Improving of the Thermoelectric Efficiency of LaCoO3 by Double

Article pubs.acs.org/JPCC

Improving of the Thermoelectric Efficiency of LaCoO3 by Double Substitution with Nickel and Iron V. Vulchev,† L. Vassilev,† S. Harizanova,‡ M. Khristov,‡ E. Zhecheva,‡ and R. Stoyanova*,‡ †

Faculty of Physics, University of Sofia, 1164 Sofia, Bulgaria Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

‡

ABSTRACT: LaCoO3 displays a high Seebeck coefficient that makes it a potential candidate for future thermoelectric applications. In this study we provide data on the improvement of the thermoelectric efficiency of LaCoO3 by double substitution with nickel and iron. The improvement is achieved by balancing the opposite effects of nickel and iron ions. Double-substituted perovskites LaCo1−x(Ni0.5Fe0.5)xO3 with compositions having equal amounts (0 < x ≤ 0.25) of Ni and Fe are examined. The perovskites are obtained from freeze-dried citrate precursors at 900 °C. Structural and morphological characterizations are carried out by powder XRD and SEM analyses. The thermoelectric efficiency of the perovskites is determined by the dimensionless figure of merit (ZT) calculated from the independently measured Seebeck coefficient, electrical resistivity, and thermal conductivity. Compared with LaCoO3, the double-substituted perovskites display a higher electrical conductivity which does not depend on the total Ni+Fe content. This is a consequence of the increase in the carrier density. It is shown that the effect of Ni ions (for partial electron delocalization) is more pronounced in comparison with that of Fe ions (for electron localization). The synergic effect of Ni and Fe is demonstrated by the effective reduction of the thermal conductivity in comparison with the single-substituted perovskites. As a result, the perovskite with a composition LaCo0.8Ni0.1Fe0.1O3 exhibits the best thermoelectric efficiency with ZT = 0.16, which is an order of magnitude higher than that of LaCoO3 at room temperature.

1. INTRODUCTION The performance of thermoelectric materials is usually characterized by the dimensionless figure of merit ZT = S2T/ (ρλ), where S is the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and λ is the thermal conductivity. The improvement of their properties can be achieved by rational control of carrier density and electrical and thermal transport.1 Classic thermoelectric materials including tellurium-, antimony-, germanium-, and silicon-based compounds exhibit larger figures of merit, but they have some drawbacks, such as low stability and high toxicity. On the contrary, oxide-based materials are usually more stable and less toxic, which makes them alternative candidates for future thermoelectric applications.2−4 Lantanum cobaltate, LaCoO3, with a perovskite-type structure, was recently considered as a material with potential application in thermoelectricity due to its high Seebeck coefficient (|S| > 500 μV/K at room temperature).5−7 The transport properties of LaCoO3 are determined (to a great extent) by the ability of Co3+ ions to adopt low-spin, intermediate-spin, and high-spin configurations (t2g6eg0, t2g5eg1, and t2g4eg2) in the perovskite structure, leading to an additional spin entropy effect.8,9 However, the electrical resistivity is high (about 10 Ω cm at room temperature),10 which lowers the thermoelectric activity (ZT < 0.01 at T = 300 K).4,6 Therefore, state-of-the-art research is mainly devoted to enhancement of © 2012 American Chemical Society

the thermoelectric efficiency of LaCoO3 by single substitution for the La and Co sites.11 The chemical approach mainly comprises replacement of the La3+ ions with alkaline-earth metals or rare-earth elements as well as replacement of Co3+ ions with 3d or 4d transition metals (such as Ni, Fe, Mn, Cu, Rh, etc). From a practical point of view, the most interesting isovalent ions are Ni3+ and Fe3+. It has been reported that the iron ions predominantly affect the magnetic properties of the perovskites,12,13 while the Ni ions play an important role in their transport properties.12 Recently, it has been demonstrated that creation of vacancies in La/O sites can effectively reduce the thermal conductivity, thus leading to an increase in the figure of merit of La1−xCoO3−y.14 Pure LaNiO3 is a perovskite with an enhanced Pauli paramagnetic susceptibility, and its eg electron is highly mobile: t2g6eg1.15,16 On the other hand, pure LaFeO3 is an AFM insulator (TN ≈ 740 K) with localized eg electrons: t2g3eg2.15,17 The Seebeck coefficient for LaNiO3 is low, −25 μV K−1,18,19 while for LaFeO3 the Seebeck coefficient changes from positive to negative around 650 K with increasing temperature.20,21 LaCoO3 interacts with LaNiO3 and LaFeO3 forming solid solutions LaCo1−xNi/FexO3 in the concentration range of 0 < x Received: March 5, 2012 Revised: May 23, 2012 Published: June 4, 2012 13507

dx.doi.org/10.1021/jp3021408 | J. Phys. Chem. C 2012, 116, 13507−13515

The Journal of Physical Chemistry C

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≤ 0.5.22,23 Replacement of Co by Ni and Fe proceeds in the framework of the rhombohedrally distorted perovskite-type structure, Co, Ni, and Fe being randomly distributed.22 It is found that Ni additives promote the transition of the Co ions from low- to high-spin states,24−26 while the iron ions are suggested to contribute to the low-spin state of the cobalt ions.27,28 As a result, LaCo1−xNixO3 solid solutions display enhanced electrical conductivity and improved thermoelectric activity at a low level of doping.29,30 Concerning the LaCo1−xFexO3 series, there are few works devoted to their transport properties.31,28 Taking into account the literature data on single-substituted LaCoO3, one can expect to modify the thermoelectric efficiency by balancing the effects of nickel and iron ions. The aim of this contribution is to evaluate the effect of double substitution with Ni and Fe on the thermoelectric efficiency of LaCoO3. LaCo1−x(Ni0.5Fe0.5)xO3 compositions having equal amounts of Ni and Fe are the subject of the investigations. All perovskites are obtained from freeze-dried citrate precursors at 900 °C. This method is shown to be effective in the preparation of single-substituted perovskites, where Ni and Fe are randomly distributed (LaCo1−xNixO3 and LaCo1−xFexO3).22 Structural and morphological characterizations are carried out by powder XRD and SEM analyses. The thermoelectric efficiency of the perovskites is determined by the dimensionless figure of merit, calculated from the independently measured Seebeck coefficient (S), electrical resistivity (ρ), and thermal conductivity (λ).

