (Taskin et al., 2005; Tarancón et al., 2008; Streule, Podlesnyak, Sheptyakov et al., 2006). In this way, linked Co—O—Co channels subsist in CoO2 layers even at high temperatures. Similarly to other perovskite-related oxides, the electronic conductivity in LnBaCo2O6−δ occurs via electron hopping along Co—O—Co bonds with a double exchange process (Zhang et al., 2008). Thus, the presence of linked Co—O—Co channels permits high values of electrical conductivity to be achieved at high temperatures.
Despite this promoting characteristic of layered perovskites, recent work by Kim et al. (2009) shows that cationic disordered La0.5Ba0.5CoO3−δ displays better ionic and electronic conductivities than cationic ordered LnBaCo2O6−δ (Ln = Pr, Nd, Sm), possibly because the former has a less distorted lattice. However, it is not clear if the improvement of transport properties is a consequence of cationic disorder or Ln nature.
Controlled syntheses of cationic ordered/disordered phases for the case Ln = La (rLa3+ = 1.36 Å) (Nakajima et al., 2005; Rautama et al., 2009, 2008) have given the opportunity to study the effect of structure (independently of Ln nature) on high-temperature transport properties. Three characteristic structures were reported by Rautama et al. (2008) for this composition at room temperature: disordered cubic perovskite La0.5Ba0.5CoO3, perfectly ordered layered LaBaCo2O6 and a nanoscale-ordered LaBaCo2O6. These structures were studied by neutron diffraction (Nakajima et al., 2005), X-ray diffraction and electron diffraction (Rautama et al., 2009, 2008) below room temperature. However, to the best of our knowledge, structural characterization of cationic disordered La0.5Ba0.5CoO3 and cationic ordered LaBaCo2O6 structures at high temperatures has not been performed yet. Recently, S. Pang and co-workers have reported a comparative study of electrical conductivity at high temperature (Pang, Li et al., 2012) and area-specific resistance (ASR) (Pang, Jiang, Li, Su et al., 2012; Pang, Jiang, Li, Wang & Su, 2012) properties for La0.5Ba0.5CoO3−δ and LaBaCo2O6−δ. These authors found that, while the electrical conductivity for the cationic disordered La0.5Ba0.5CoO3−δ phase is higher than that of ordered LaBaCo2O6−δ, the ASR is lower. The ASR reaches values of 0.0086 Ω cm−2 for the ordered phase (Pang, Jiang, Li, Su et al., 2012) and 0.013 Ω cm−2 for the disordered phase (Pang, Jiang, Li, Wang & Su, 2012) at 1073 K in air. These results suggest a strong influence of structural features on the electrochemical performance.
The electrochemical performance of cobaltites is influenced by O-ion and electron transport. The O-ion transport takes place with an O-vacancy mechanism involving hopping through the O sublattice, whereas the electron transport arises via electron holes in the Co—O 3d–2p band. Neutron diffraction makes it possible to obtain information about the oxygen sublattice of crystalline oxides given its high sensitivity to oxygen in the presence of heavier rare earth and transition metal elements. This structural characterization, complemented with X-ray diffraction, gives useful information about oxygen and cation sublattices. Therefore, significant structural information could be obtained by combining the two techniques, and this structural information could be related to O-ion and electron-hole transport. Thus, for example, the effects of oxygen vacancies on structures and the determination of the O-diffusion path in perovskites and related structures such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (McIntosh et al., 2006; Itoh et al., 2009, 2010), SrFeOx (Schmidt & Campbell, 2002), NdBaCo2O5+x (Hu et al., 2012), PrBaCo2O5+δ (Chen et al., 2013) and Sr3Fe(Fe,Co,Ni)O6+δ (Mogni et al., 2009) were studied from in situ neutron diffraction techniques.
In this work, we perform an in situ neutron powder diffraction study for cationic ordered LaBaCo2O6−δ and cationic disordered La0.5Ba0.5CoO3−δ at room temperature and 673 K, in order to obtain detailed structural information that can be correlated with the intermediate-temperature properties of these oxides. The choice of 673 K as intermediate temperature is based on the fact that, at this temperature, the oxygen exchange and bulk diffusion in these materials are high enough to promote oxygen vacancy generation.
