2Fe-Oxide in Oxidative and - American

Article pubs.acs.org/cm

Electronic Origin of Conductivity Changes and Isothermal Expansion of Ta- and Ti-Substituted La1/2Sr1/2Fe-Oxide in Oxidative and Reducing Atmosphere Artur Braun,*,† Selma Erat,†,‡,§,∥ Defne Bayraktar,† Ashley Harvey,‡ and Thomas Graule†,⊥ †

Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Testing & Research, CH−8600 Dübendorf, Switzerland ‡ Department for Nonmetallic Inorganic Materials, ETH Zürich, Swiss Federal Institute of Technology, CH-8037 Zürich, Switzerland § Advanced Technologies Research & Application Center, Mersin University, TR-33343 Yenisehir, Mersin, Turkey ∥ Faculty of Engineering, Electrical-Electronics Department, Toros University, TR-33140 Yenisehir, Mersin, Turkey ⊥ Technische Universität Bergakademie Freiberg, D-09596 Freiberg, Germany ABSTRACT: Iron perovskites of the ABO 3 type with stoichiometry La1/2Sr1/2Fe(1−x)MexO3−δ have been substituted with 10% Ta and 20% Ti %, and heat treated in air or in argon. 10% substitution of iron by tantalum in the hole-doped La1/2Sr1/2FeO3 decreases the electric conductivity by around 3 orders of magnitude. The conductivity decrease is well correlated with a decrease of the isothermal lattice expansion, a decrease of the formal and actual Fe4+/Fe3+ ratio, and with a shift of the spectral weight within the O(2p)-Fe(3d) mixed states from levels with eg symmetry near the Fermi energy to levels with t2g symmetry. Systematic and corresponding differences in the Fe (2p) and O (1s) X-ray absorption spectra reveal that the preparation in an oxygen-rich atmosphere promotes the formation of states with increased covalence in the initial state and states which contain O (2p) character. The electron hole doping peak in the oxygen pre-edge X-ray near edge absorption fine structure (NEXAFS) spectra is virtually absent when the samples were heated in argon. The corresponding Fe core level spectra show no significant differences other than in the branching ratio of L2 and L3 peaks, suggesting that the (3d) states are localized, and that hole states are responsible for electric transport changes in the material. KEYWORDS: perovskites, SOFC, oxygen separation membrane, valence band, NEXAFS, conductivity, chemical expansion, thermal expansion, hole doping

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INTRODUCTION Management of defects in ceramic materials for energy applications has been a long-standing problem in materials technology. Defects may cause structural disintegration and mechanical failure, but may also be necessary as charge carriers in ceramics with electric functions, particularly at high temperatures. Volume changes in perovskites can be brought about not only by thermal, but also by chemical expansion.1 The latter process typically depends on the oxygen content and goes along with reduction of the B-site cation; both processes are not necessarily coupled with each other.2 Says Adler: “Chemical expansion is a new and exciting physical property of ceramics that has not been thoroughly studied or exploited”.2 The ionic conductivity, which correlates positively with the thermal expansion,3 may then conflict with the stability, or reversible volume changes may incur cracks and loss of conductivity.4 Tailoring of materials is particularly critical, then, for ceramic fuel cells, sensors, and gas membranes. One strategy to limit thermal expansion of ABO3-type perovskites is the substitution of the B-site transition metal with a more stable one. In the aforementioned applications, oxygen activity may change rapidly across time scales or length scales.5 This influences © 2012 American Chemical Society

the oxygen vacancy formation at high temperatures and thus the conductivity. We present here a B-site doping study of Asite substituted LaFeO3, a perovskite with ABO3 structure, where the La3+ is to 50% substituted with Sr2+, and the Fe is substituted with 20% Ti or 10% Ta. La1/2Sr1/2FeO3 is the A-site substituted material with the highest electric conductivity; Ta and Ti are substitutional cations with a higher valence (Ti4+, Ta5+) than Fe, which would force the Fe ions to remain Fe3+, and have been shown to cause the least thermal expansion upon change of the oxygen concentration.6,7 We elucidate the interrelation of the electronic structure of the materials with respect to the substitution and the oxygen concentration under which they were treated. LaFeO3 is an antiferromagnetic charge transfer insulator with 2 eV charge gap energy. Heterovalent substitution of La3+ in the insulating LaFeO3 with Sr2+ (LaSrFe-oxide; LSF) creates electron hole states (hole doping) with substantial O (2p) character near the Fermi level.8 The electrical conductivity of La(1−x)SrxFeO3 Received: February 7, 2012 Revised: March 29, 2012 Published: March 29, 2012 1529