position, and oxygen is in the 18e position. Determination of occupancy factors in this system is a complicated procedure, as demonstrated by neutron diffraction studies on singlesubstituted perovskites.22,31 In order to simplify the fitting procedure the oxygen occupation in LaCo1−xNixO3−δ was determined by iodometric titration, whereas the total occupancy of La on 6a sites and Co/Ni on 6b sites was constrained to 1/6. Details of the fitting procedure are given elsewhere.22 It is important that the procedure suitable for describing the cationic distribution in LaCo1−xNixO3 is also applicable for the LaCo1−xFexO3 series.22 Irrespective of the uncertainty in the Rietveld refinement model for determination of the oxygen content, the results give evidence that the oxygen deficiency for iron-substituted perovskites (i.e., both LaCo1−xFexO3 and LaCo1−x(Ni0.5Fe0.5)xO3) is limited and close to 3. This is in agreement with our previous data on hydrogen reduction of LaCo1−xFexO3.31 The transport properties were measured on pellets sintered at 900 °C for 90 h. This temperature was chosen in order to avoid the disproportionation of the iron-rich cobaltates when heated at T > 900 °C. Pellet density was determined by the Archimedes method. Porosity was evaluated by comparison with the theoretical density of LaCoO3 (7.299, JCPDS No. 251060). For the pellets with different perovskite compositions the porosity varied between 20% and 25% despite the Ni, Fe, and Ni,Fe content. SEM analysis was undertaken to analyze the pellet porosity (Figure 1). SEM images of pellets coated with

2. EXPERIMENTAL SECTION A precursor-based method was used for preparation of cobaltates following the procedure described elsewhere.22,31 Homogeneous La−Co−Ni, La−Co−Fe, and La−Co−Ni−Fe citrate precursors were obtained by freeze drying. Lanthanum− cobalt−nickel (or iron) citrates were prepared by adding a 5 M aqueous solution of citric acid (CA) to a suspension of CoCO3, NiCO3 (or Fe(NO3)3) in an aqueous solution of La(NO3)3·6H2O (1 M La). The ratio between the components was La:Co1−xMx:CA = 1:1:10. After stirring, a clear solution was obtained, which was diluted to 0.25 M La (Co1−xMx). For preparation of freeze-dried precursors this solution was instantly frozen with liquid nitrogen and dried in vacuo (20− 30 mbar) at −20 °C in an Alpha-Christ Freeze-Dryer. Thermal decomposition of the La−Co−Ni(Fe) precursors was achieved at 400 °C for 3 h in air. The obtained solid residue was annealed at 900 °C for 40 h in air and then cooled to room temperature at a rate of 5°/min. The lanthanum, cobalt, nickel, and iron content of the initial salts used was determined complexometrically. The redox chemical method based on iodometric titration after dissolution of the powdered sample in HCl under argon was used for determination of the oxygen content in perovskites: LaCo1−xMxO3−δ. We demonstrated that this method gives reliable data on the oxygen content for pure and nickelsubstituted LaCoO3, but it is not applicable for iron-substituted cobaltates.22,31 X-ray structural analysis was performed on a Bruker Advance 8 diffractometer with Cu Kα radiation. Step-scan recordings for structure refinement by the Rietveld method were carried out using 0.02° 2θ steps of 5 s duration. XRD patterns are analyzed by a structural model comprising rhombohedrally distorted perovskite-type structure (R3̅c space group) where La occupies the 6a position (0, 0, 1/4), Co/Ni/Fe are in the octahedral 6b

Figure 1. SEM images of the pellet sintered at 900 °C for 90 h: top of the pellet (a) and cross section (b).

gold were obtained by a Zeiss DSM 962 microscope and Philips XL30 scanning electron microscope. SEM images demonstrate the pellet porosity in two directions: top of the pellet and cross section (Figure 1). Electrical resistivity, density and mobility of charge carriers were determined by a MMR's Variable Temperature Hall System (K2500-5SLP-SP) using the Van der Pauw method over a temperature range from 90 to 600 K. The benchtop permanent magnet (5 T) is used. Thermal conductivity was determined at room temperature on a Thermal Conductivity Analyzer TCi (SETARAM). In order to compare the thermal conductivities of samples having different pellet porosity, the thermal conductivity is normalized to 95% of the theoretical density (λt) using the following density correction:32λt = λ(0.951.5)/(1 − P)1.5, where λ is the measured thermal conductivity and P is the fractional porosity of the pellet.