Powder samples were obtained by the solid state reaction method (SSR). Stoichiometric amounts of La2O3 and Co3O4 oxides and BaCO3 carbonate were mixed using an agate ball mill. The cationic ordered LaBaCo2O6−δ phase was obtained by annealing at 1423 K under Ar flow for 24 h, with heating and cooling rates of 2 K min−1. Then, the powders were annealed at 673 K in air for 6 h in order to reach the equilibrium oxygen content. The cationic disordered La0.5Ba0.5CoO3−δ phase was obtained by heat treating at 1373 K in air for 12 h, with heating and cooling rates of 5 K min−1.
X-ray powder diffraction (XRD) patterns were collected at room temperature with a Philips PW1700 diffractometer using Cu Kα radiation and a graphite monochromator. All reflection peaks of La0.5Ba0.5CoO3−δ were indexed according to the cubic symmetry (space group ) previously reported for this compound (Nakajima et al., 2005), while those corresponding to LaBaCo2O6−δ were assigned to the tetragonal unit cell (space group P4/mmm) reported by the same authors.
Neutron powder diffraction (NPD) measurements were carried out at Institut Laue–Langevin (ILL), Grenoble, France, using the D2B powder diffractometer (λ = 1.594 Å). A vanadium sample holder was used for the measurements at room temperature and a quartz tube for the measurements at high temperature. The quartz tube was open, allowing oxygen exchange with the surrounding air. The high resolution of this two-axis diffractometer allowed a precise determination of the positions and occupation numbers of O atoms. All patterns were refined by the Rietveld method using the FullProf suite tools (Rodríguez-Carvajal, 2000). Details of the refinements are given in the supporting information.1 The XRD and NPD room-temperature data were refined simultaneously in order to obtain complementary information. The Bond_Str (bond-valence parameters) and GFourier tools of FullProf were used for calculating the distances and angles in the crystal structures and for obtaining the Fourier maps, respectively.
The oxygen content of samples was determined in situ as a function of temperature in air by thermogravimetry (TG) using a symmetrical thermobalance based on a Cahn 1000 electrobalance. The mass change was associated with oxygen content by a posteriori in situ reduction under 10% H2/Ar atmosphere. The final products of reduction, La2O3, BaO and Co, were checked by XRD. This oxygen content determination was used as an independent measure of total oxygen vacancies. The oxygen content for the cationic disordered phase was also determined under an oxygen partial pressure pO2 = 2 × 10−3 atm (1 atm = 101 325 Pa), making use of an electrochemical system of an oxygen pump and a sensor in Ar as gas carrier.
Dense samples were obtained by pressing La0.5Ba0.5CoO3−δ and LaBaCo2O6−δ powders followed by a heat treatment at 1373 K for 6 h in air and 1423 K for 6 h in Ar, respectively. Resistivity measurements on these dense samples were performed by the four-probe technique. Resistivity (ρ) curves as a function of temperature (T) were collected in air for temperatures ranging between 273 and 1173 K. Conductivity data were obtained from the relation σ = ρ−1.
The diffraction pattern profile depends on structural parameters (symmetry, lattice parameters, atom positions, occupancies and vibration, etc.) and microstructural parameters (grain size and microstrain effect). The structural models used for La0.5Ba0.5CoO3−δ and LaBaCo2O6−δ are based on those of cubic La0.5Sr0.5CoO3−δ (Setevich et al., 2012) and tetragonal PrBaCo2O6−δ (Nakajima et al., 2005; Frontera et al., 2005), respectively. The models tested to refine the crystal structures are discussed in the supplementary information . These models included microstrain effects, considerations about isotropic or anisotropic atomic relative vibrational motion, and the location and values of oxygen vacancy defects.
The goodness of fits for the samples were improved by considering the phenomenological Stephens model (Stephens, 1999) for the anisotropic microstrain effect and including anisotropic and isotropic Debye–Waller coefficients for thermal atomic vibration for LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ. Besides, in both compounds, the O content was fixed to equal the values obtained from independent TG measurement to avoid any correlation between the O occupation and thermal displacement.
Fig. 2 shows the normalized O content obtained from TG as a function of T for LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ. The data at room temperature and 673 K are used to fit the NPD profile, obtaining the best agreement at 673 K. Figs. 3 and 4 show the NPD measured and calculated profiles, and the difference between the two, for La0.5Ba0.5CoO3−δ and for LaBaCo2O6−δ, respectively. The structural data obtained from these refinements are discussed below.
