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Oxygen nonstoichiometry and electrical conductivity of LaNi0.6Fe0.4O3 − δ at high temperatures under various oxygen partial pressures
Solid State Ionics 274 (2015) 119–122
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Oxygen nonstoichiometry and electrical conductivity of LaNi0.6Fe0.4O3 − δ at high temperatures under various oxygen partial pressures Eiki Niwa a, Chie Uematsu b, Junichiro Mizusaki c, Takuya Hashimoto a a b c
Department of Physics, College of Humanities and Sciences, Nihon University, Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan Department of Integrated Sciences in Physics and Biology, College of Humanities and Sciences, Nihon University, Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 11 December 2014 Received in revised form 27 February 2015 Accepted 28 February 2015 Available online 27 March 2015 Keywords: LaNi0.6Fe0.4O3 Electrical conductivity Hopping conduction Oxygen nonstoichiometry Mobility Carrier density
a b s t r a c t Temperature and oxygen partial pressure, P(O2), dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ, which is a new candidate cathode material for solid oxide fuel cells, has been determined by the equilibrium thermogravimetry. The P(O2) and temperature dependence of the electrical conductivity of LaNi0.6Fe0.4O3 − δ has also been measured. It has been clarified that the oxygen content, 3 − δ of LaNi0.6Fe0.4O3 − δ at temperatures between 300 °C and 700 °C under 10−4 bar ≤ P(O2) ≤ 1 bar is around 2.90, which can be explained by Ni and Fe valence of +2 and +4, respectively. Temperature and P(O2) dependence of the oxygen nonstoichiometry and the electrical conductivity is small for LaNi0.6Fe0.4O3 − δ, which can be ascribed to small variation of the average valence of B-site ion by temperature and P(O2) due to occurrence of charge compensation between Ni ion and Fe ion. The easy occurrence of the charge compensation corresponds to hopping conduction with small, temperature and P(O2) independent activation energy. By combination of oxygen nonstoichiometry and electrical conductivity, it has been clarified that LaNi0.6Fe0.4O3 − δ is p-type conductor with mobility of 0.257 cm2 V−1 s−1, which is independent on temperature and P(O2). Hole concentration at temperatures between 300 °C and 800 °C under −4 ≤ log(P(O2)/bar) ≤ 0 is 3–4 × 1021 cm−3. © 2015 Elsevier B.V. All rights reserved.
1. Introduction LaNi1 − xFexO3 − δ has been expected as a cathode material of solid oxide fuel cells, SOFCs, because these oxides have high electrical conductivity and high stability against YSZ, which is the most employed material as SOFC electrolyte [1,2]. The electrical conductivity of LaNi1 − xFexO3 − δ with 0.4 ≤ x ≤ 1.0 increases continuously with increasing Ni content and LaNi0.6Fe0.4O3 − δ shows the highest conductivity among LaNi1 − xFexO3 − δ. However, the electrical conductivity of LaNi1 − xFexO3 − δ with 0.0 ≤ x ≤ 0.4 decreases with increasing Ni content because Ni-rich specimens show low stability at high temperature, resulting in low sintering property [2]. So far, the electrical conduction mechanism of LaNi1 − xFexO3 − δ has been explained as small polaron hopping conduction model [1,3,4]. Activation energy for hopping conduction, Ea, of LaNi1 − xFexO3 − δ, which was estimated from temperature dependence of electrical conductivity in air, gradually decreases with increasing Ni content. It has been also clarified that Ea is independent on sintering density, indicating that Ea is not determined by grain boundary but by intraparticle.
E-mail address: [email protected] (E. Niwa).
http://dx.doi.org/10.1016/j.ssi.2015.02.022 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Since cathode materials are exposed to atmospheres with various oxygen chemical potentials, information on electrical conduction behavior and oxygen nonstoichiometry under various oxygen partial pressures, P(O2), is inevitable for application. However, no reliable report on oxygen nonstoichiometry and its effect on electrical conduction behavior of LaNi1 − xFexO3 − δ has appeared. Chen and coworkers [5] reported electrical conductivity of LaNi0.6Fe0.4O3 − δ under various P(O2); however, they carried out electrical conduction measurements above 800 °C, where the decomposition to La2(Ni,Fe)O4 or La4(Ni,Fe)3O10 phase was probable [6,7]. In this study, temperature and oxygen partial pressure dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ, which is the most promising as cathode material for SOFC among LaNi1 − xFexO3 − δ, has been determined by equilibrium thermogravimetry. The measurements were carried out below 700 °C under −4 ≤ log(P(O2)/bar) ≤ 0, where the decomposition is not probable [6,7]. Also, temperature dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ was measured under various P(O 2) and the variation of Ea on P(O2 ) was investigated. With combination of obtained oxygen nonstoichiometry and electrical conductivity under various P(O2), electrical conduction model of LaNi0.6 Fe 0.4 O 3 − δ considering defect structure was discussed.