dx.doi.org/10.1021/cm300423m | Chem. Mater. 2012, 24, 1529−1535

Chemistry of Materials

Article

increases with increasing substitution parameter x, with SrFeO3 being an antiferromagnetic metallic conductor with temperature-dependent conductivity.9 In addition to the hole doping effect, the increasing ionic radius of the A-site ion from La to Sr decreases the typical rhombohedral lattice distortion, which in turn increases the hybridization of the O(2p)-Fe(3d) states, and thus the one-electron bandwidth ω,9 and which is a function of the superexchange angle θ, formed by the Fe−O− Fe configuration: ω ∼ cos2 θ.10 Because La and Sr are believed to retain their formal valence of La3+ and Sr2+, the charge balance is primarily maintained by adjustment of the oxidation state of the Fe with the formal electron hole concentration τ = [Fe4+]/[Fe3+], and secondarily by an onset of oxygen deficiency δ. The formal valence of Fe in LaFeO3 and SrFeO3 is Fe3+ and Fe4+, respectively. The actual Fe4+ concentration in SrFeO3−δ is only about 40%11,12 The Fe4+ ions represent the electron holes and control the conductivity, whereas the Fe3+ ions control the magnetic properties via their spin structure.13 The electronic ground state of the hole-doped LSF system has been discussed previously.14 The band gap in LSF decreases with increasing Sr substitution. Chu et al.15 found for La0.6Sr0.4FeO3 (LSF60) that the Fe−O−Fe superexchange angle θ is larger for reduced samples, and smaller for oxidized samples, in line with the perception that the bandwidth ω is proportional to cos2 θ, and that samples prepared in excess air (oxidized samples) have a higher conductivity than reduced samples. LSF with x = 0.5 (La1/2Sr1/2FeO3, LSF50) has the maximum oxygen ion, electron, and hole conductivity.16 LSF50 has a rhombohedral structure with lattice parameter a = 3.889 Å, α = 90.26°, and a Néel temperature of TN = 230 K.11,17 The stoichiometry of LSF50 imposes a formal valence of 3.5 for iron, with Fe3+ and Fe4+ present in equal amounts in the unit cell. The actual Fe4+ content in LSF60 and in LSF50 were reported to be only around 40%.11,13 LSF, like many perovskites, can accommodate a high concentration of disordered oxygen vacancies in the crystal lattice.7 The oxygen partial pressure during high temperature synthesis of LSF is a very critical parameter. Particularly for LSF with high Sr content, a sufficiently high oxygen partial pressure is mandatory to achieve stoichiometric LSF compounds.17 While hole doping by A-site substitution is a well understood process, substitution on the B-site is less so. Different from the hole doping is homovalent substitution of the B-site cation, where empty states near the Fermi level may be redistributed because of the mismatch of the substituting cations.8 The formation of oxygen vacancies at high temperatures goes along with dimensional changes in the unit cell. This effect can propagate across all length scales of a material and may, in the extreme case, cause total structural disintegration. To put it simply, “the stability of perovskites is inversely proportional to their non-stoichiometry”.18 We present here a study of a total of six samples, that is, LSF50 and iron substituted derivatives with a heat treatment either in air or in argon. The samples treated at 300 K in air have an overall higher conductivity than those treated in argon. Heterovalent substitution of the iron on the B-site with Ta or Ti as well as annealing the samples in argon causes a reduction of holes and hence a decreasing conductivity. It was found that B-site substitution with Ta and Ti decreases the isothermal expansion upon changing the gas from air to argon and vice versa, and expands the linear range for isothermal expansion toward higher temperatures.6 On the other hand, substitution with Ta and Ti decreases the electronic conductivity

considerably. The interplay of gas partial pressures constituting oxidizing and reducing conditions with substituted perovskites is particularly important for the functionality and operation of high temperature electrochemical devices such as solid oxide fuel cells and electrolyzers, gas sensors and oxygen separation membranes. We have therefore applied macroscopic analytical methods to determine the thermal and chemical expansion, and the electric conductivity, and X-ray spectroscopy to analyze the electronic structure and its relation with the transport properties.