Tables 1 and 2 show the structural and microstructural parameters determined from Rietveld analysis for La0.5Ba0.5CoO3−δ and LaBaCo2O6−δ, respectively. Structural features such as lattice parameters (a, b and c), specific atomic positions (x, y, z) and occupancies (gatom), and the equivalent thermal displacements (Biso), are shown for both samples at room temperature and 673 K. Values of anisotropic atomic displacement parameters and information about the refinements are included in the supplementary data . In both samples, as the temperature increases, the O occupancies (gO) decrease, while the mean atomic displacements increase. The two parameters might be strongly correlated, in which case, fixing the oxygen content with an independent technique would allow a better resolution of Biso. For the cationic ordered phase, three different oxygen positions can be distinguished, O1, O2 and O3, located in BaO, CoO2 and LaO layers, respectively. Initially, the O vacancies were fully located on O3 sites. The Biso values of the O1 and O2 sites were larger than that of the O3 site, suggesting that these O-atom sites could collaborate with the oxygen migration diffusion path. Considering this, the occupancies gO1 and gO2 were also released to be adjusted, and it was found that only the O2 and O3 sites can accept O vacancies. Therefore, it is possible to assume that, similar to the isostructural LnBaCo2O6 (Ln = Pr, Nd, Sm, Gd) compounds, the oxygen vacancies are mainly located in LaO layers (Streule, Podlesnyak, Pomjakushina et al., 2006), while the O atoms located in CoO2 layers facilitate the oxygen diffusion process. Recent studies of the diffusional pathway of O2− ions in PrBaCo2O6−δ (Chen et al., 2013) and NdBaCo2O6−δ layered perovskites (Hu et al., 2012) by in situ neutron diffraction at temperatures higher than 873 K are consistent with our results.
Comparing the O content of cationic ordered and disordered phases in Fig. 2 and Tables 1 and 2, one can observe that the concentration of oxygen vacancies is higher for the LaBaCo2O6−δ ordered phase than for the La0.5Ba0.5CoO3−δ phase over the whole temperature range. However, the O-atom displacement in the cationic disordered phase is larger than that in the ordered one, suggesting a higher delocalization or mobilization degree for O in the La0.5Ba0.5CoO3−δ structure.
From the point of view of microstructural features, it was possible to obtain information about size and microstrain effects on peak profile by using a calibration function. While the size of domain seems to have a negligible effect on the broadening, the microstrain effect is evident in both structures. The maximum strains (Max-strain) are the average values calculated using the reciprocal lattice directions. In this way, the symmetry of each phase is taken into account and the standard deviations of Max-strain are a measure of the degree of anisotropy. Therefore, despite the fact that LaBaCo2O6−δ shows less stress than La0.5Ba0.5CoO3−δ, the cationic order renders a more anisotropic strain. Fig. 5 shows the Williamson–Hall plots of integral breadth as a function of reciprocal distance for LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ samples at 673 K. The relative strain of the two samples and its anisotropic characteristic can be observed in these plots. These results are similar to those obtained at room temperature. The lower degree of anisotropy for the cationic disordered phase compared to ordered LaBaCo2O6−δ might be related to the cubic symmetry of this phase. In the cationic ordered phase, the more strained reflections are those corresponding to Miller indices h00, 0k0 and hk0. The preferential strain direction suggests that some kind of local cationic disorder should be present in these directions despite the global ordering along the c axis. Besides, additional strain effects associated with some degree of ordering in the O sublattice should not be discarded. The O vacancies located in LaO layers could be ordered in chains along the a or b direction like in other isostructural LnBaCo2O6 (Ln = Pr, Nd, Sm, Gd) perovskites (Streule, Podlesnyak, Pomjakushina et al., 2006). In the LnBaCo2O6 (Ln = Pr, Nd, Sm, Gd) compounds, the O-vacancy order is responsible for the existence of a low-temperature orthorhombic phase. Likewise, but with a minor degree of anisotropy, the highest strain is present along the (h00) plane and the equivalent (0k0) and (00l) planes in the cubic phase. The high strain present in the La0.5Ba0.5CoO3−δ sample might suggest that the cationic ordered LaBaCo2O6−δ phase is structurally more favorable.
