Structural and Electrical Properties of the Perovskite Oxide

octahedra render the real structure of these perovskite oxides very flexible. .... I4/m is the correct structure for some similar perovskites.25 T...
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Chem. Mater. 2004, 16, 2309-2316

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Structural and Electrical Properties of the Perovskite Oxide Sr2FeNbO6 Shanwen Tao, Jesus Canales-Va´zquez, and John T. S. Irvine* School of Chemistry, University of St. Andrews, Fife KY16 9ST, Scotland, U.K. Received January 13, 2004. Revised Manuscript Received March 22, 2004

The mixed perovskite Sr2FeNbO6 has been shown to have a tetragonal structure with space group I4/m (87), a ) 5.6078(1) Å, c ) 7.9658(1) Å, V ) 250.51(1) Å3 according to electron and X-ray diffraction. The material is redox stable and maintains its structure in a reducing atmosphere. After reducing in 5% H2 at 900 °C for 6 h, Sr2FeNbO6 still exhibits a tetragonal structure with space group I4/m (87), a ) 5.6051(1) Å, c ) 7.9644(1) Å, V ) 250.22(1) Å3, i.e., with a slightly higher degree of tetragonality. A lattice volume contraction of 0.12% was observed during the reduction, which may be attributed to the loss of lattice oxygen. TGA analysis indicates that Sr2FeNbO6 starts to lose oxygen at 300 °C and the total weight loss is about 0.2 wt % from room temperature to 950 °C in 5% H2. The morphology of this material does not significantly change on reduction according to SEM observation. The conductivities of this material in air and 5% H2 were 3.13 × 10-2 and 2.39 S/cm, respectively at 900 °C. The apparent conduction activation energy of Sr2FeNbO6 in air is 0.74 ( 0.02 eV between 400 and 900 °C and in 5% H2 is 0.28 ( 0.02 eV between 140 and 560 °C and 0.58 ( 0.02 eV between 560 and 900 °C, indicating it is a semiconductor in both atmospheres. The increase of dc conductivity of Sr2FeNbO6 at low p(O2) indicates n-type electronic conduction. The dc conductivity of Sr2FeNbO6 at low p(O2) exhibits a p(O2)-1/6 dependence that is interpreted by a simple defect chemistry model.

1. Introduction Mixed ionic-electronic conductors (MIECs) that exhibit both electronic and ionic conductivity have numerous technological applications, such as electrodes for solid oxide fuel cells (SOFCs), gas semipermeation membranes, water electrolysis, and electrocatalytic reactions.1-5 It has been reported that some nonstoichiometric mixed perovskites with formula A3B′1+xB′′2-xO9-δ, where A is an alkali earth element Ca, Sr, Ba, etc., B′ is an element with valence +2 or +3, and B′′ is an element with valence +5, such as Nb and Ta, exhibit quite high proton and oxygen ion conductivity.6-11 It is expected that mixed ionic and electronic conductors may be found in these compounds if the B-sites are partially substituted by the appropriate first row transition elements. Such mixed conductors may provide potential anode materials for fuel cell applications or oxygen separation. It has been reported that Fe-containing * Corresponding author. Tel.: +44 1334 463817. Fax: +44 1334 463808. E-mail: [email protected]. (1) Tuller, H. L. Solid State Ionics 1992, 52, 135. (2) Irvine, J. T. S.; Fagg, D. P.; Labrincha, J.; Marques, F. M. B. Catal. Today 1997, 38, 467. (3) Ma, B.; Balachandran, U.; Park, J.-H. J. Electrochem. Soc. 1996, 143, 1736. (4) Stoukides, M. Catal. Rev. 2000, 42, 1. (5) Tao, S. W.; Irvine, J. T. S. J. Solid State Chem. 2002, 165, 12. (6) Liang, K. C.; Du, Y.; Nowick, A. S. Solid State Ionics 1994, 69, 117. (7) Nowick, A. S.; Du, Y. Solid State Ionics 1995, 77, 137. (8) Bohn, H. G.; Schober, T.; Mono, T.; Schilling, W. Solid State Ionics 1999, 117, 219. (9) Nowick, A. S.; Du, Y.; Liang, K. C. Solid State Ionics 1999, 125, 303. (10) Glockner, R.; Neiman, A.; Larring, Y.; Norby, T. Solid State Ionics 1999, 125, 369. (11) Nowick, A. S.; Liang, K. C. Solid State Ionics 2000, 129, 201.