3. RESULTS AND DISCUSSIONS LaCo1−x(Fe0.5Ni0.5)xO3 Solid Solutions. Figure 2 gives the XRD patterns of single- and double-substituted perovskites. All diffraction peaks are fitted in the framework of a structural model comprising rhombohedrally distorted LaCoO3-type 13508



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spin state of the cobalt ions.24−26 The ionic radii of high-spin Co3+ and Ni3+ are close (0.61 and 0.60 Å, respectively), whereas low-spin Ni3+ has a slightly larger ionic radius than that for lowspin Co3+ (0.56 and 0.545 Å, respectively). In addition, nickelsubstituted compounds display oxygen deficiency that depends on the nickel amount: LaCo1−xNixO3−δ with δ = 0.02, 0.02, and 0.09 for x = 0, 0.10, and 0.5, respectively. The compensation of the oxygen deficiency has been achieved through stabilization by Ni2+ ions instead of Co2+ ions.22 The Ni2+ ion exhibits a larger ionic dimension (0.69 Å, respectively) in comparison with that for low- and high-spin Co3+ and Ni3+ ions. Therefore, all these features account for lattice expansion upon replacement of Co by Ni. In the case of iron-substituted cobaltates, it has been found that iron additives are stabilized as Fe3+ ions in high-spin states, while cobalt ions remain in the low-spin configuration.28 The high-spin Fe3+ ions exhibit a larger ionic radius (0.645 Å), which matches well the observed lattice expansion for LaCo1−xFexO3 (Figure 2). In general, both Ni3+ and Fe3+ ions yield lattice expansion of the cobaltates, but the electronic state of the cobalt ions is different for the LaCo1−xNiO3 and LaCo1−xFexO3 compositions. When Ni and Fe are present together in the LaCoO3 structure, it seems that the electronic states of the cobalt ions bring all features of the single-substituted perovskites. Transport Properties of Single- and Double-Substituted Cobaltates. Electrical Resistivity. Figures 3, 4, and 5

Figure 2. XRD patterns of LaCo 0.50 Ni 0.50 O 3 (a) and LaCo0.50Ni0.25Fe0.25O3 (b). Unit cell parameters for single- and doublesubstituted perovskites (c): LaCo1−xNixO3 (full lines), LaCo1−xFexO3 (dotted lines), and LaCo1−x(Ni0.5Fe0.5)xO3 (dashed lines).

Figure 3. Variation of the electrical resistivity (300 K) as a function of the amount of metal additives: LaCo1−xNixO3 (circles), LaCo1−xFexO3 (squares), and LaCo1−x(Ni0.5Fe0.5)xO3 (triangles).

structure. Lattice parameters display a dependence on the content of metal additives (Figure 2). The a parameter that reflects the distance between two neighboring metal ions increases with the Ni and Fe content, this increase being more pronounced for Fe. The c parameter also increases. Lattice expansion upon Ni and Fe substitution is consistent with previous data on formation of solid solutions in the LaCoO3− LaNiO3 and LaCoO3−LaFeO3 systems.22,23 The new finding is the concentration dependence of the lattice parameters a and c for double-substituted perovskites. It appears that the a and c parameters for LaCo1−x(Ni0.5Fe0.5)xO3 are a superposition of the lattice parameters of the single-substituted perovskites LaCo1−xNixO3 and LaCo1−xFexO3 (Figure 2). The observed lattice expansion is a consequence from the dimension mismatch between the cobalt, nickel, and iron ions. Substitution of Ni for Co has been shown to promote the high-

compare the transport properties of the single- and doublesubstituted cobaltates. At 300 K, the electrical resistivity of LaCoO3 decreases after replacement of Co with Ni, while the iron additives cause an increase in the resistivity (Figure 3). For the double-substituted perovskites LaCo1−x(Ni0.5Fe0.5)xO3, the resistivity decreases and seems to be independent of the total Ni,Fe content (Figure 3). To understand the observed transport properties, the temperature dependencies of the resistivity, carrier density, and mobility are analyzed. The temperature variation of the electrical resistivity of perovskites is summarized in Figure 4. On cooling from 600 to 100 K there is an increase in the resistivity for all cobaltates. This behavior is typical for semiconducting materials. For unsubstituted LaCoO3, the temperature-induced changes in the resistivity can be divided in two temperature ranges: between 100 and 400 K and above 400 K. As a result, the temperature 13509



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Figure 4. Temperature dependence of the electrical resistivity as well as the “ln(ρ/T) vs T−1” function for LaCo1−xFexO3 (a, a′), LaCo1−‑xNixO3 (b, b′), and LaCo1−x(Ni0.5Fe0.5)xO3 (c, c′). (Inset) “ln(ρ) vs T−1/4” function for LaCo0.75Ni0.25O3.

equivalent amounts give rise to the electronic structure of LaCoO3 and determine its short-range semiconductor behavior.5 By increasing the temperature, a gradual electronic delocalization takes place, leading to an intermediate-spin configuration of Co3+ (IS, t2g5eg1). These IS cobalt ions are responsible for stabilization of a metallic phase formed above 650 K. On the other hand, it has been suggested that the IS cobalt ions start to form above 90 K and the Jahn−Teller effect

dependence of the electrical resistivity of LaCoO3 cannot be fitted with one model in the whole temperature range from 100 to 600 K. The complex electrical transport in LaCoO3 has been already established and related to the temperature-induced spin crossover of Co3+ ions.5,33 Irrespective of the fact that LaCoO3 is a subject of intensive studies, there are controversial interpretations of its electrical resistivity. It has been proposed that at 350 K both low- and high-spin states of Co3+ in 13510



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Figure 5. Temperature dependence of the carrier density and carrier mobility for LaCo1−xFexO3 (a, a′), LaCo1−xNixO3 (b, b′), and LaCo1−x(Ni0.5Fe0.5)xO3 (c, c′).