Fig. 2 shows that the concentration of oxygen vacancies is higher in the cationic ordered LaBaCo2O6−δ phase than in cationic disordered La0.5Ba0.5CoO3−δ in the whole temperature range. Therefore, we might assume that oxygen is more strongly bonded to the lattice in La0.5Ba0.5CoO3−δ than in LaBaCo2O6−δ. Besides, as has been observed from the structural analysis in the previous section, the oxygen vacancies of LaBaCo2O6−δ are preferentially located in O3 sites along LaO layers. These behaviors might be explained in the frame of the bond valence formalism.
Table 3 shows the bond valence sum (BVS) and some characteristic distances between atoms for both structures at room temperature and 673 K. The BVS of the oxygen sites follows the sequence O3P4/mmm < O2P4/mmm ≃ OPmm < O1P4/mmm. In addition, these values decrease as temperature increases for both samples. Lower BVS values indicate that O is weakly bonded to the lattice (Chen et al., 2013). Therefore, this result is in agreement with the fact that cationic order induces a highly defective structure with oxygen vacancies distributed along LaO layers.
In addition, the atomic displacements become anisotropic as a consequence of cationic order. Fig. 6 shows the atomic displacement (75% probability surface) at 673 K for both phases. Two unit cells of La0.5Ba0.5CoO3−δ were included for comparison with LaBaCo2O6−δ. The anisotropic atomic displacement in layered LaBaCo2O6−δ suggests that the O1 sites (U11 = U22 > U33) move parallel to the BaO plane, whereas the O2 sites (U33 > U11 × U22) distort the CoO6 octahedra, moving along the c axis toward the O3 sites, which exhibit a shorter and almost isotropic atomic displacement. The O-atom displacement in the La0.5Ba0.5CoO3−δ structure is larger than those of LaBaCo2O6−δ. Therefore, we might assume that the O atoms in La0.5Ba0.5CoO3−δ are more mobile than O2 and O3 in LaBaCo2O6−δ, because although they are more strongly bonded to the lattice (high BVS) their mean displacements are larger. The possible O-migration pathways are indicated by arrows in Fig. 6. The O-migration paths were proposed considering the anisotropic or isotropic atomic displacement and the Fourier maps shown in Fig. 7. The Fourier maps were obtained from the structure factors observed (NPD-Fobs) and the phase calculated from Rietveld analysis of NPD data using GFourier tools. Fig. 7(a) shows the Fourier maps of the (200) Co—O plane in cubic perovskite at 673 K. The (200) plane is equivalent to the (020) and (002) planes because of the cubic symmetry. Figs. 7(b) and 7(c) show the Fourier maps of the (004) and (200) [or (020)] Co—O planes, respectively. As observed, while all the Co—O planes in La0.5Ba0.5CoO3−δ and that normal to the c direction in LaBaCo2O6−δ exhibit an isotropically distributed neutron density, the neutron densities of Fourier maps parallel to the c direction are concentrated toward LaO planes. The Fourier maps and the O-atom displacements suggest a two-dimensional character for the O-migration path in LaBaCo2O6−δ in contrast to the three dimensions in La0.5Ba0.5CoO3−δ. Besides, considering the O—O distances in Table 3, it might be expected that the O migration for LaBaCo2O6−δ structures involves mainly O2—O3 and O2—O2 hopping because these distances are shorter than the O3—O3 distances.
Kim et al. (2009) compared the ionic and electronic conductivity of disordered La0.5Ba0.5CoO3−δ with those of cationic ordered LnBaCo2O6−δ (Ln = Pr, Nd, Sm). They found that O-permeation flow in these materials is bulk limited by O-ion diffusion and that the cubic perovskite La0.5Ba0.5CoO3−δ has the higher ionic (σion) and electronic (σel) conductivity. On the other hand, Zhang et al. (2008) compared these properties for the layered perovskite series LnBaCo2O6−δ (Ln = La, Pr, Nd, Sm, Gd, Y). Despite the difference between absolute values of ionic conductivity reported in the two works, σion follows the series La0.5Ba0.5CoO3−δ > PrBaCo2O6−δ > NdBaCo2O6−δ >> SmBaCo2O6−δ > GdBaCo2O6−δ > LaBaCo2O6−δ. Therefore, the diffusion mechanism proposed in this work, involving a three-dimensional pathway for La0.5Ba0.5CoO3−δ and a two-dimensional one for LaBaCo2O6−δ, would explain the higher ionic conductivity for the cationic disordered phase in spite of their lower O-vacancy concentration.