perovskite exhibits quite high O2- ion and electronic conductivity.12,13 To the best of our knowledge, reports about the conductivity of Fe-doped niobates are scarce. Therefore, the conductivity of Sr2FeNbO6 in air and 5% H2 was investigated. On the other hand, the structure of Sr2FeNbO6 has been reported as cubic and orthorhombic according to different reports.14-16 As listed in Table 1, Sr2FeNbO6 was first prepared by a solid-state reaction and quenching method. The structure of Sr2FeNbO6 was first reported as a primitive cubic perovskite with a ) 3.987(1) Å.14 Akhtar et al.15 reported that a simple cubic phase may be obtained by heating the mixture of SrCO3, Fe2O3, and Nb2O5 at 1350 °C with a slightly smaller lattice parameter. This is not strange, because it is believed that the quenched sample exhibits the property of high-temperature phase. However, it was reported that Sr2FeNbO6 is not a primitive but a double perovskite with orthorhombic GdFeO3 structure16 prepared by solid-state reaction with the same starting materials. Therefore, in this paper, the crystal structure of Sr2FeNbO6 is reevaluated by Rietveld refinement using XRD, and its electrical properties are examined as a potential fuel cell electrode material. (12) Iwahara, H.; Esaka, T.; Mangahara, T. J. Appl. Electrochem. 1988, 18, 173. (13) Jurado, J. R.; Figueiredo, F. M.; Gharbage, B.; Frade, J. R. Solid State Ionics 1999, 118, 89. (14) Rodriguez, R.; Fernandez, A.; Isalgue, A.; Rodriguez, J.; Labarta, A.; Tejada, J.; Obradors, X. J. Phys. C: Solid State Phys. 1985, 18, L401. (15) Akhtar, M. J.; Akhtar, Z. N.; Dragun, J. P.; Catlow, C. R. A. Solid State Ionics 1997, 104, 147. (16) Tezuka, K.; Henmi, K.; Hinatsu, Y. J. Solid State Chem. 2000, 154, 591.

10.1021/cm049923+ CCC: $27.50 © 2004 American Chemical Society Published on Web 05/11/2004

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Table 1. Comparison of Synthesis Conditions and Structure of Sr2FeNbO6 by Different Reports

preparation process heated mixture of SrCO3, Fe2O3, and Nb2O5 at 1300 °C and then quenched into liquid nitrogen heated mixture of SrCO3, Fe2O3, and Nb2O5 at 1350 °C heated mixture of SrCO3, Fe2O3, and Nb2O5 at 1200 °C and finally at 1300 °C for 12 h heated mixture of SrCO3, Fe2O3, and Nb2O5 at 1400 °C for 16 h twice

structure

lattice parameter (Å)

ref

simple perovskite cubic

a ) 3.987(1)

14

simple perovskite cubic (?)

a ) 3.951(5)

15

double perovskite a ) 5.6082(9) 16 othorhombic with b ) 7.9642(1) sp gr Pnma (62) double perovskite tetragonal with

c ) 5.6084(9) a ) 5.6078(1) this c ) 7.9658(1) work

sp gr I4/m (87)

2. Experimental Section Sr2FeNbO6 was synthesized by solid-state reaction using SrCO3, Fe2O3, and Nb2O5 as starting materials. The compounds were dried at 500 °C overnight to remove absorbed H2O and CO2. Stoichiometric amounts were mixed and ballmilled in zirconia containers with zirconia balls for two 15min periods. A single phase was obtained by firing the mixture at 1400 °C for 16 h without intermediate grinding, cooling at 5 °C min-1 to 900 °C, and then air-cooling in a platinum crucible. To study the phase stability of the material in reducing atmospheres, the powders obtained at 1400 °C were further heated at 900 °C for 6 h and cooled to room temperature in 5% H2. The pellets for conductivity were prepared by hand grinding the powders formed at 1400 °C, pressing at 3.7 × 105 MPa into 13 mm pellets, and then firing at 1300 °C for 36 h. The relative density of the as-prepared pellets is about 85%. The lower sintering temperature was used as better quality pellets were obtained than at 1400 °C. XRD analysis of powders reacted at different temperatures was carried out on a Stoe Stadi-P diffractometer to determine phase purity and measure crystal parameters. Structure refinement was performed by the Rietveld method using the program General Structure Analysis System (GSAS).17 Thermal analysis of Sr2FeNbO6 was carried out on a Rheometric Scientific TG 1000M Plus and a TA Instruments SDT2960 from room temperature to 950 °C (5 °C/min), holding at 950 °C for 30 min and then cooling to 30 °C (10 °C/min) under flowing 5% H2 in argon at a rate of 35 mL/min. Selected area electron diffraction (SAED) was carried out on a JEOL 2011 HRTEM operating at 200 keV. SEM observations were carried out on a JSM 5600 scanning electron microscopy. For ac impedance measurements in air, a Schlumberger Solartron 1260 frequency response analyzer coupled with a 1287 electrochemical interface controlled by Zplot electrochemical impedance software was used over the frequency range from 1 MHz to 100 mHz. Ac impedance measurements were made in 50 °C steps in air between 150 and 900 °C. The dc conductivity was measured by a conventional four-terminal method using a Keithley 220 programmable current source to control current and a Schlumberger Solartron 7150 digital multimeter to measure the voltage. The Sr2FeNbO6 samples were mounted with four Pt wire electrodes to measure the dc conductivity dependence upon p(O2) in a slowly varying atmosphere, which was monitored by a zirconia oxygen sensor. The conductivity was measured by the four-terminal dc method in air and 5% H2.