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conductivity was evaluated. Activation energy of hopping conduction, Ea, of LaNi0.6Fe0.4O3 − δ was estimated from temperature dependence of electrical conductivity. For small polaron hopping conduction model, the following equation is satisfied,
2. Experimental 2.1. Preparation procedure LaNi0.6Fe0.4O3 − δ was prepared by Pechini method [8] with which homogeneous specimens could be prepared [2,9]. The starting materials were dilute nitric acid solution of La2O3 (99.9%, Furuuchi Chemical Corp.), aqueous solution of Fe(NO3)3·9H2O and Ni(NO3)2·6H2O. Purity of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O as compounds was evaluated from the weight of NiO and Fe2O3 remained after heat-treatment at 1000 °C in air. La2O3 was heated at 1000 °C for 10 h before the dissolution to decompose the second phases such as La(OH)3 and La2(CO3)3. The solutions were mixed with the nominal amount of the cation ratio. After addition of citric acid and ethylene glycol, the mixed solution was heated at 450 °C by a mantle heater until the residual materials were fired. The obtained precursors were calcined at 750 °C for 24 h in static air. The obtained samples were confirmed to be single phase with X-ray diffraction analysis [2,4].
σ∝μ ¼
A E exp − a T kT
ð3Þ
where A and k are pre-exponential constant and Boltzmann constant. From the electrical conductivity measured under various P(O2), dependence of Ea of LaNi0.6Fe0.4O3 − δ on P(O2) was investigated. P(O2) was monitored with zirconia oxygen sensor at downstream of the apparatus. The conduction mechanism considering defect structure was discussed by the combination of the obtained oxygen nonstoichiometry and the electrical conductivity. 3. Results and discussion 3.1. Oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ
2.2. Equilibrium thermogravimetry of LaNi0.6Fe0.4O3 − δ Temperature and P(O2) dependence of oxygen nonstoichiometry was measured at equilibrium state with high-resolution electric micro-balance (D-200, Cahn). Approximately 1 g of the single phase LaNi0.6Fe0.4O3 − δ powder was hand-pressed using carbide die with 10 mm diameter and sintered at 800 °C for 5 h resulting in a porous pellet employed as measurement specimen. The measurement was carried out for temperature range between 300 °C and 700 °C under −4.0 ≤ log(P(O2)/bar) ≤ 0.0 to avoid decomposition of the specimen. After more 12 h passed since the specimen was set at measurement temperature and P(O2), equilibrium state was realized and weight relaxation was observed. The change of oxygen content, Δδ, can be estimated from the weight change (Δws) of the sample with following equation.
Δδ ¼
MS ΔwS M O wS
Fig. 1 shows P(O2) dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ measured with the equilibrium thermogravimetry at various temperatures. The absolute value of oxygen content of LaNi0.6Fe0.4O3 − δ, which was determined from weight loss by reduction under pure H2 atmosphere, is around 2.90, showing correspondence with the results of iodometric titration in our preceding reports [10, 11]. LaNi0.6Fe0.4O3 − δ shows rather high δ, which is more than 3 mol%, even in oxidative atmosphere. Due to the high δ, large oxide ion diffusion can be expected, which was confirmed by Nishi and coworkers [12]. Assuming that all the Ni ion and Fe ion are divalent, NiNi′, and tetravalent, Fe•Fe, respectively, oxygen content, 3 − δ, in LaNi0.6Fe0.4O3 − δ is equal to 2.9, which shows correspondence with the observed oxygen content in LaNi0.6Fe0.4O3 − δ. The valence of Fe ion approaching + 4 with increasing Ni content has also been proposed by Mössbauer spectroscopy [13]. Clear plateau is not observed around 2.90 in Fig. 1, suggesting that the hole on Fe 3d orbital, represented as Fe·Fe, is easily
ð1Þ
where Ms, Mo and ws are the molar weight of the sample, that of oxygen gas and the sample weight set in micro-balance apparatus, respectively. In order to evaluate the reversibility of Δδ, the weight variation with heating and cooling procedure under constant P(O2) was compared and confirmed to be identical. Also, identical weight variation was observed between increasing and decreasing P(O2) at constant temperature. The absolute value of oxygen content in LaNi0.6Fe0.4O3 − δ was estimated from the weight decrease by reductive decomposition of the specimen at 800 °C under pure H2 atmosphere expressed as following formula (2). 1 3−2δ O2 LaNi0:6 Fe0:4 O3−δ → La2 O3 þ 0:6Ni þ 0:4 Fe þ 2 4
ð2Þ
2.3. Electrical conduction measurement Electrical conductivity of LaNi0.6Fe0.4O3 − δ was measured by DC four probe method from 300 °C to 800 °C under −4 ≤ log(P(O2)/bar) ≤ 0. The sintering specimen was prepared by pressing the pellet and sintered at 1000 °C for 10 h in air. Platinum was used as material for electrode wire. Shape of the samples employed for the measurements was the rectangular solid with 2 mm × 2 mm × 14 mm. The length between voltage terminals was approximately 12.5 mm. The holding time at each temperature was more than 12 h, which was enough to realize equilibrium state. After confirmation of linear relationship between voltage and current between − 100 mA and + 100 mA, electrical
Fig. 1. Dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ on P(O2) measured by equilibrium thermogravimetry.