for the t2g5eg1 configuration is mainly responsible for the semiconductor behavior of LaCoO3.34−36 Finally, the appearance of the IS configuration of Co ions in LaCoO3 is not in agreement with the magnetic and thermal expansion measurements of Asai et al.37 The resistivity of LaCoO3 undergoes significant changes during Co substitution (Figure 4a). For low-substituted LaCo1−xFexO3 (0 < x ≤ 0.10), the resistivity increases more quickly below 400 K, while above 400 K the resistivity tends to that of the unsubstituted LaCoO3. A higher level of Fe additives (x > 0.10) causes a growth in the resistivity in the whole temperature range from 100 to 600 K. For highly substituted LaCo0.5Fe0.5O3, the temperature dependence of the electrical resistivity is analyzed by a model based on the nearest neighbor hopping of small polarons, which is described by ρ = ρoT exp(−Ep/kT), where Ep is the polaron hopping energy. The “ln(ρ/T) vs 1/T” curves are shown in Figure 4a′. For LaCo0.5Fe0.5O3, the small polaron model describes relatively well the electrical resistivity in the whole temperature range, while for low-substituted ones there is a deviation from this model at least at high temperatures (above 400 K). The calculated polaron hopping energy gradually increases with the Fe content: 0.129(8), 0.134(8), 0.155(5), and 0.170(3) eV for x = 0.05, 0.10, 0.20, and 0.5, respectively. To rationalize these values, it is worth mentioning the hopping energy for iron containing perovskites, where magnetic Co3+ ions are replaced with nonmagnetic ions such as Ga3+: for LaFe0.5Ga0.5O3 the Ep value is 0.32 eV.38 This value is significantly higher than that for the cobalt analogue LaFe0.5Co0.5O3, thus providing doubt on the low-spin configuration of Co ions in LaCo0.5Fe0.5O3. This will be the subject of later investigations. The observed changes in the resistivity for iron-substituted perovskites are related to the density and mobility of the charge carriers. As shown in Figure 5, the carrier density decreases with iron content while the carrier mobility remains the same. Both the density and the mobility of charge carriers show a tendency to increase with the temperature; this trend is more pronounced for the carrier mobility (Figure 5a and 5a′). The electron transport properties of LaCo1−xFexO3 can be interpreted supposing that the iron ions act as electron-trapping centers, giving rise to electron localization in the cobaltates. For

slightly substituted perovskites, the higher resistivity in the lowtemperature range (below 400 K) is consistent with the stabilization of the low-spin state of cobalt ions, whereas above 400 K there is a spin transition for cobalt ions and a delocalization of electrons as in the case of unsubstituted LaCoO3. For highly substituted perovskites, the electron localization and, most probably, low-spin state of cobalt ions become more stable, resulting in the semiconducting behavior between 100 and 600 K. Comparison of the temperature dependence of the electrical resistivity for LaCoO3 and ironsubstituted LaCoO3 indicates that Fe3+ ions shift the spin transition of the cobalt ions to a higher temperature. The effect of Fe3+ on the spin state of Co3+ has been observed by Troyanchuk et al.27,28 for LaCo1−yFeyO3 obtained by solid state reaction at temperatures higher than 1000 °C. It has been found that LaCo0.5Fe0.5O3 undergoes a structural transition between 200 and 300 K from a rhombohedral phase to an orthorhombic phase.27,28 The electrical resistivity of LaCo0.5Fe0.5O3 obtained by us at 900 °C does not show any peculiarities between 200 and 300 K. In contrast to iron, replacement of Co by Ni causes a significant reduction in the resistivity of the perovskites (Figure 3). For x ≤ 0.1, the model based on the nearest neighbor hopping of small polarons describes well the temperature dependence of the resistivity, the Ep values being 0.0608 (6) and 0.0692 (6) eV for x = 0.05 and 0.10, respectively (Figure 4b). It appears that the hopping energy is insensitive toward the Ni content up to x < 0.1. By further increasing in the Ni content (x = 0.25), the Mott’s variable range hopping model [ρ = ρo exp(−T0/T)1/4] describes more adequately the temperature dependence of the resistivity (Figure 4b′, inset). At x = 0.5, the resistivity displays a slight dependence on the temperature between 100 and 600 K, indicating a transition from semiconductive to metallic conductivity. This is in agreement with earlier studies where LaCo0.5Ni0.5O3 is considered as a metallic ferromagnet.24−26 For LaCo1−xNixO3 with x ≤ 0.2, Li et al. analyzed the electrical conductivity assuming a small polarons hopping mechanism,30 while Androulakis et al.25 used Mott’s variable range hopping model for x ≥ 0.2. Data obtained by us show that the 13511