3. Results and Discussion As stated above, Sr2FeNbO6 prepared by solid-state reaction using the same starting materials SrCO3, Fe2O3, and Nb2O5 has been reported before with different structures.14-16 The structure of perovskite oxides

may be fairly complicated. A wide range of structures from high cubic symmetry to the lowest triclinic symmetry has been observed. For example, Sr2CrNbO6 is a double perovskite with alternate B-site ordering.18 Ca3CaNb2O9 prepared under different conditions give 1:1, 1:2, and 1:3 orderings, respectively.19 The ordering of B-site cations and tilting of BO6 octahedra render the real structure of these perovskite oxides very flexible. Splitting of the B-site is commonly observed because two (or more) different elements at B-sites are involved. It is believed that the cation ordering at B-sites is related to the synthesis conditions and elements involved. Large differences in charge and radius of cations may cause long-range ordering;20 however, the radius of the iron ions is related to their charge number and spin state of orbital electrons. Both disordering and ordering may happen under different synthesis conditions and the state of the Fen+ ions. Therefore, the structure of Sr2FeNbO6 is related to the preparation process. This could be the possible reason different structures were observed in this and previous studies. The tilting of BO6 octahedra in perovskite oxides may also change the symmetry and thus the structure as well. As for the structure of Sr2FeNbO6, primitive cubic Sr2FeNbO6 was previously observed in both quenched14 and unquenched15 samples. Although an orthorhombic Sr2FeNbO6 with GdFeO3 structure was reported later by Tezuka,16 the reported short axis a and c values are essentially identical (see Table 1) considering the standard deviation. The model was tried with our XRD data and convergence could not be achieved when refining the lattice parameter, because a and c are so close as to be identical. XRD investigation of Sr2FeNbO6 prepared in our lab indicates that Sr2FeNbO6 exhibits a tetragonal structure. Electron diffraction patterns of Sr2FeNbO6 along [001]c and [112 h ]c direction are shown in Figure 1. The identical distance from [100]/c and [010]/c to the origin indicating that in the new unit cell, a and b are equal or very close to each other. The absence of superlattice spots along the [100]/c and [010]/c does not mean it is a primitive perovskite, because, in some symmetries, the (h00) and (0k0) diffractions are not allowed with h ) 2n + 1 or k ) 2n + 1 due to systematic absences; however, superstructure diffraction along the [111]/c axis was observed (Figure 1b), indicating that Sr2FeNbO6 is not a primitive perovskite, which is consistent with the XRD pattern. The XRD pattern of Sr2FeNbO6 obtained at 1400 °C may be indexed as a tetragonal lattice parameter of x2ap × x2ap × 2ap, where ap ≈ 4 Å, which is the lattice parameter of cubic primitive perovskite (Figure 3). The systemic absence of h + k + l ) 2n + 1 reflections indicates that it is body-centered tetragonal. The appearance of the (311)t peak at 2θ ≈ 37° and the splitting of (200)c peak at 2θ ≈ 45° confirm that it is not a primitive cubic (17) Larson, A. C.; Von Dreele, R. B. GSAS-Generalised Crystal Structure Analysis System, Los Alamos National Laboratory Report No. LA-UR-86-748, 1994. (18) Choy, J. H.; Hong, S. T.; Choi, K. S. J. Chem. Soc., Faraday Trans. 1996, 92, 1051. (19) Levin, I.; Chan, J. Y.; Geyer, R. G.; Maslar, J. E.; Vanderah, T. A. J. Solid State Chem. 2001, 156, 122. (20) Fesenko, E. G.; Filip’ev, V. S.; Kupriyanov, M. F. Izv. Akad. Nauk. SSSR Ser. Fiz. 1964, 28, 669.

Properties of the Perovskite Oxide Sr2FeNbO6

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Figure 1. Electron diffraction pattern of Sr2FeNbO6.