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EXPERIMENTAL SECTION

Synthesis. Samples, phase pure within the detection limit of X-ray diffractograms (XRD), were prepared using the conventional solid state route, by mixing La(OH)3, SrCO3 and Fe2O3, Ta2O5, and TiO2, in the stoichiometric amounts, followed by ball-milling in isopropanol and subsequent calcinations at 1523 K in air for 4 h. The XRD matched the structure with R-3C symmetry (JCPDS 01-082-1962 reference pattern with a = 5.511 Å, c = 13.4158 Å). More details on the synthesis of these particular samples are available in ref 19. Structural data on LSF in general are available in ref . Because of the larger cation size for Ti4+ and Ta5+ compared to Fe4+, the unit cell of LSF expands upon substitution with Ti and Ta, as evidenced by XRD.19 The mass loss during and after the reaction, as obtained by thermogravimetry, was used to estimate the oxygen deficiency δ. Electrical Characterization. The 4-point direct current (DC) conductivity of the samples was obtained from bars of prismatic shape, 3 mm × 4 mm × 25 mm that had been sintered for 4 h at 1703 K. The bars were coated on both ends with Pt-paste (CL11-5100, W. C. Heraeus GmbH & Co. KG, Germany) and contacted by attaching Ptwires around the bars, approximately 15 mm distant from each other. Conductivity data were recorded between ambient temperature and 1173 K with the sample being in a muffle furnace supplied with air or argon. Conductivity data were acquired with a meter (Resistomat Model 2318, Burster Präzisionsmesstechnik GmbH & Co KG, Germany). X-ray spectroscopy. X-ray near edge absorption fine structure (NEXAFS) spectra were recorded at the soft X-ray beamline 1.1 at the Daresbury Laboratory Synchrotron Radiation Source, United Kingdom. The samples were obtained from sintered pellets that were ground to fine powder and dispersed over sticky carbon tape prior to transfer into the vacuum recipient (p ∼5 × 10−10 Torr). Spectra were recorded with the total electron yield at an energy resolution of ΔE/E ∼ 1/5000, typically around 0.5 eV. The carbon tape has a characteristic oxygen peak at 532 eV, which was used for the energy calibration,21 but removed from the spectra after deconvolution into Voigt functions and arc tangent functions. The Fermi edge was determined by the leading low energy edge of the well conducting LSF50 sample.21 We briefly review how the formal valence of Fe and τ = Fe4+/Fe3+, the formal hole ratio, are obtained from the stoichiometry as a function of the substitution parameter x. The calculation is based on the assumption that La, Sr, and Ta and Ti are redox-inactive in comparison to Fe, and that δ is either equal to 0, or the actual value as determined gravimetrically. The formal valence for the B-site pair Fe1−xTax is 3.5 in La1/2Sr1/2Fe1−xTaxO3−δ xOx Ta + (1 − x)Ox Fe = 3.5,

and

Ox Ta = + 5

For the valence of iron, OxFe, and the relative Fe4+ concentration c, we find Ox Fe =

3.5 − 5x = 4c + 3(1 − c) = c + 3 1−x

The formal Fe4+/Fe3+ ratio is then τ = c/(1 − c). On the basis of this calculation, x is to be constrained as follows: 0 ≤ x ≤ 0.25. 1530



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Table 1. Crystallographic Structure Parameters Measured at Ambient Temperature of Samples Heated in Air and in Argon at 1123 Ka heated in air

a

heated in argon

a (Å)

c (Å)

V (Å3) normed to cubic

a′ (Å) c′ (Å)

α′ (deg)

a (Å)

V (Å3)

LSF50

5.51061

13.41577

58.8017

90.23

3.9073

59.6527

LSFTi20

5.53027

13.45271

59.3850

90.26

3.9114

59.8407

LSFTa10

5.54756

13.49234

59.9333

3.89659 3.87279 3.91049 3.88346 3.92272 3.89490

90.27

3.9237

60.4070

sample

Reproduced from ref 17.