E. Niwa et al. / Solid State Ionics 274 (2015) 119–122
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delocalized according to the following equation with small energy variation. 0
•
NiNi þ Fe Fe ↔NiNi þ Fe Fe
ð4Þ
3+ × and Fe3 + in LaNi0.6Fe0.4O3 − δ, Here, Ni× Ni and FeFe represent Ni respectively. The small energy variation of reaction (4) shows agreement with small activation energy for hopping conduction of LaNi0.6Fe0.4O3 − δ reported in our reports [4,11]. Compared with other perovskite-type oxide with transition metal on B-site cation such as La1 − xSrxCo1 − yFeyO3 − δ [14,15], the variation of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ by temperature and P(O2) is smaller. The smaller variation of oxygen nonstoichiometry can be attributed to small variation of the average valence of B-site cation. Due to small energy of reaction (4) to left side, valence change of Ni ion from 3 + to 2 + by reduction can be easily compensated by the valence variation of Fe ion from 3 + to 4+ with little variation of oxygen content.
3.2. P(O2) dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ Fig. 2(a) shows P(O2) dependence of the electrical conductivity of LaNi0.6Fe0.4O3 − δ at various temperatures. It has been revealed that the variation of log σ on temperature and P(O2) is rather small. The electrical conductivity of LaNi0.6Fe0.4O3 − δ increases with increasing P(O2), suggesting that the dominate carrier of LaNi0.6Fe0.4O3 − δ under the measurement P(O2) is hole. Fig. 2(b) shows Arrhenius plot of hopping conduction of LaNi0.6Fe0.4O3 − δ under various P(O2). Linear relationship between 1/T and log(σT) is observed regardless of P(O2), indicating that the small polaron hopping model can be applicable. At 800 °C under logP(O2) below − 4, observed is deviation from linear relationship, which can be attributed to decomposition to La4(Ni,Fe)3O10 phase [6,7] and thus omitted from further discussion. Activation energy for hopping conduction, Ea, of LaNi0.6Fe0.4O3 − δ was estimated from Eq. (3) and Fig. 3(b). Table 1 lists Ea of LaNi0.6Fe0.4O3 − δ under various P(O2). Ea is independent on P(O2) and approximately 0.049 eV. The obtained Ea shows agreement with the result of the temperature dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ in air [4,11]. In the preceding paper, it was reported that Ea of LaNi0.6Fe0.4O3 − δ had linear relationship with hole mobility, μh, in intraparticle. Therefore, it can be concluded that hole mobility, μh, is independent on P(O2) and oxygen content. Hole hopping of LaNi1 − xFexO3 − δ from Ni ion to Fe ion can be represented as following equations.
0
•
ð5Þ
•
•
ð6Þ
NiNi ↔NiNi þ h
Fe Fe þ h ↔Fe Fe
Combining Eqs. (5) and (6), Eq. (4) can be obtained. Therefore, Ea should correspond to activation energy of chemical reaction represented as Eq. (4). For LaNi0.6Fe0.4O3 − δ, Ea is as small as 0.049 eV, indicating that kinetics of chemical reaction represented as Eq. (4) is so high that charge compensation by valence variation immediately occurs. This prompt charge compensation corresponds to the small variation of oxygen nonstoichiometry by temperature and P(O2). 3.3. Electrical conduction mechanism considering defect structure of LaNi0.6Fe0.4O3 − δ Fig. 3 shows the relationship between the oxygen content and electrical conductivity of LaNi0.6Fe0.4O3 − δ prepared from the results of Figs. 1 and 2(a). Linear relationship was obtained regardless of temperature. This indicates that the variation of mobility, which should be dependent on temperature, is negligibly low compared to that of carrier density, which should depend on oxygen content. Thus revealed
Fig. 2. (a) P(O2) dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ at various temperatures. Right axis represents hole carrier density evaluated in this study. (b) Arrhenius plot of hopping conduction of LaNi0.6Fe0.4O3 − δ under various P(O2).