Figure 2. Scheme showing the relationship between some primitive perovskite and tetragonal double perovskite oxides with space groups I4/mmm and I4/m.

ABO3 perovskite. The super-reflection associated with 1/2[111]/c observed by electron diffraction was very weak at 2θ ≈ 19° in the XRD pattern. Electron diffraction cannot judge whether the 1/2[111]/c superlattice diffractions come from the 1:1 ordering of B-site cations or the antiphase tilting of BO6 octahedron, because they may be caused by either or even the combination due to the dynamic double diffractions in TEM. However, the difference of the similar structures may be, to some extent, evaluated by Rietveld refinement. First, the possible space group should be selected. A schematic showing the transformations from primitive cubic perovskite to body-centered tetragonal perovskite is shown in Figure 2. The primitive cubic perovskite exhibits SrTiO3 structure with space group Pm3 h m. One-to-one ordering of the B-site cations requires the unit cell to be doubled along all three crystallographic directions in order to maintain translational symmetry. The lattice also changes to a facecentered cubic double perovskite with space group Fm3 h m. If a slight distortation occurs, for example, the lattice along 〈001〉 directions becomes longer or shorter, and the structure may distort to face-centered tetragonal, space group F4/mmm. This is not a standard setting and the unit cell 2ap × 2ap × 2ap is not the smallest.

Therefore, the unit cell must be redefined in order to describe the structure in terms of the standard group setting, I4/mmm with lattice parameter x2ap × x2ap × 2ap. The tilting of BO6 octahedron may significantly lower the symmetry as well and has been described in detail by Glazer.21 The tilt system a0a0c- allows the neighboring BO6 octahedra to rotate at opposite directions with the same magnitude along the 4-fold axis parallel to the z axes, which results in a double cell with tetragonal structure, space group F4/mmc. Similarly, this is not a standard space group setting. Redefinition of the space group gives reduced unit cell parameters x2ap × x2ap × 2ap and space group I4/mcm. In this arrangement, the B-cations (if different elements are involved) are still disordered and they share the same positions, e.g. the 4c (0,0,0). With further 1:1 ordering of the B-cations, the positions need to be split into two groups (0,0,0) and (0,0,1/2), and the mirror plane perpendicular and the c glide planes parallel to the 4-fold axis are destroyed, lowering the symmetry from I4/mcm to I4/m.22 In brief, the space groups for the known bodycentered tetragonal double perovskite with lattice pa(21) Glazer, A. M. Acta Crystallogr. 1972, B28, 3384. (22) Woodward, P. M. Acta Crystallogr. 1997, B53, 32.

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rameter x2ap × x2ap × 2ap obtained by XRD may be I4/mmm from 1:1 B-cation ordering, I4/mcm from a0a0ctilting, and I4/m from the combination of both. Therefore, these three space groups were chosen as the models for Rietveld refinement. Starting from the lowest symmetry I4/m, the structure may not be refined if the 2a (0,0,0) and 2b (0,0,1/2) positions are occupied by Fe and Nb, respectively, supposing that the B-cations are totally ordered. In fact, the ordering of Fe/Nb ions may not be complete as the case in Sr2CrNbO6 with alternate B-site ordering.18 The structure may be refined when the 2a and 2b B-sites are shared by Fe and Nb and a reasonable χred2 (goodness of fit) value (2.552) is obtained. The partially ordering of the B-site has ruled out the I4/mcm model, which involves only octahedral tilting, although refinement with the model I4/mcm where the Fe and Nb disorder on the 4c (0,0,0) positions may be carried out with a χred2 value 2.537. Better fits and smaller χred2 value (1.823) were obtained when the I4/mmm model was applied. There is no Jahn-Teller effect in the Fe3+ and Nb5+ ions and the oxygen vacancies are unlikely, as Sr2FeNbO6 obtained in air is close to stoichiometric. Therefore, there is no obvious driving force for the distortion of Fe3+ and/or Nb5+ centered octahedral, which implies that the tetragonal distortion may come from the tilting of the octahedra. In this case, the structure of Sr2FeNbO6 would be I4/m rather than I4/ mmm as illustrated in Figure 2. It was reported that the structure of Sr2FeMoO6 at room temperature is I4/ mmm based on X-ray single-crystal analysis;23 however, neutron diffraction indicates that its structure is I4/m.24 It is difficult to experimentally distinguish I4/mmm and I4/m, because they have the same systematic absences. It is proposed by Howard et al. that I4/m is the correct structure for some similar perovskites.25 The real structure for Sr2FeNbO6 could therefore be I4/m, but the model may not give a significantly better fit to the XRD data due to the low sensitivity to the light oxygen atoms. Therefore, we choose I4/m as the structure of Sr2FeNbO6, although a slightly higher goodness-tofit value was observed for the model I4/mmm. Highresolution neutron diffraction is required to definitely distinguish between these two space groups. Refinement of the oxygen site occupancy proved unstable and made no difference to the quality of the fits, illustrating the insensitivity of powder X-ray diffraction to the small variations in oxygen content. The observed, calculated, and difference profiles for the refinement of Sr2FeNbO6 are shown in Figure 3. The final refined structural data are given in Table 2. This phase shows a similar rocksalt type site order to the double perovskite series Sr2FeMoO6.23 and Sr2CrMoO6,18 which have been widely studied for their interesting magnetic properties; however, unlike these related phases, Sr2FeNbO6 shows significant tetragonal distortion above any magnetic transition. For Sr2FeNbO6, at least, the systematic absences observed from XRD and electron diffraction clearly indicate body centered symmetry. Unlike these (23) Tomioka, Y.; Okuda, T.; Okimoto, Y.; Kobayashi, K.-I.; Tokura, Y. Phys. Rev. B 2000, 61, 422. (24) Chmaissem, O.; Kruk, R.; Dabrowski, B.; Brown, D. E.; Xiong, X.; Kolesnik, S.; Jorgensen, J. D. Phys. Rev. B 2000, 62, 14197. (25) Howard, C. J.; Kennedy, B. J.; Woodward, P. M. Acta Crystallogr. 2003, B59, 463.