The equilibrium electrical conductivities (σ) of LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ dense samples have been measured as a function of temperature between 293 and 1173 K in air. Usually in these cobalt-rich perovskites, the ionic conductivity ranges from 10−2 to 10−1 S cm−1 and the electronic conductivity is between 100 and 1000 S cm−1 (Zhang et al., 2008; Kim et al., 2009); therefore it is a good approximation to consider that the electron conductivity almost equals the total electrical conductivity. Fig. 8(a) shows the σ versus T curves. As observed, the conductivity remains almost constant with T below 523 K for both samples. On increasing T above 523 K, the conductivities decrease, exhibiting a metallic like behavior. Besides, the electrical conductivity of LaBaCo2O6−δ is lower than that of La0.5Ba0.5CoO3−δ, but both are higher than those reported for other perovskites, which achieve maximum conductivity values around 200–500 S cm−1 at 673 K (Tai et al., 1995; Zhang et al., 2008; Suntsov et al., 2011). The fact that the electrical conductivity of LaBaCo2O6−δ is lower than that of La0.5Ba0.5CoO3−δ is in agreement with the previous results reported by Pang, Li et al. (2012).
Comparing Figs. 2 and 8(a), it is possible to correlate the behavior of σ with T to the change of oxygen content. In such a case, the conductivity would be p-type, with electron-hole (h·) charge carriers (Pang, Li et al., 2012) hopping from Co4+ to Co3+ ions through the Co 3d–O 2p band (Zhang et al., 2008). Therefore, using the Kroger–Vink notation, oxygen vacancies VO·· are created as T increases, consuming the electron holes according to
Combining electrical conductivity data with oxygen content data as a function of T, the dependence of σ versus `normalized' oxygen content (6−δ) is plotted in Fig. 8(b), showing that, effectively, for both samples the conductivity increases with 6−δ.
One can notice from Fig. 2 that the oxygen content in La0.5Ba0.5CoO3−δ is higher than that of LaBaCo2O6−δ at the same temperature, rendering higher conductivity values (see Fig. 8). However, one additional factor influencing the conductivity behavior seems to be present. With the aim of discriminating some structural effect on conductivity, in addition to the O-vacancy effect, a different in situ condition was selected for the cationic disordered phase. The oxygen content for this sample was determined using the thermobalance coupled to a gas blending system. This system allowed the determination of the oxygen content at pO2 = 0.002 atm between room temperature and 1173 K. From these experiments we were able to determine that, at 673 K and pO2 = 0.002 atm, the unit formula of the cationic disordered phase is La0.5Ba0.5CoO2.36. This 3−δ value corresponds to the same normalized oxygen content as for the cationic ordered phase LaBaCo2O5.72 at 673 K in air. Then, fixing pO2 at 0.002 atm the equilibrium electrical conductivity was measured at 673 K. This unique point is also included in Figs. 8(a) and 8(b) (marked as open triangles). It was found that the electronic conductivity in the cationic disordered phase (La0.5Ba0.5CoO3−δ) is higher than that in cationic ordered LaBaCo2O6−δ, regardless of the oxygen vacancy concentration, the structural properties being key to controlling the electron transport.
The metallic like behavior and the decrease of conductivity as the concentration of oxygen vacancies decreases (at constant temperature) suggest the presence of an electron-hole mechanism controlling the electrical transport. Most cobaltites with perovskite structure show a small polaron mobility controlling the electron-hole mechanism (Tai et al., 1995; Zhang et al., 2008; Sehlin et al., 1995). In these cases, the electron holes move slowly along the polar oxides, dragging a polarization cloud. Then, a thermally activated dependence is observed for conductivity data:
where e is the electron charge, μ is the electron-hole mobility, p is the electron-hole concentration per unit-cell volume, Ea is the activation energy and kB is the Boltzmann constant. Under constant electron-hole concentration, if the activation energy is low, the exponential pre-factor depending on T would dominate, causing the conductivity to decrease as T increases.