constant mobility showed correspondence with P(O2) independent E a as listed in Table 1. Fig. 3 also indicates that the carrier of LaNi 0.6 Fe0.4O 3 − δ is hole under examined conditions because the electrical conductivity increases with increasing oxygen content. Here, hole carrier density, p, can be expressed as following from Eqs. (5) and (6), • NA • N A N p¼ h ¼ NiNi þ Fe Fe ¼ 2ð0:3−δÞ A Vm Vm Vm
ð7Þ
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E. Niwa et al. / Solid State Ionics 274 (2015) 119–122
temperature and P(O2) can be estimated from electrical conductivity, σ. The hole concentration thus estimated are shown in the right vertical axis in Fig. 2(a). It has been clarified that the hole concentration of LaNi0.6Fe0.4O3 − δ is 3–4 × 1021 cm−3, which is relatively high among electrical conducting oxides. 4. Conclusions
Fig. 3. Relationship between oxygen content, 3 − δ and electrical conductivity, σ of LaNi0.6Fe0.4O3 − δ.
Oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ has been evaluated by equilibrium thermogravimetry. It has been clarified that oxygen content is about 2.90 at temperatures between 300 °C and 700 °C under − 4 ≤ log{P(O2)/bar} ≤ 0, indicating that oxide ion vacancy is stable under this conditions. Temperature and P(O2) dependence of oxygen nonstoichiometry and electrical conductivity is small for LaNi0.6Fe0.4O3 − δ, probably because the variation of average valence of B-site ion by temperature and P(O2) is small since charge compensation between Ni and Fe is easy to occur. The easy occurrence of the charge compensation corresponds to hopping conduction with small, temperature and P(O2) independent activation energy. By the combination of oxygen nonstoichiometry and electrical conductivity, it has been revealed that LaNi0.6Fe0.4O3 − δ is p-type conductor with mobility of 0.257 cm2V−1 s−1, which is independent on temperature and P(O2). Hole concentration at temperatures between 300 °C and 800 °C under −4 ≤ log{P(O2)/bar} ≤ 0 is calculated to be 3–4 × 1021 cm−3. Acknowledgment This study was financially supported by grants from the “Strategic Research Base Development” Program for Private Universities (S0901022) subsidized by MEXT (2009) and from Nihon University Strategic Projects for Academic Research “Nanotechnology ExcellenceNanomaterial based Photonic and Quantum Technologies”. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.
Table 1 The activation energy for hopping conduction, Ea of LaNi0.6Fe0.4O3 − δ estimated from Fig. 2(b). log(P(O2)/bar)
Activation energy, Ea/eV
−0.03 −1.09 −1.96 −2.95 −4.22
0.050 0.049 0.049 0.049 0.048
References
where NA and Vm , are Avogadro's constant and molar volume of LaNi0.6Fe0.4O3 − δ, respectively. The total electrical conductivity, σ, can be expressed as σ ≅σ h ¼ peμ h ¼ 2ð0:3−δÞ
NA eμ : Vm h
ð8Þ
The slope of relationship between oxygen content and electrical conductivity can be written as ∂σ N ¼ −2 A eμ h : Vm ∂δ
ð9Þ
Therefore, linear relationship between σ and δ should be observed if temperature and P(O2) dependence of μh is low. For LaNi0.6Fe0.4O3 − δ, linear relationship is observed as shown in Fig. 3, indicating that μh is independent on temperature and P(O2). The μh can be calculated from the slope of Fig. 3 using Eq. (9) to be 0.257 cm2V− 1 s−1. By using Eq. (8) and calculated μh, the dependence of hole concentration on
[1] R. Chiba, F. Yoshimura, Y. Sakurai, Solid State Ionics 124 (1999) 281. [2] E. Niwa, C. Uematsu, E. Miyashita, T. Ohzeki, T. Hashimoto, Solid State Ionics 201 (2011) 87. [3] M. Bevilacqua, T. Montini, C. Tavagnacco, G. Vicario, P. Fornasiero, M. Graziani, Solid State Ionics 177 (2006) 2957. [4] E. Niwa, H. Maeda, C. Uematsu, T. Hashimoto, ECS Trans. 50 (11) (2013) 117. [5] J.Y. Chen, J. Rebello, V. Vashook, D.M. Trots, S.R. Wang, T.L. Wen, J. Zosel, U. Guth, Solid State Ionics 192 (2011) 424. [6] E.A. Kiselev, V.A. Cherepanov, J. Solid State Chem. 183 (2010) 1992. [7] Y. Morishima, E. Niwa, T. Hashimoto, J. Therm. Anal. Calorim. (2015) (submitted for publication). [8] M.P. Pechini, US Patent 3,330,697 (1967). [9] E. Niwa, C. Uematsu, T. Hashimoto, Mater. Res. Bull. 48 (1) (2013) 1. [10] E. Niwa, C. Uematsu, J. Mizusaki, T. Hashimoto, ECS Trans. 57 (1) (2013) 2133–2140. [11] E. Niwa, H. Maeda, C. Uematsu, T. Hashimoto, Mater. Res. Bull. (2015) (submitted for publication). [12] M. Nishi, T. Horita, K. Yamaji, H. Yokokawa, T. Shimonosono, H. Kishimoto, M.E. Brito, D. Cho, F. Wang, ECS Trans. 45 (1) (2012) 171. [13] T. Hashimoto, E. Niwa, C. Uematsu, E. Miyashita, T. Ohzeki, K. Shozugawa, M. Matsuo, Hyperfine Interact. 206 (2012) 47. [14] S. Hashimoto, Y. Fukuda, M. Kuhn, K. Sato, K. Yashiro, J. Mizusaki, Solid State Ionics 181 (2010) 1713. [15] M. Kuhn, Y. Fukuda, S. Hashimoto, K. Sato, K. Yashiro, J. Mizusaki, J. Electrochem. Soc. 160 (2013) F34.