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Figure 3. X-ray diffraction profiles of Sr2FeNbO6 formed at 1400 °C. Table 2. Structure Parameters of Sr2FeNbO6 Prepared at 1400 °C from X-ray Diffraction Dataa atom site occupancy Sr Fe Nb Fe Nb O(1) O(2)

4d 2a 2a 2b 2b 4e 8h

1 0.68(1) 0.32(1) 0.32(1) 0.68(1) 1 1

x

y

z

Uiso (Å2)

0 0 0 0 0 0 0.222(9)

0.5 0 0 0 0 0 0.277(7)

0.25 0 0 0.5 0.5 0.248(8) 0

0.0209(1) 0.0112(10) 0.0112(10) 0.0220(9) 0.0220(9) 0.0044(13) 0.0044(13)

a Space group I4/m (87); a ) 5.6078(1) Å, c ) 7.9658(1) Å, V ) 250.51(1) Å3, Z ) 2. Rwp ) 6.75%, Rp ) 5.19%, χred2 ) 2.552.

Figure 4. TGA analysis of Sr2FeNbO6 in 5% H2.

M/Mo phases,18,23,24 much more site disorder is present in the M/Nb variant. Figure 4 shows the thermogravimetric programmed reduction of Sr2FeNbO6 performed in 5% H2. The sample weight remained constant below 300 °C. There is a rapid weight loss occurring between 300 and 950 °C in two steps that may be attributed to the loss of lattice oxygen with the reduction of Fe3+ to Fe2+. The total weight loss in the range of 20-950 °C is only 0.2 wt %, indicating that Sr2FeNbO6 is very stable. Assuming the charge of Fe in Sr2FeNbO6 is +3 in the sample prepared in air, then the average charge of Fe in the reduced sample is +2.9. It is assumed that niobium maintains its +5 valence, as NbV is more difficult to reduce than FeIII. On the basis of TGA analysis of other Nb-containing perovskites, no significant mass change due to the reduction of niobium would be expected. The formula of the reduced sample may therefore be written as

Properties of the Perovskite Oxide Sr2FeNbO6

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Figure 5. X-ray diffraction profiles of Sr2FeNbO6 after reduction at 900 °C in 5% H2 for 6 h. Table 3. Structure Parameters of Sr2FeNbO6 after Heating in 5% H2 at 900 °C for 6 h atom site occupancy Sr Fe Nb Fe Nb O(1) O(2)

4d 2a 2a 2b 2b 4e 8h

1 0.67(1) 0.33(1) 0.33(1) 0.67(1) 1 1

x

y

z

Uiso (Å2)

0 0 0 0 0 0 0.221(4)

0.5 0 0 0 0 0 0.284(3)

0.25 0 0 0.5 0.5 0.254(5) 0

0.0091(5) 0.0219(4) 0.0219(4) 0.0120(4) 0.0120(4) 0.0084(14) 0.0084(14)

a Space group I4/m (87); a ) 5.6051(1) Å, c ) 7.9644(1) Å, V ) 250.22(1) Å3, Z ) 2. Rwp ) 4.18%, Rp ) 3.03%, χred2 ) 2.583.