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conductive mechanism is changed with increasing Ni content from small polarons hopping to Mott’s variable range hopping. The carrier density displays a complex dependence for LaCo1−xNixO3: small amounts of nickel cause a strong increase in the carrier density, followed by a slight decrease for high levels of the Ni additive (Figure 5b). This behavior can be related to the oxygen deficiency and appearance of aliovalent Ni2+ ions as charge compensators. Contrary to the carrier density, the carrier mobility seems to be insensitive toward the nickel content (Figure 5b′). These results reveal that the enhancement in the electrical conductivity for nickelsubstituted perovskites results from an increase in density of the charge carriers. The enhancement of the conductivity of LaCoO3 has been also observed when La3+ ions were replaced by aliovalent Sr2+ ions. Because the transport properties of LaCo1−xNixO3 and La1−xSrxCoO3 are usually interpreted in the same manner,24−26 there is a need to compare the density of the charge carriers for both LaCo1−xNixO3 and La1−xSrxCoO3 compositions. For La1−xSrxCoO3, higher density of the charge carriers is established for x = 0.15 with a maximum value of 7− 9 × 1021 cm−3 at 300 K.39 For the Ni-substituted cobaltates studied by us, LaCo1−xNixO3 with x = 0.10 displays the highest density with a value of 1 × 1020 cm−3, which is 1 order of magnitude lower than that for Sr-substituted cobaltates. In addition, the Sr2+ ions induce an increase in the carrier mobility, while the Ni ions do not have any measurable effect. Therefore, the enhanced electrical conductivity for Sr-substituted perovskites can be regarded as a consequence from the increased carrier density and mobility, while the carrier density accounts only for the electrical conductivity of Ni-substituted cobaltates. The important feature of the transport properties of singlesubstituted cobaltates is that the nickel ions give rise to electron delocalization, which is opposite to the effect of the iron ions. While the transport properties of single-substituted cobaltates have been already studied, the double-substituted cobaltates are, to a great extent, unexplored. Figures 4 and 5 present the temperature dependence of the electrical resistivity, carrier density, and carrier mobility of LaCo1−x(Ni0.5Fe0.5)xO3. With a low level of doping (x = 0.05), there is a strong reduction in the resistivity (Figure 4c). It is noticeable that the resistivity remains the same despite the further increase in the Ni,Fe content (Figure 4c). The most adequate model that describes the changes in the resistivity between 100 and 600 K is based on the nearest neighbors hopping of small polarons [ρ = ρoT exp(−Ep/kT)] (Figure 4c′). The calculated hopping energy Ep is as follows: 0.0738(5), 0.0909(5), and 0.0865(5) eV for x = 0.05, 0.1, and 0.25. Contrary to the single-substituted LaCo1−xNixO3, this model is valid for highly substituted perovskites (i.e., for LaCo1−x(Ni0.5Fe0.5)xO3 with x = 0.5 where 50% of Co is replaced). It is clear that the hopping energy does not depend on the total Ni,Fe content, and its value is an intermediate between those calculated for the singlesubstituted cobaltates. This means that when Ni and Fe ions are in equal amounts, their effects are balanced, which leads to formation of LaCo1−x(Ni0.5Fe0.5)xO3 of lower resistivity, which is concentration insensitive despite replacement of 50% of Co ions. The reduction in the resistivity is a result of the increase in the carrier density induced by double substitution (Figure 5c). The carrier mobility seems to be unchanged during replacement of the cobalt ions (Figure 5c′). Figure 6 compares the dependence of the carrier density determined at 300 K for single- and double-substituted perovskites. The comparison shows that the effect of Ni ions (for partial electron

Figure 6. Carrier density as a function of the metal additives: LaCo 1−x Ni x O 3 (squares), LaCo 1−x Fe x O 3 (circles), and LaCo1−x(Ni0.5Fe0.5)xO3 (triangles).

delocalization) is more pronounced in comparison with that of Fe ions (for electron localization). Thermal Conductivity. The thermal conductivities of the single- and double-substituted perovskites are shown in Figure 7. In general, the thermal conductivity decreases upon

Figure 7. Thermal conductivity as a function of the metal additives: LaCo 1−x Ni x O 3 (circles), LaCo 1−x Fe x O 3 (squares), and LaCo1−x(Ni0.5Fe0.5)xO3 (triangles).

increasing the metal content (Ni, Fe and Ni,Fe, respectively). It is shown that the double substitution yields a more significant decrease in the thermal conductivity in comparison with the single substitution. The thermal conductivity comprises two contributions that are due to conductive carriers (λe) and phonon scattering. If the electron conduction is due to one type of charge carrier only then λe can be calculated by the Wiedemann−Franz law: λe = LT/ρ (the Lorentz number, L, is 2.45 × 10−8 V2 K−2). It is obvious that the κe term increases for Ni-substituted perovskites (3.10−4, 0.014, 0.014, and 0.055 W/ (m K) for LaCo1−xNixO3 with x = 0, 0.05, 0.10, and 0.25, respectively) but remains lower than the total thermal conductivity (0.44, 0.55, 0.43, and 0.33 W/(m.K) for LaCo1−xNixO3 with x = 0, 0.05, 0.10 and 0.25, respectively). This means that even in this case the total thermal conductivity is governed by the lattice contribution. As a result, the thermal 13512



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has already been observed for LaCo1−xNixO3 solid solutions.29,30,43 Contrary to the nickel ions, the iron ions show a tendency to increase the value of S. For double-substituted perovskites, the magnitude of S is intermediate between the S values of single-substituted perovskites. It is noticeable that after 50% of Co substitution the sign of S remains positive, reaching a value of 108 μV/K for LaCo0.5Ni0.25Fe0.25O3. In cobaltates, the larger thermopower is driven by the strong electronic correlations and large configurational entropy (typical for cobalt ions). Both contributions determine a dependence of S on the carrier concentration, which in turn is easily modified by Co site substitution. In the high-temperature limit, the Seebeck coefficient (S) is expressed by the Heikes formula44 extended by Koshibae et al.45 in order to include the spin and orbital degrees of freedom: S = −kB/e ln[(g3/g4){x/(1 − x)], where kB is the Boltzmann constant, e is the elementary electric charge, x denotes the substitution amount, and g3/g4 are the spin and orbital degeneracy of the cobalt ions. Experimentally it has been found that for cobaltates this formula qualitatively predicts the trends in the changes of both the magnitude and the sign of S.3 For the samples studied by us, the hole conductivities account for the electrical transport properties for single- and double-substituted perovskites. Therefore, the observed changes in the carrier density can simply describe the dependence of S on the amount of Ni, Fe, and Ni−Fe ions. Figure 8 gives the calculated S values for single- and double-substituted perovskites neglecting the contribution of the spin and orbital degeneracy. This assumption is reasonable, since the carrier concentration is less than 0.9% (even for LaCo0.9Ni0.10O3 with higher carrier density, Figure 6). As one can see, the calculated S values satisfactorliy reproduce the experimentally obtained dependence “S vs concentration” for the LaCo1−xFexO3 series. A large discrepancy is observed for highly substituted LaCo1−xNixO3 and LaCo1−x(Ni0.5Fe0.5)xO3. The results suggest that both spin and orbital degeneracy play a dominant role in the case when nickel appeared in the perovskite structure. The electronic structure of LaCo1−xNixO3 has been a subject of dispute during recent years.23−26 It has been proposed that the cobalt and nickel ions in LaCo1−xNixO3 remain in the trivalent state, but the nickel ions promote the spin transition for Co3+ ions.23−25 Another explanation is associated with an electronic transfer between Ni3+ and Co3+, leading to a disproportionation into Ni2+ and Co4+ ions.24−26 In addition, LaCo1−xNixO3 exhibits a higher level of oxygen deficiency, which could also contribute to the electronic structure. The problem of choosing the right cations as well as the various types of interactions between them does not allow applying correctly the extended Heikes formula in the case of LaCo1−xNixO3.46,47 However, the main trends in the changes of S with the Fe, Ni, and Fe+Ni content are fulfilled. It is worth noting that the Heikes formula is successfully applied for describing the Seebeck coefficient for low-doped perovskites,3 while for highly doped perovskites the Heikes formula is not applicable due to the increased spin entropy and the appearance of more than one charge carrier.6,47,48 Taking into account the Seebeck coefficient and electrical resistivity data, the power factor is calculated PF = S2/ρ (Figure 9a). It is clear that PF dramatically increases for low-doped LaCoO3 with Ni (about 1.4 μW/(K2 cm) for LaCo0.95Ni0.05O3), after which there is a significant decrease. For iron-doped cobaltates, PF smoothly decreases with the iron content. Double-substituted LaCoO3 displays a dependence of PF on