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Oxygen nonstoichiometry and electrical conductivity of LaNi0.6Fe0.4O3 − δ at high temperatures under various oxygen partial pressures Eiki Niwa a, Chie Uematsu b, Junichiro Mizusaki c, Takuya Hashimoto a a b c
Department of Physics, College of Humanities and Sciences, Nihon University, Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan Department of Integrated Sciences in Physics and Biology, College of Humanities and Sciences, Nihon University, Sakurajousui, Setagaya-ku, Tokyo 156-8550, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 11 December 2014 Received in revised form 27 February 2015 Accepted 28 February 2015 Available online 27 March 2015 Keywords: LaNi0.6Fe0.4O3 Electrical conductivity Hopping conduction Oxygen nonstoichiometry Mobility Carrier density
a b s t r a c t Temperature and oxygen partial pressure, P(O2), dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ, which is a new candidate cathode material for solid oxide fuel cells, has been determined by the equilibrium thermogravimetry. The P(O2) and temperature dependence of the electrical conductivity of LaNi0.6Fe0.4O3 − δ has also been measured. It has been clarified that the oxygen content, 3 − δ of LaNi0.6Fe0.4O3 − δ at temperatures between 300 °C and 700 °C under 10−4 bar ≤ P(O2) ≤ 1 bar is around 2.90, which can be explained by Ni and Fe valence of +2 and +4, respectively. Temperature and P(O2) dependence of the oxygen nonstoichiometry and the electrical conductivity is small for LaNi0.6Fe0.4O3 − δ, which can be ascribed to small variation of the average valence of B-site ion by temperature and P(O2) due to occurrence of charge compensation between Ni ion and Fe ion. The easy occurrence of the charge compensation corresponds to hopping conduction with small, temperature and P(O2) independent activation energy. By combination of oxygen nonstoichiometry and electrical conductivity, it has been clarified that LaNi0.6Fe0.4O3 − δ is p-type conductor with mobility of 0.257 cm2 V−1 s−1, which is independent on temperature and P(O2). Hole concentration at temperatures between 300 °C and 800 °C under −4 ≤ log(P(O2)/bar) ≤ 0 is 3–4 × 1021 cm−3. © 2015 Elsevier B.V. All rights reserved.
1. Introduction LaNi1 − xFexO3 − δ has been expected as a cathode material of solid oxide fuel cells, SOFCs, because these oxides have high electrical conductivity and high stability against YSZ, which is the most employed material as SOFC electrolyte [1,2]. The electrical conductivity of LaNi1 − xFexO3 − δ with 0.4 ≤ x ≤ 1.0 increases continuously with increasing Ni content and LaNi0.6Fe0.4O3 − δ shows the highest conductivity among LaNi1 − xFexO3 − δ. However, the electrical conductivity of LaNi1 − xFexO3 − δ with 0.0 ≤ x ≤ 0.4 decreases with increasing Ni content because Ni-rich specimens show low stability at high temperature, resulting in low sintering property [2]. So far, the electrical conduction mechanism of LaNi1 − xFexO3 − δ has been explained as small polaron hopping conduction model [1,3,4]. Activation energy for hopping conduction, Ea, of LaNi1 − xFexO3 − δ, which was estimated from temperature dependence of electrical conductivity in air, gradually decreases with increasing Ni content. It has been also clarified that Ea is independent on sintering density, indicating that Ea is not determined by grain boundary but by intraparticle.