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RESULTS AND DISCUSSION We briefly recall for the reader the crystallographic structure data of the present samples,6,7 as listed in Table 1. The samples heated in air have rhombohedral symmetry, whereas those heated in argon have cubic symmetry. To compare lattice parameters and unit cell volumes of both data sets, the rhombohedral structure parameters have been renormalized to simple cubic symmetry. La1/2Sr1/2FeO3 (LSF50) heated in air has the overall smallest lattice volume of 58.8017 Å3, followed by La1/2Sr1/2Fe0.8Ti0.2O3 (LSFTi20) with 59.3850 Å3 and by La1/2Sr1/2Fe0.8Ta0.1O3 (LSFTa10) with 59.9333 Å3. When the samples were heated in argon, the unit cell volumes are overall larger than those prepared in air, that is, LSF50 with 59.6527 Å3, LSFTi20 with 59.8407 Å3, and LSFTa10 with 60.4070 Å3, the largest unit cell volume of all samples. This trend is comparable with that of La1−xSrxMnO3.22 The thermal expansion Δl/l0 has been determined as a function of temperature and also isothermally at 1173 K for changing the ambient gas concentration from air to argon.6 Figure 1 shows the isothermal expansion Δl/l0 from air to

Figure 2. Electric conductivity of the air-heated and argon-heated samples in Arrhenius law representation from ambient temperature to ∼1053 K. The conductivity of samples heated in argon is plotted with red open circles. Solid black lines denote Arrhenius least-squares fits. Criterion for the Arrhenius plot was whether a straight line could be fitted starting from 300 K.

remain strikingly parallel for LSFTa10. The conductivity of LSFTa10 heated in air follows an Arrhenius law from 300 to 588 K (300 to 526 K for argon), as indicated by the straight line. For LSFTi20 heated in air, an Arrhenius behavior is observed from 300 to 400 K only. For LSF50 heated in air, Arrhenius behavior is found from 300 to 357 K; when heated in argon, LSF50 shows Arrhenius behavior from 300 to 392 K. The electric conductivity σ of the samples measured at ambient temperature and ambient oxygen partial pressure is summarized in Table 2. Comparison with Table 1 shows that the conductivity decreases when the lattice volume increases, at least for the samples that were heated in air. Just by substituting the B-site cation from Fe to Ta by 10%, the conductivity decreases by almost 2 orders of magnitude. LSF50 has the highest overall conductivity with around 50 S/cm and activation energy as low as 180 meV. The conductivity gradually decreases in the order LSF50 (50 S/cm; 180 meV) > LSFTi20 (2.28 S/cm; 262 meV) > LSFTa10 (0.82 S/cm; 342 meV). Not only the substitution parameter x, which basically constitutes the relative hole concentration τ, but also the oxygen concentration during sample preparation has an impact on the conductivity. The LSF50 heated in argon has a conductivity of only 0.075 S/cm, 3 orders of magnitude lower than LSF50 heated in air. LSFTa10 appears less sensitive to exchange of the gas, corresponding to its smaller thermal expansion, but still has a conductivity ratio of 0.025, when compared with the well conducting LSF50. The samples heated in argon had an overall lower conductivity, that is, LSF50 (0.075 S/cm) > LSFTa10 (0.03 S/cm). The conductivity for

Figure 1. Relation between isothermal expansion (from air to argon from ref 6 and oxygen deficiency at 1173 K (from ref 5).

argon at 1173 K for LSF50, LSFTi20, and LSFTa10. Δl/l0 scales linearly with the gravimetrically obtained oxygen deficiency δ, revealing that LSFTa10 has the smallest oxygen deficiency among these three samples and also the smallest isothermal expansion. The electric conductivity behavior of the samples heated in air and in argon (for LSFTi20 in air only) is shown in Figure 2 in an Arrhenius law representation for temperatures from ambient to approximately 1053 K. The Arrhenius behavior for the low temperature branches are highlighted by solid lines. Visual inspection shows that the slopes for these branches differ significantly for the air and argon heated LSF50, whereas they 1531