Sr2FeNbO5.95 in 5% H2 at 950 °C; however, the reduced Sr2FeNbO6 sample regained weight on cooling from 950 °C, and this process stopped at about 500 °C. The weight gain for Sr2FeNbO6 during cooling may be attributed to the reoxidization of the sample. In the overall reduction cycle, the overall weight loss is only 0.08 wt %. The loss of lattice oxygen in a reducing atmosphere may provide oxygen vacancies for possible oxygen migration. TGA itself cannot directly determine the phase stability of materials at high temperatures in the case of phase segregation. To further determine the chemical stability of Sr2FeNbO6 in a reducing atmosphere, the as-prepared Sr2FeNbO6 obtained in air was reduced in 5% H2/95% Ar at 900 °C for 6 h and cooled to room temperature under 5% H2. XRD analysis indicates that the reduced Sr2FeNbO6 is a single phase with a tetragonal structure, indicating that Sr2FeNbO6 is stable in 5% H2 at 900 °C. A close profile fit was achieved when the same model I4/m (87) was used for Rietveld refinement (Figure 5). The same strategy as for the unreduced sample was applied during the refinement. Table 3 lists the final refined structure data. It was found that the lattice parameters a and c both decrease slightly and that the degree of tetragonality was slightly increased. The decrease of the lattice parameter c may be due to the loss of apical oxygen (4e site). The introduction of oxygen vacancies on reduction may cause the distortion and tilting of octahedra. Therefore, I4/m is also likely to be the correct structure for reduced Sr2FeNbO6-δ. The cell volume contracts 0.12% during the reduction. Loss of lattice oxygen can cause such a lattice contraction. On the other hand, the reduction of Fe from 3+ to 2+ may cause lattice expansion, assuming that the spin state of the Fe orbital electrons does not change. The ionic sizes for Fe3+ are 0.55 Å (LS, low spin) and 0.645

Figure 6. SEM pictures of Sr2FeNbO6 as obtained at 1400 °C (a) and after heating at 900 °C in 5% H2 for 6 h (b).

Å (HS, high spin) and for Fe2+ are 0.61 Å (LS) and 0.77 Å (HS).26 The size of Fe2+ ions in low-spin configuration is smaller than that of Fe3+ in high-spin configuration (0.645 Å); however, the configuration of a small number of created Fe2+ ions in a lattice such as this is unlikely to be low-spin. Such a contraction in lattice parameters is commonly associated with Ti or Nb d0 states being reduced to introduce some d1 electrons that may introduce a degree of metallic bonding, which means that the contribution of any M-O bond length expansion on reduction is often exceed by opposing contributions from factors such as oxygen loss leading to net contraction. Thus, it may be that some of the electrons introduced by FeIII reduction may reside a little on Nb sites, which is consistent with the mixed B-site occupation. To observe any possible microstructure change during reduction, scanning electron microscopy was applied. The SEM pictures of the powders before and after the reduction are shown in Figure 6. The sample is quite homogeneous with secondary particle size of about 5 µm (Figure 6a). The morphology of the sample did not change significantly after the reduction (Figure 6b). The ac impedance spectra of Sr2FeNbO6 indicate that electronic conduction is dominant, because only one arc was observed, as shown in Figure 7. If oxide ionic

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Figure 9. The dc conductivity of Sr2FeNbO6 in air and 5% H2. Figure 7. Ac impedance plots, -Z′′ versus Z′, obtained at 150 °C (0) and 200 °C (O) for the Sr2FeNbO6 sample.

Figure 8. The ac conductivity of Sr2FeNbO6 in air.

conductivity were dominant, then some indication of diffusion limitation would have been expected in the impedance spectrum. This semicircle is broadened and is associated with a capacitance of around 1 × 10-10 F cm-1. The arc relates to both bulk and grain boundary resistances, with the grain boundary being dominant at temperatures where the arc is observable. Unfortunately, the bulk component only just becomes discernible on reducing temperature to 100 °C, when the total resistance moves out of range of the impedance analyzer. The general trend in relative bulk and grain boundary resistances did indicate that the grain boundary component was associated with higher activation energy than the bulk, however. For most temperatures studied in air and all in hydrogen, there was no possibility to separate bulk and grain boundary components, so only the total bulk and grain boundary value have been considered. The total ac conductivity of Sr2FeNbO6 is shown in Figure 8. The conduction activation energy is 0.75 ( 0.01 and 0.64 ( 0.01 eV, respectively, below and above 400 °C. The change of activation energy (26) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925.