E-mail address: [email protected] (E. Niwa).
http://dx.doi.org/10.1016/j.ssi.2015.02.022 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Since cathode materials are exposed to atmospheres with various oxygen chemical potentials, information on electrical conduction behavior and oxygen nonstoichiometry under various oxygen partial pressures, P(O2), is inevitable for application. However, no reliable report on oxygen nonstoichiometry and its effect on electrical conduction behavior of LaNi1 − xFexO3 − δ has appeared. Chen and coworkers [5] reported electrical conductivity of LaNi0.6Fe0.4O3 − δ under various P(O2); however, they carried out electrical conduction measurements above 800 °C, where the decomposition to La2(Ni,Fe)O4 or La4(Ni,Fe)3O10 phase was probable [6,7]. In this study, temperature and oxygen partial pressure dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ, which is the most promising as cathode material for SOFC among LaNi1 − xFexO3 − δ, has been determined by equilibrium thermogravimetry. The measurements were carried out below 700 °C under −4 ≤ log(P(O2)/bar) ≤ 0, where the decomposition is not probable [6,7]. Also, temperature dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ was measured under various P(O 2) and the variation of Ea on P(O2 ) was investigated. With combination of obtained oxygen nonstoichiometry and electrical conductivity under various P(O2), electrical conduction model of LaNi0.6 Fe 0.4 O 3 − δ considering defect structure was discussed.
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E. Niwa et al. / Solid State Ionics 274 (2015) 119–122
conductivity was evaluated. Activation energy of hopping conduction, Ea, of LaNi0.6Fe0.4O3 − δ was estimated from temperature dependence of electrical conductivity. For small polaron hopping conduction model, the following equation is satisfied,
2. Experimental 2.1. Preparation procedure LaNi0.6Fe0.4O3 − δ was prepared by Pechini method [8] with which homogeneous specimens could be prepared [2,9]. The starting materials were dilute nitric acid solution of La2O3 (99.9%, Furuuchi Chemical Corp.), aqueous solution of Fe(NO3)3·9H2O and Ni(NO3)2·6H2O. Purity of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O as compounds was evaluated from the weight of NiO and Fe2O3 remained after heat-treatment at 1000 °C in air. La2O3 was heated at 1000 °C for 10 h before the dissolution to decompose the second phases such as La(OH)3 and La2(CO3)3. The solutions were mixed with the nominal amount of the cation ratio. After addition of citric acid and ethylene glycol, the mixed solution was heated at 450 °C by a mantle heater until the residual materials were fired. The obtained precursors were calcined at 750 °C for 24 h in static air. The obtained samples were confirmed to be single phase with X-ray diffraction analysis [2,4].
σ∝μ ¼
A E exp − a T kT
ð3Þ
where A and k are pre-exponential constant and Boltzmann constant. From the electrical conductivity measured under various P(O2), dependence of Ea of LaNi0.6Fe0.4O3 − δ on P(O2) was investigated. P(O2) was monitored with zirconia oxygen sensor at downstream of the apparatus. The conduction mechanism considering defect structure was discussed by the combination of the obtained oxygen nonstoichiometry and the electrical conductivity. 3. Results and discussion 3.1. Oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ
2.2. Equilibrium thermogravimetry of LaNi0.6Fe0.4O3 − δ Temperature and P(O2) dependence of oxygen nonstoichiometry was measured at equilibrium state with high-resolution electric micro-balance (D-200, Cahn). Approximately 1 g of the single phase LaNi0.6Fe0.4O3 − δ powder was hand-pressed using carbide die with 10 mm diameter and sintered at 800 °C for 5 h resulting in a porous pellet employed as measurement specimen. The measurement was carried out for temperature range between 300 °C and 700 °C under −4.0 ≤ log(P(O2)/bar) ≤ 0.0 to avoid decomposition of the specimen. After more 12 h passed since the specimen was set at measurement temperature and P(O2), equilibrium state was realized and weight relaxation was observed. The change of oxygen content, Δδ, can be estimated from the weight change (Δws) of the sample with following equation.