conductivities of the nickel- and iron-substituted perovskites are close. The dopant-induced decrease in the thermal conductivity of the cobaltates can mainly be explained by the enhanced phonon scattering due to the disordering of Co−Ni or Co−Fe ions in the octahedral 6b position (with coordinates 0, 0, 0) of the perovskite structure. This is a consequence of the ionic mismatch of the nickel, iron, and cobalt ions (Figure 2). It is worth mentioning that the simultaneous appearance of three ions (nickel, iron, and cobalt ions) in the 6b position more effectively reduces the heat transport in the cobaltates in comparison with that of two ions (Co−Ni or Co−Fe pairs). The decrease in the thermal conductivity is an important issue with a potential to design new thermoelectric materials. Recently, it has been reported that nanowires exhibit dramatic reduction of the thermal conductivity as compared to bulk materials, thus leading to a significant improvement of the figure of merit.40−42 Another chemical approach includes creation of vacancies in the La/O sites, which are responsible for the large thermal conductivity reduction observed for La1−xCoO3−y compositions.14 Our studies demonstrate that by rational double substitutions in perovskites a reduction of the thermal conductivity can also be achieved. Thermoelectric Efficiency. The Seebeck coefficient is strongly dependent on the amount of nickel and iron dopants (Figure 8). All perovskite compositions (with the exception of

Figure 8. Seebeck coefficient as a function of the metal additives: LaCo 1−x Ni x O 3 (circles), LaCo 1−x Fe x O 3 (squares), and LaCo1−x(Ni0.5Fe0.5)xO3 (triangles). Dotted lines correspond to the calculated Seebeck coefficient using Heikes formula.

nickel-rich cobaltates) display a positive sign of the Seebeck coefficient (S), thus indicating that the predominant mobile charge carriers are holes. For pure LaCoO3, the magnitude of S is 600 μV/K at room temperature. This value coincides with the data already reported in the literature for LaCoO3.5,29 It is important to note that the thermopower of LaCoO3 is extremely sensitive toward its oxygen content, LaCoO3‑δ:6 both positive and negative values of S have been reported for powders and single crystals of LaCoO3. For LaCoO3 studied by us, the oxygen deficiency is 0.02, which determines the observed positive S value. The magnitude of S decreases with the nickel content, reaching a negative value of −2.68 μV/K for LaCo0.5Ni0.5O3. In the same order, the oxygen deficiency also increases, suggesting a possible contribution of oxygen nonstoichiometry to the type of charge carriers in Ni-rich compositions. The trend of decreasing of S with the Ni content 13513



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conductivity of the LaCo1−xFexO3 series: LaCoO3 doped with 5% of Fe exhibits a ZT value between 0.02 and 0.03. The new finding of our studies is the impact of the double substitution on the figure of merit (Figure 9b). The thermoelectric activity strongly increases for double-substituted perovskites. This means that the thermoelectric activity is a result from the synergic effect of the simultaneous appearance of Fe and Ni in the cobalt perovskite structure. The perovskite with LaCo0.8Ni0.1Fe0.1O3 composition exhibits the best thermoelectric activity with ZT = 0.16, which is an order of magnitude higher than that of LaCoO3 at room temperature. The increased thermoelectric activity can mainly be related with the decreased thermal conductivity for double-substituted perovskites. This figure of merit is comparable with that reported for Sr-substituted cobaltates (La0.95Sr0.05CoO3 with ZT = 0.18 at room temperature), which has been considered as an efficient room-temperature thermoelectric oxide.50 In addition, a higher ZT value (of about 0.18) has also been found for La0.92CoO2.93, which contains vacancies in both lanthanum and oxygen sites.14 The comparison shows that double-substituted cobaltates, LaCo1−x(Ni0.5Fe0.5)xO3, exhibit a relatively good figure of merit, which makes them interesting as materials with thermoelectric properties.