around 400 °C could indicate a possible phase transformation, or most probably, a change from grain boundary to bulk domination of conduction. The total conductivities in air are 3.80 × 10-4 and 2.48 × 10-2 S/cm at 400 and 900 °C, respectively. The conductivity of Sr2FeNbO6 was also measured by the four-terminal dc method. The conductivity of Sr2FeNbO6 is higher in 5% H2 than in air. The total conductivities of Sr2FeNbO6 in air and 5% H2 were measured as 3.13 × 10-2 and 2.39 S/cm, respectively, at 900 °C (Figure 9). At 400 °C, the total conductivity is 1.36 × 10-1 S/cm in 5% H2, which is almost 3 orders of magnitude higher that that in air (2.15 × 10-4 S/cm) at the same temperature. The apparent conduction activation energy of Sr2FeNbO6 in air is 0.74 ( 0.02 eV between 400 and 900 °C and in 5% H2 is 0.28 ( 0.02 eV between 140 and 560 °C and 0.58 ( 0.02 eV between 560 and 900 °C, indicating it is a semiconductor in both atmospheres. The conductivity and activation energy in air measured by ac and dc methods are consistent with each other. The slope change at around 560 °C on the conductivity curve in 5% H2 (Figure 9) can be attributed to a change in the extent of reduction and is wholly consistent with the thermogravimetric measurements in the same atmosphere. The high electronic conductivity of Sr2FeNbO6 in 5% H2 must involve the 3d orbitals. The Jahn-Teller splitting is not considered, because neither Fe3+ nor Fe2+ ions have the d9,d7 (LS) or d4 (HS) configuration. If the iron is low spin, then the energy level configurations for iron ions are as follows: Fe2+, t2g6eg0; Fe3+, t2g5eg0. As illustrated by Tuller,27 in perovskite, the 3eg orbital of B-site cations, e.g. Fen+, may overlap with the nearby 2pσ orbital from the split O2p to form σ-bonds (eg-pσ-eg bond); meanwhile, the 3t2g orbital of iron ions may overlap with the 2pπ of O2- ions to form weaker π-bonds (t2g-pπ-t2g bond). The third possibility of Fe d-orbital interactions may be the direct t2g-t2g overlap as the case in TiO,28 which is unlikely because of the large Fe-Fe distance across the face diagonal. It is supposed that electrons (e′) or electron holes (h•) may hop from one Fe ion through the σ or π bonds to a nearby (27) Tuller, H. L. Solid State Ionics 1997, 94, 63. (28) Duffy, J. A. Bonding, Energy & Bands in Inorganic Solids; Longman Scientific & Technical: Essex, 1990.

Properties of the Perovskite Oxide Sr2FeNbO6

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Fe ion, resulting in electronic conduction. In Sr2FeNbO6, iron and niobium are partially ordered across the B-sites. To form a percolation path, the iron content must be above a minimum value or electrons shared between Nb and Fe. The quite high electronic conductivity of Sr2FeNbO6 in air certainly indicates that the Fe-O-Fe bonds (σ and/or π) do form a percolation path for electrons or electron holes to move through the crystals. The electronic conduction may relate to the eg electron structure if the σ bonds are the bridge. Although the strong antiferromagnetic superexchange coupling for the Fe-O-Fe linkage may discourage electron transport, the Fe-O-Nb bonds (σ and/or π) are unlikely to be the major pathway for electron transport, because it was found that, in the Sr2Fe1+xNb1-xO6-δ series, increased niobium in the materials results in lower conductivity in air. The material is an n-type conductor at low oxygen partial pressure, as its conductivity increases at low p(O2). Therefore, it is likely that iron is mainly in highspin state in Sr2FeNbO6 with the energy level configurations for iron ions being as follows: Fe2+, t2g4eg2; Fe3+, t2g3eg2. Neither the t2g nor the eg orbitals are fully occupied, even when all the irons are Fe3+, i.e., under high oxygen partial pressures. Therefore, electrons may transfer through both the eg-pσ-eg and t2g-pπ-t2g bonds. In a reducing atmosphere, some iron ions were reduced and some electrons were released, which increases the concentration of charge carriers, resulting in higher electronic conductivity. 1 × •• 2Fe× Fe + OO ) 2Fe′Fe + VO + /2O2(g)

(1)