Δδ ¼
MS ΔwS M O wS
Fig. 1 shows P(O2) dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ measured with the equilibrium thermogravimetry at various temperatures. The absolute value of oxygen content of LaNi0.6Fe0.4O3 − δ, which was determined from weight loss by reduction under pure H2 atmosphere, is around 2.90, showing correspondence with the results of iodometric titration in our preceding reports [10, 11]. LaNi0.6Fe0.4O3 − δ shows rather high δ, which is more than 3 mol%, even in oxidative atmosphere. Due to the high δ, large oxide ion diffusion can be expected, which was confirmed by Nishi and coworkers [12]. Assuming that all the Ni ion and Fe ion are divalent, NiNi′, and tetravalent, Fe•Fe, respectively, oxygen content, 3 − δ, in LaNi0.6Fe0.4O3 − δ is equal to 2.9, which shows correspondence with the observed oxygen content in LaNi0.6Fe0.4O3 − δ. The valence of Fe ion approaching + 4 with increasing Ni content has also been proposed by Mössbauer spectroscopy [13]. Clear plateau is not observed around 2.90 in Fig. 1, suggesting that the hole on Fe 3d orbital, represented as Fe·Fe, is easily
ð1Þ
where Ms, Mo and ws are the molar weight of the sample, that of oxygen gas and the sample weight set in micro-balance apparatus, respectively. In order to evaluate the reversibility of Δδ, the weight variation with heating and cooling procedure under constant P(O2) was compared and confirmed to be identical. Also, identical weight variation was observed between increasing and decreasing P(O2) at constant temperature. The absolute value of oxygen content in LaNi0.6Fe0.4O3 − δ was estimated from the weight decrease by reductive decomposition of the specimen at 800 °C under pure H2 atmosphere expressed as following formula (2). 1 3−2δ O2 LaNi0:6 Fe0:4 O3−δ → La2 O3 þ 0:6Ni þ 0:4 Fe þ 2 4
ð2Þ
2.3. Electrical conduction measurement Electrical conductivity of LaNi0.6Fe0.4O3 − δ was measured by DC four probe method from 300 °C to 800 °C under −4 ≤ log(P(O2)/bar) ≤ 0. The sintering specimen was prepared by pressing the pellet and sintered at 1000 °C for 10 h in air. Platinum was used as material for electrode wire. Shape of the samples employed for the measurements was the rectangular solid with 2 mm × 2 mm × 14 mm. The length between voltage terminals was approximately 12.5 mm. The holding time at each temperature was more than 12 h, which was enough to realize equilibrium state. After confirmation of linear relationship between voltage and current between − 100 mA and + 100 mA, electrical
Fig. 1. Dependence of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ on P(O2) measured by equilibrium thermogravimetry.
E. Niwa et al. / Solid State Ionics 274 (2015) 119–122
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delocalized according to the following equation with small energy variation. 0
•
NiNi þ Fe Fe ↔NiNi þ Fe Fe
ð4Þ
3+ × and Fe3 + in LaNi0.6Fe0.4O3 − δ, Here, Ni× Ni and FeFe represent Ni respectively. The small energy variation of reaction (4) shows agreement with small activation energy for hopping conduction of LaNi0.6Fe0.4O3 − δ reported in our reports [4,11]. Compared with other perovskite-type oxide with transition metal on B-site cation such as La1 − xSrxCo1 − yFeyO3 − δ [14,15], the variation of oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ by temperature and P(O2) is smaller. The smaller variation of oxygen nonstoichiometry can be attributed to small variation of the average valence of B-site cation. Due to small energy of reaction (4) to left side, valence change of Ni ion from 3 + to 2 + by reduction can be easily compensated by the valence variation of Fe ion from 3 + to 4+ with little variation of oxygen content.
3.2. P(O2) dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ Fig. 2(a) shows P(O2) dependence of the electrical conductivity of LaNi0.6Fe0.4O3 − δ at various temperatures. It has been revealed that the variation of log σ on temperature and P(O2) is rather small. The electrical conductivity of LaNi0.6Fe0.4O3 − δ increases with increasing P(O2), suggesting that the dominate carrier of LaNi0.6Fe0.4O3 − δ under the measurement P(O2) is hole. Fig. 2(b) shows Arrhenius plot of hopping conduction of LaNi0.6Fe0.4O3 − δ under various P(O2). Linear relationship between 1/T and log(σT) is observed regardless of P(O2), indicating that the small polaron hopping model can be applicable. At 800 °C under logP(O2) below − 4, observed is deviation from linear relationship, which can be attributed to decomposition to La4(Ni,Fe)3O10 phase [6,7] and thus omitted from further discussion. Activation energy for hopping conduction, Ea, of LaNi0.6Fe0.4O3 − δ was estimated from Eq. (3) and Fig. 3(b). Table 1 lists Ea of LaNi0.6Fe0.4O3 − δ under various P(O2). Ea is independent on P(O2) and approximately 0.049 eV. The obtained Ea shows agreement with the result of the temperature dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ in air [4,11]. In the preceding paper, it was reported that Ea of LaNi0.6Fe0.4O3 − δ had linear relationship with hole mobility, μh, in intraparticle. Therefore, it can be concluded that hole mobility, μh, is independent on P(O2) and oxygen content. Hole hopping of LaNi1 − xFexO3 − δ from Ni ion to Fe ion can be represented as following equations.