The inset in Fig. 8(a) shows both Arrhenius plots [ln(σT) versus T−1]. The Ea values of LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ were computed between room temperature and 523 K, where the oxygen content and the electron-hole concentration remain almost constant. The activation energies obtained, = 0.019 eV and = 0.030 eV, are almost negligible, suggesting that these are not thermally activated processes. Therefore, the electron-hole transport should correspond to a large polaron.
The main difference between small and large polarons is their mobility values. In the first case the interaction between electron hole and lattice is strong, rendering the highest localization of charge. The electron holes, occupying a narrow band, hop between Co4+ and Co3+ sites. The characteristic mobility values for small polarons are between 10−4 and 10−5 cm2 V−1 s−1. In the case of the large-polaron mechanism, the charge carriers are itinerant, moving almost freely along a wide band. The behavior is metallic like, but the mobility (1–10 cm2 V−1 s−1) is still lower than that of metallic conductors. Therefore, from the mobility for both phases computed using equation (2) and conductivity data, the electron transport mechanism could be proposed.
In both La–Ba–Co–O systems studied in this work, the negative charges produced when La3+ ions are partially replaced by Ba2+ ions must be compensated for by electron-hole creation at the Co sites (h·) and oxygen vacancy generation at O sites (VO··). The charge balance is
The electron-hole concentration p = h· /Vuc (Vuc is the unit-cell volume) can be estimated considering VO·· = δ and equal to 1 and 0.5 for LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ, respectively. Therefore, the mobility values are obtained from equation (2) using the electron-hole concentration estimated from oxygen content. In the third column of Table 4 the electron-hole mobility values estimated at 293 and 673 K are listed. On one hand, in both samples the mobility decreases with T. This behavior and the mobility values themselves are typical of large-polaron behavior. On the other hand, the electron-hole mobility in the cationic disordered phase is almost twice that of the cationic ordered phase at both temperatures. Analyzing the Co—O distances and angles given in Table 3, the differences between them are not sufficient to explain the difference between mobility values.
If the two-dimensional character of layered perovskites as opposed to the three-dimensional structure of cubic perovskites is considered, it might be probable that, instead of six equivalent directions to electron-hole movements, in layered perovskites there are only four. The electronic density of these compounds was studied from the XRD Fourier maps. The results of this study are shown in Fig. 9. Figs. 9(a) and 9(b) show the Co—O layers corresponding to the (002) and (200) planes of La0.5Ba0.5CoO3−δ, respectively, whereas Figs. 9(c) and 9(d) show the Co—O2 and Co—O (O1, O2 and O3) layers corresponding to the (004) and (200) planes of LaBaCo2O6−δ, respectively. The electronic density is more isotropically distributed in the cationic disordered phase than in the cationic ordered one, where the c direction shows a lack of electronic density around the O3 site. Therefore, it is plausible to assume that the oxygen vacancies, preferentially located in the LaO layer, work as a barrier blocking the charge carrier movement through the Co 3d–O 2p band in the c-axis direction. This way, it is possible to define a lineal mobility such as = μ/6 for the cubic symmetry and = μ/4 for the tetragonal one. The lineal mobility values are also shown in Table 4.
The lineal mobility of the cationic ordered phase is slightly lower than that of the cationic disordered phase. The differences between electron-hole mobility arise from the octahedral distortion produced by La–Ba cationic order. As observed in Table 3 and Fig. 9, in cationic ordered phases the O—Co—O angles are lower than 180°, locating these atoms out of the ab plane. This fact, as well as the increment in the Co—O distances (see Table 3), reduces the overlapping between the Co 3d and O 2p orbitals in LaBaCo2O6−δ compared to La0.5Ba0.5CoO3−δ, decreasing the electron-hole mobility.
Therefore, although these cobaltites are better electronic conductors than other perovskites, the structural difference between them improves the electronic conduction in La0.5Ba0.5CoO3−δ compared to LaBaCo2O6−δ. This would be due to the fact that, in LaBaCo2O6−δ, the conductivity is two dimensional instead of three dimensional. Besides, the loss of overlapping between the Co 3d and O 2p orbitals due to the octahedral distortion hampers the electron-hole transport.