4. CONCLUSIONS Replacement of cobalt with nickel and iron in LaCo1−x(Ni0.5Fe0.5)xO3 in the concentration range 0 ≤ x ≤ 0.25 yields oxides with a rhombohedrally distorted perovskite structure. The transport properties of LaCoO3 are effectively modified by single and double substitution. The electrical resistivity (ρ) significantly decreases during progressive replacement of cobalt by nickel. This is a consequence of the increased carrier density. In the same order, there is a decrease in the values of the Seebeck coefficient (S) and the thermal conductivity (λ). As a result, low-doped LaCo1−xNixO3 oxides (0.05 ≤ x ≤ 0.10) display a higher thermoelectric efficiency as compared to LaCoO3: ZT = 0.07. Substitution of iron for cobalt leads to an increase in ρ and S, while λ decreases. As a result, low-doped LaCo1−xFexO3 (x = 0.05) exhibits a dimensionless figure of merit ZT = 0.04, which is slightly higher than that of LaCoO3. Using the specific effects of Ni and Fe additives on S, ρ, and λ, new perovskite-type thermoelectric materials with double substitution (i.e., LaCo1−x(Ni0.5Fe0.5)xO3) are proposed. In these perovskites, the effect of nickel ions on the Seebeck coefficient is more pronounced than that of the iron ions. It is noticeable that the double-substituted perovskites have an enhanced electrical conductivity as compared to that of LaCoO3, which does not depend on the total Ni+Fe content. The synergic effect of Ni and Fe is demonstrated by the effective reduction of the thermal conductivity in comparison with the single-substituted perovskites. As a result, the perovskite of composition LaCo0.8Ni0.1Fe0.1O3 exhibits the best thermoelectric efficiency with ZT = 0.16, which is an order of magnitude higher than that of LaCoO3 at room temperature. The double-substitution strategy opens new possibilities for optimization of the thermoelectric activity of LaCoO3-based ceramics.

Figure 9. Power factor (PF = S2/ρ, a) and figure of merit (ZT = S2T/ (ρκt), b) determined at 300 K as a function of metal additives: LaCo 1−x Ni x O 3 (circles), LaCo 1−x Fe x O 3 (squares), and LaCo1−x(Ni0.5Fe0.5)xO3 (triangles).

metal additives, which is close to that of single substituted with nickel LaCoO3. The PF value reaches about 0.9 μW/(K2 cm) for LaCo0.90Ni0.05Fe0.05O3. This reveals that nickel additives have a stronger effect as compared to iron ones. It is noticeable that in comparison with single-substituted perovskites doublesubstituted LaCoO3 have a higher PF for highly doped samples. The observed power factor for double-substituted cobaltates LaCo1−x(Ni0.5Fe0.5)xO3 is lower than the unusually high power factor established for SrTiO3 (20 μW/(K2 cm)40,49). However, the extremely large thermal conductivity of SrTiO3 (about 10 W/(m K)) reduces its figure of merit.41,49 The figures of merit for single- and double-substituted perovskites are shown in Figure 9b. As one can expect, the lower level of nickel substitution causes a strong increase in thermoactivity due to the enhanced electrical conductivity. Further increases in the nickel content lead to a decrease in ZT as a consequence of the decreased Seebeck coefficient. The best thermoelectric activity is observed for LaCoO3 doped with 5− 10% of Ni, where the ZT value reaches about 0.07 at 295 K. These results are in good agreement with previous data on the thermoelectric properties of LaCo1−xNixO3 obtained by a softchemistry process and a solid state reaction:25,26 for ceramic LaCo0.9Ni0.1O3 a maximum of ZT = 0.031 at 50 °C has been reported.26 For LaCo1−xFexO3, the ZT value increases with a small amount of iron additives, whereas for higher iron contents a decrease becomes visible. This dependence reflects the increased Seebeck coefficient and the lower electrical

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*Phone: 0 359 2 9793915. Fax: 0 359 2 8705024. E-mail: [email protected]. 13514



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Notes

(32) Schulz, B High Temp. High Press. 1981, 13, 649−660. (33) Sehlin, S. R.; Anderson, H. U.; Sparlin, D. M. Phys. Rev. B 1995, 52, 11681. (34) Korotin, M. A.; Ezhov, S; Solovyev, Yu, IV; Anisimov, V. I.; Khomskii, D. I.; Sawatzky, G. A. Phys. Rev. B 1996, 54, 5309. (35) Yamaguchi, S; Okimoto, Y; Tokura, Y Phys. Rev. B 1997, 55, R8666. (36) Saitoh, T; Mizokawa, T; Fujimori, A; Takeda, Y; Takano, M Surf. Sci. Spectra 1999, 6, 302. (37) Asai, K; Yoneda, A; Yokokura, O; Tranquada, J. M.; Shirane, G; Kohn, K J. Phys. Soc. Jpn. 1998, 67, 290. (38) Komine, S; Iguchi, E J. Phys. Chem. Solids 2007, 68, 1504−1507. (39) Iwasaki, K; Ito, T; Nagasaki, T; Arita, Y; Yoshino, M; Matsui, T J. Solid State Chem. 2008, 181, 3145−3150. (40) Chen, R; Delgado, R. D.; Garnett, E. C.; Hochbaum, A. I.; Liang, W; Majumdar, A; Najarian, M; Yang, P Nature 2008, 451, 734− 167. (41) Yadav, G. G.; Zhang, G; Qiu, B; Susoreny, J. A.; Ruan, X; Wu, Y Nanoscale 2011, 3, 4078−4081. (42) Wang, Y; Fan, H. J. J. Phys. Chem. C 2010, 114, 13947−13953. (43) Robert, R; Bocher, L; Trottmann, M; Reller, A; Weidenkaff, A J. Solid State Chem. 2006, 179, 3893−3899. (44) Chaikin, P. M.; Beni, G Phys Rev B 1976, 13, 647−651. (45) Koshibae, W; Tsutsui, K; Maekawa, S Phys. Rev. B 2000, 62, 6869. (46) Doumerc, J.-P. J. Solid State Chem. 1994, 110, 419−420. (47) Jirák, Z.; Hejtmánek, J.; Knížek, K; Veverka, M Phys. Rev. B 2008, 78, 014432. (48) Wang, Y; Sui, Y; Ren, P; Wang, L; Wang, X; Su, W; Fan, H. J. Inorg. Chem. 2010, 49, 3216−3223. (49) Chateigner, D; Chong, K; Funahashi, R; Guilmeau, E; Mikami, M Appl. Phys. Lett. 2004, 85, 1490. (50) Androulakis, J; Migiakis, P; Giapintzakis, J Appl. Phys. Lett. 2004, 84, 1099.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Science Fund of Bulgaria (IDEAS No D0-02-309/2008 and National Centre for New Materials UNION, Contract No DCVP-02/2/2009).