0
•
ð5Þ
•
•
ð6Þ
NiNi ↔NiNi þ h
Fe Fe þ h ↔Fe Fe
Combining Eqs. (5) and (6), Eq. (4) can be obtained. Therefore, Ea should correspond to activation energy of chemical reaction represented as Eq. (4). For LaNi0.6Fe0.4O3 − δ, Ea is as small as 0.049 eV, indicating that kinetics of chemical reaction represented as Eq. (4) is so high that charge compensation by valence variation immediately occurs. This prompt charge compensation corresponds to the small variation of oxygen nonstoichiometry by temperature and P(O2). 3.3. Electrical conduction mechanism considering defect structure of LaNi0.6Fe0.4O3 − δ Fig. 3 shows the relationship between the oxygen content and electrical conductivity of LaNi0.6Fe0.4O3 − δ prepared from the results of Figs. 1 and 2(a). Linear relationship was obtained regardless of temperature. This indicates that the variation of mobility, which should be dependent on temperature, is negligibly low compared to that of carrier density, which should depend on oxygen content. Thus revealed
Fig. 2. (a) P(O2) dependence of electrical conductivity of LaNi0.6Fe0.4O3 − δ at various temperatures. Right axis represents hole carrier density evaluated in this study. (b) Arrhenius plot of hopping conduction of LaNi0.6Fe0.4O3 − δ under various P(O2).
constant mobility showed correspondence with P(O2) independent E a as listed in Table 1. Fig. 3 also indicates that the carrier of LaNi 0.6 Fe0.4O 3 − δ is hole under examined conditions because the electrical conductivity increases with increasing oxygen content. Here, hole carrier density, p, can be expressed as following from Eqs. (5) and (6), • NA • N A N p¼ h ¼ NiNi þ Fe Fe ¼ 2ð0:3−δÞ A Vm Vm Vm
ð7Þ
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E. Niwa et al. / Solid State Ionics 274 (2015) 119–122
temperature and P(O2) can be estimated from electrical conductivity, σ. The hole concentration thus estimated are shown in the right vertical axis in Fig. 2(a). It has been clarified that the hole concentration of LaNi0.6Fe0.4O3 − δ is 3–4 × 1021 cm−3, which is relatively high among electrical conducting oxides. 4. Conclusions
Fig. 3. Relationship between oxygen content, 3 − δ and electrical conductivity, σ of LaNi0.6Fe0.4O3 − δ.
Oxygen nonstoichiometry of LaNi0.6Fe0.4O3 − δ has been evaluated by equilibrium thermogravimetry. It has been clarified that oxygen content is about 2.90 at temperatures between 300 °C and 700 °C under − 4 ≤ log{P(O2)/bar} ≤ 0, indicating that oxide ion vacancy is stable under this conditions. Temperature and P(O2) dependence of oxygen nonstoichiometry and electrical conductivity is small for LaNi0.6Fe0.4O3 − δ, probably because the variation of average valence of B-site ion by temperature and P(O2) is small since charge compensation between Ni and Fe is easy to occur. The easy occurrence of the charge compensation corresponds to hopping conduction with small, temperature and P(O2) independent activation energy. By the combination of oxygen nonstoichiometry and electrical conductivity, it has been revealed that LaNi0.6Fe0.4O3 − δ is p-type conductor with mobility of 0.257 cm2V−1 s−1, which is independent on temperature and P(O2). Hole concentration at temperatures between 300 °C and 800 °C under −4 ≤ log{P(O2)/bar} ≤ 0 is calculated to be 3–4 × 1021 cm−3. Acknowledgment This study was financially supported by grants from the “Strategic Research Base Development” Program for Private Universities (S0901022) subsidized by MEXT (2009) and from Nihon University Strategic Projects for Academic Research “Nanotechnology ExcellenceNanomaterial based Photonic and Quantum Technologies”. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.
Table 1 The activation energy for hopping conduction, Ea of LaNi0.6Fe0.4O3 − δ estimated from Fig. 2(b). log(P(O2)/bar)
Activation energy, Ea/eV
−0.03 −1.09 −1.96 −2.95 −4.22
0.050 0.049 0.049 0.049 0.048
References
where NA and Vm , are Avogadro's constant and molar volume of LaNi0.6Fe0.4O3 − δ, respectively. The total electrical conductivity, σ, can be expressed as σ ≅σ h ¼ peμ h ¼ 2ð0:3−δÞ
NA eμ : Vm h
ð8Þ
The slope of relationship between oxygen content and electrical conductivity can be written as ∂σ N ¼ −2 A eμ h : Vm ∂δ
ð9Þ
Therefore, linear relationship between σ and δ should be observed if temperature and P(O2) dependence of μh is low. For LaNi0.6Fe0.4O3 − δ, linear relationship is observed as shown in Fig. 3, indicating that μh is independent on temperature and P(O2). The μh can be calculated from the slope of Fig. 3 using Eq. (9) to be 0.257 cm2V− 1 s−1. By using Eq. (8) and calculated μh, the dependence of hole concentration on
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