The effect of cationic ordering or disordering on ionic and electronic transport properties of cobalt perovskites was analyzed in relation to the structural features determined by NPD and XRD techniques. Two compounds with the same cationic ratio but different structures have been studied: cationic disordered La0.5Ba0.5CoO3−δ with cubic symmetry and cationic ordered LaBaCo2O6−δ with tetragonal symmetry.
The cationic LaBaCo2O6−δ ordered phase shows lower strain values but a higher degree of anisotropy than the cationic La0.5Ba0.5CoO3−δ disordered one. The minor degree of anisotropy for the cationic disordered phase was related to its cubic symmetry. The origin of the stress has been associated with the local order/disorder of La and Ba and the local concentration of oxygen vacancies, which produce a distribution of lattice parameters of the unit cell. Therefore, the minor strain in LaBaCo2O6−δ suggests that the distribution of cations in layers is structurally more favorable than cations randomly distributed.
NPD has allowed us to obtain precise information on the O sublattice and O-migration path. The oxygen vacancies of LaBaCo2O6−δ are mainly located in LaO layers. However, the O ions located in the CoO2 layer are highly delocalized, contributing to the O-diffusion process. Rietveld refinement and bond valence sum methods suggest that O3 is more weakly bonded than the O1 and O2 O-atom sites in the LaBaCo2O6−δ structure and also than the O atoms in cubic La0.5Ba0.5CoO3−δ. The atomic diaplacement and Fourier maps suggest a two-dimensional diffusional path for oxygen migration in LaBaCo2O6−δ, whereas this migration is isotropic along three dimensions in La0.5Ba0.5CoO3−δ. The lowest dimensionality for the O-migration path harms the ionic conductivity in the tetragonal LaBaCo2O6−δ structure compared to the cubic La0.5Ba0.5CoO3−δ.
Both LaBaCo2O6−δ and La0.5Ba0.5CoO3−δ compounds exhibit high electrical conductivity values with a metallic like behavior. The combination of electrical conductivity with oxygen content data indicates that the conductivity mechanism is via electron-hole charge carriers. Two structural features are detrimental to the electrical conductivity of the cationic ordered phase in contrast to La0.5Ba0.5CoO3−δ:
(a) The two-dimensional instead of three-dimensional conductivity. This lower dimensionality for the electron-hole transport was associated with the blocking of the c direction owing to the existence of a high concentration of oxygen vacancies in the LaO layers.
(b) The loss of overlap between the Co 3d and O 2p orbitals, since the octahedral distortion enlarges the Co—O2 distance and decreases the O2—Co—O2 angles.
|CubicPm3m||V = 58.93 (1) Å3|
|Hall symbol: -P 4 2 3||Constant Wavelength Neutron Diffraction radiation|
|a = 3.89151 (5) Å|
|Radiation source: nuclear reactor||2θmin = 0.297°, 2θmax = 160.247°, 2θstep = 0.050°|
|Rp = 4.367||4001 data points|
|Rwp = 5.947||34 parameters|
|Rexp = 2.847||0 restraints|
|RBragg = 2.308|
|χ2 = NOT FOUND|
|CubicPm3m||V = 58.93 (1) Å3|
|a = 3.89151 (5) Å||Constant Wavelength Neutron Diffraction radiation|
|2θmin = 0.297°, 2θmax = 160.247°, 2θstep = 0.050°|
|Crystal system, space group||CubicPm3m|
|a (Å)||3.89151 (5)|
|V (Å3)||58.93 (1)|
|Radiation type||Constant Wavelength Neutron Diffraction|
|Specimen shape, size (mm)||?, ? × ? × ?|
|Data collection mode||?|
|2θ values (°)||2θmin = 0.297 2θmax = 160.247 2θstep = 0.050|
|R factors and goodness of fit||Rp = 4.367, Rwp = 5.947, Rexp = 2.847, RBragg = 2.308, χ2 = NOT FOUND|
|No. of data points||4001|
|No. of parameters||34|
Computer programs: FULLPROF.
This work was supported by CONICET, CNEA, ANPCyT and Fundación Balseiro from Argentina, through PICT 2010-0850 and PIP 11220080100120, and by Institut Laue–Langevin through ILL research proposal 47269. We also thank Professor N. Gonzalez and M. C. Ferreiro from Balseiro Institute for their help in the English revision of the manuscript.
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