From this point of view, the conductivity of Sr2FeNbO6 in H2 should be higher than that in air, which is consistent with our data. If the electron transfer is only through the σ-bonds, the electron conductivity of Sr2FeNbO6 should not be affected by the change of iron t2g electron structure, because the eg orbital is unchanged. Therefore, the t2g-pπ-t2g bond is an important path for electrons. The Fe-O distance becomes slightly smaller, which makes the hopping of electrons easier, resulting in lower conduction activation energy and higher conductivity in H2 than in air. The tilting of the BO6 octahedra affects the overlap of the Fe-O orbitals and may thus influence the conductivity also. Another consideration that may influence the electronic and ionic conductivities is the concentration of oxygen vacancies. With increasing oxygen deficiency, the 3D hopping path for electrons or holes through Fe-O-Fe close contacts might be affected because some are blocked by oxygen vacancies, leading to lower electron conductivity. With an increase of ionic charge carriers V•• O, the oxygen ion conductivity will increase if the formation of defect clusters and ordering of oxygen vacancies are negligible. As shown in Figure 10, at p(O2) values below 10-12 atm, the conductivity increases with decreasing p(O2), indicating that the material exhibits an n-type conduction. The log σ vs log p(O2) relation of n-type electronic conductivity at low p(O2) gives a slope of -1/6. The mass action equation for the release of lattice oxygen at low p(O2) with reduction of iron according to eq 1 is

Figure 10. Isothermal conductivity vs p(O2) diagram for Sr2FeNbO6 at 900 °C.

KR(T) )

[Fe′Fe]2[V••O]p(O2)1/2 2 × [Fe× Fe] [OO]

(2)

Because, at lowp(O2), only limited Fe3+ is reduced to Fe2+, i.e., [Fe× Fe] . [Fe′Fe], and the concentration of oxygen and vacancies is much smaller than that of the lattice oxygen, eq 2 can be rearranged to give eq 3, explaining the observed conductivity dependence upon p(O2), if mobility is invariant (eq 4).

ne = [Fe′Fe] ) (12KR(T))1/3p(O2)-1/6

(3)

σe ) qeµe‚(12KR(T))1/3p(O2)-1/6

(4)

where qe is the elementary charge of electron carriers and µe is mobility of electrons. It should be noted that only the reduction of iron ions is considered in this model, but a small residence of electrons on niobium ions could have a very large effect on the conductivity as the case for SrTiO3-δ, where even the δ cannot be measured. Although low-temperature impedance data indicate that electronic conduction is dominant, possibly even via the grain boundary element, oxide ion conductivity may become more important at high temperatures. The lack of dependence of conductivity upon p(O2) under oxidizing conditions would be consistent with ionic conduction; however, it is very likely that an electronic mechanism is responsible. 4. Conclusions The mixed perovskite Sr2FeNbO6 was synthesized by a solid-state reaction. It exhibits a tetragonal structure with space group I4/m (87), a ) 5.6078(1) Å, c ) 7.9658(1) Å, V ) 250.51(1) Å3 according to electron and X-ray diffraction. This material is stable in 5% H2 until at least 900 °C. In a reducing atmosphere, it maintains the tetragonal structure. The reduced sample has a tetragonal structure with space group I4/m (87), a ) 5.6051(1) Å, c ) 7.9644(1) Å, V ) 250.22(1) Å3. A lattice contraction of 0.12% was observed during the reduction. TGA experiments indicate that Sr2FeNbO6 begins to lose oxygen at about 300 °C, and the total weight loss is about 0.2 wt % from room temperature to 950 °C in 5% H2. The morphology of this material does not

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Chem. Mater., Vol. 16, No. 11, 2004

significantly change on reduction according to SEM observation. The conductivities of this material in air and 5% H2 were 3.13 × 10-2 and 2.39 S/cm, respectively, at 900 °C. The decrease of dc conductivity of Sr2FeNbO6 at p(O2) below 10-12 atm indicates n-type electronic conduction. The electrons may hop through the Fe-O eg-pσ-eg σ bonds and/or t2g-pπ-t2g π bonds. The lower apparent conduction activation energy and higher conductivity in H2 than in air may be due to the reduction of some of the iron ions, which release electrons and increases the charge carrier concentration, thus increasing the conductivity. The effect of oxygen vacancies on electron conductivity is small as long as the Fe-O-Fe links are sufficient for an uninterrupted 3D hopping

Tao et al.

path. Sr2FeNbO6 might be a candidate for oxygen permeation if the oxygen ion conductivity is reasonably high, which needs further investigation. The material might be a candidate for SOFC anode application if a thin film anode could be applied, because the conductivity is reasonable at low p(O2) and the chemical stability is acceptable. The dc conductivity of Sr2FeNbO6 at low p(O2) exhibits a p(O2)-1/6 dependence that is interpreted by a simple defect chemistry model. Acknowledgment. We thank EPSRC and NEDO for funding. CM049923+