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REFERENCES

(1) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2009, 48, 8616−8639. (2) Terasaki, I; Sasago, Y; Uchinokura, K Phys. Rev. B 1997, 56, R12685. (3) Maignan, A; Wang, L. B.; Hébert, S; Pelloquin, D; Raveau, B Chem. Mater. 2002, 14, 1231−1235. (4) Weidenkaff, A; Robert, R; Aguirre, M; Bocher, L; Lippert, T; Canulescu, S Renewable Energy 2008, 33, 342−347. (5) Señarís Rodríguez, M. A.; Goodenough, J. B. J. Solid State Chem. 1995, 116, 224−231. (6) Berggold, K; Kriener, M; Zobel, C; Reichl, A; Reuther, M; Müller, R; Freimuth, A; Lorenz, T Phys. Rev. B 2005, 72, 155116. (7) Herve, P; Ngamou, T; Bahlawane, N Chem. Mater. 2010, 22, 4158−4165. (8) Raccah, P. M.; Goodenough, J. B. Phys. Rev. 1967, 155, 932−943. (9) Berggold, K; Kriener, M; Becker, P; Benomar, M; Reuther, M; Zobel, C; Lorenz, T Phys. Rev. B 2008, 78, 134402. (10) Tokra, Y; Okimoto, Y; Yamaguchi, S; Taniguchi, H Phys. Rev. B 1998, 58, R1699−R1702. (11) Hébert, S; Flahaut, D; Martin, C; Lemonnier, S; Noudem, J; Goupil, C; Maignan, A; Hejtmanek, J Prog. Solid State Chem. 2007, 35, 457−467. (12) Dho, J; Hur, N. H. Solid State Commun. 2006, 138, 152−156. (13) Luo, X; Xing, W; Li, Z; Wu, G; Chen, X Phys. Rev. B 2007, 75, 054413. (14) Wang, Y; Li, F; Xu, L; Sui, Y; Wang, X; Su, W; Liu, H Inorg. Chem. 2011, 50, 4412−4416. (15) Goodenough, J. B. Phys. Rev. 1967, 164, 785. (16) Medarde, M. L. J. Phys.: Condens. Matter 1997, 9, 1679. (17) Scholl, A; Stohr, J; Luning, J; Seo, J. W.; Fompeyrine, J; Siegwart, H; Locquet, J-P; Nolting, F; Anders, S; Fullerton, E. E.; Scheinfein, M. R.; Padmore, H. A. Science 2000, 287, 1014−1016. (18) Funahashi, R; Mikami, M; Urata, S; Kouuchi, T; Mizuno, K; Chong, K MRS Proc. 2003, 793, S3.3. (19) Gayathri, N; Raychaudhuri, A. K.; Xu, X. Q.; Peng, J. L.; Greene, R. L. J. Phys.: Condens. Matter. 1998, 10, 1323. (20) Iwasaki, K; Ito, T; Yoshino, M; Matsui, T; Nagasaki, T; Arita, Y J. Alloys Compd. 2007, 430, 297−301. (21) Mizusaki, T; Sasamoto, W. R.; Cannon, H. K.; Bowen. J. Am. Ceram. Soc. 1982, 65, 363−368. (22) Ivanova, S; Senyshyn, A; Zhecheva, E; Tenchev, K; Stoyanova, R; Fuess, H J. Solid State Chem. 2010, 183, 940−950. (23) Yu, J; Kamazawa, K; Louca, D Phys. Rev. B 2010, 82, 224101. (24) Kyômen, T; Yamazaki, R; Itoh, M Phys. Rev. B 2003, 68, 104416. (25) Androulakis, J; Katsarakis, N; Viskadourakis, Z; Giapintzakis, J J. Appl. Phys. 2003, 93, 5484. (26) Viswanathan, M; Anil Kumar, P. S. Phys. Rev B 2009, 80, 012410. (27) Karpinsky, V; Troyanchuk, I. O.; Barner, K; Szymczak, H; Tovar, M J. Phys.: Condens. Matter 2005, 17, 7219. (28) Troyanchuk, I. O.; Karpinsky, D. V.; Szymczak, R; Szymczak, H J. Magn. Magn. Mater. 2006, 298, 19−24. (29) Robert, R; Bocher, L; Sipos, B; Dobeli, M; Weidenkaff, A Prog. Solid State Chem. 2007, 35, 447−455. (30) Li, F; Li, J-F Ceram. Int. 2011, 37, 105−111. (31) Ivanova, S; Senyshyn, A; Zhecheva, E; Tenchev, K; Nikolov, V; Stoyanova, R; Fuess, H J. Alloys Compd. 2009, 480, 279−285. 13515


Improving of the Thermoelectric Efficiency of LaCoO3 by Double

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