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Table 2. Oxygen Deficiency in Air at 1173 K, Formal Hole Concentration τ = Fe4+/Fe3+, Experimental Hole Concentration in Air at 1173 K, Conductivity at 300 K, Peak Ratio L2/L3 and Branching Ratio L3/(L2+L3) sample

τform

gas

LSF50 LSF50 LSFTi20 LSFTi20 LSFTa10 LSFTa10

1.00 1.00 0.60 0.60 0.50 0.50

argon air argon air argon air

δgrav at 1173 K

τgrav at 1173 K

0.078

0.52

0.063

0.28

0.058

0.26

σ [S/cm] Ea [meV] 0.075 50/180 meV 2.28/262 meV 0.017 0.82/342 meV

L2/L3

L3/(L2+L3)

0.24 0.29 0.23 0.27 0.24 0.28

0.81 0.67 0.81 0.68 0.81 0.66

→ 3d transition) at around 708.5 eV is stronger for the samples prepared in argon, see column L3/(L2+L3) Table 2. This suggests that the samples prepared in argon contain, quite naturally, more of the Fe3+ type iron, in line with our gravimetrical determination of δgrav and τgrav in Table 2. Here we should point out that the determination of the oxygen nonstoichiometry of a metal oxide is not a trivial task. There exist a number of alternatives to determine the oygen deficiency δ, such as thermogravimetry like applied here. Its accuracy depends on the accurate stoichiometry information of the precursors, which is not always given. High resolution neutron diffraction is very sensititve to the presence or absence of low Z elements in a crystal lattice; together with Rietveld structure refinement it allows determination of δ.1 Carrier gas coulometry25 and coulometric titration in a closed electrochemical cell allow to accurately control the oygen partial pressure and δ over a very wide range.26 The reduced form of a species generally has a smaller L2/L3 ratio,23 compare Table 2. We notice that the three samples with the highest conductivity (heated in air) also have the larger Fe L2/L3 peak height ratios (0.27−0.29) and smaller branching ratios (0.78−0.79), whereas the other samples (heated in argon) have a peak height ratio of only 0.23−0.24. For a qualitative assessment of the changes of the Fe core level, we compare the peak height ratio L2/L3 along the formal hole concentration τform = Fe4+/Fe3+ (Table 2). Although the spectral changes at the L-edges within the three air-heated and the three argon-heated samples are not substantial, their values systematically decrease with decreasing τ, revealing a change of the valence of the Fe atoms depending on the doping situation (Table 2). According to ref 24, Fe ions in LSF remain essentially in the high spin 3d5 configurations. The holes induced by substituting La3+ by Sr2+ necessarily go to states of primarily oxygen character. The authors in ref 24 conclude that the main component of the ground state in La1/2Sr1/2FeO3 is 3d5L, and not 3d4. We can assess the spin state of our materials with the branching ratio L3/(L2+L3).27 The spin state bears some significance in this context because high spin ions with parallel spins require more space than the low spin ions with antiparallel spins; this is a direct consequence from Pauli’s principle and Hund’s rules. Orbital overlap, which is important for the charge transfer, is governed by the interplay of Coulomb repulsion of electrons, exchange interaction, and spin states.28 The branching ratio for the samples prepared in air is smaller than for those prepared in argon, revealing that the samples with lower hole concentration have a larger net spin value. This finding is in line with the suggestion that the Fe4+ ion as the hole dopant controls the conductivity, whereas the Fe3+ ion controls the magnetic properties.24 For LSF50, LSFTi20, and LSFTa10 the L2 peak with eg symmetry at 720.5 eV is slightly more intense for the samples prepared in argon than for those

the other two samples heated in argon was below the resolution limit of the meter. The multiplet structure of 3d metals such as iron and its Xray spectroscopic assessment are generally well understood.23 The panels in Figure 3 display the Fe (2p) X-ray absorption

Figure 3. Left panel: Comparison of experimental Fe (2p) absorption spectra of samples (LSF50 bottom, LSFTa10 middle, LSFTi20 top) heated in argon (red solid line) and air (blue dashed line). Vertical dashed lines denote the eg and t2g orbital symmetry transitions at the Fe2p3/2 and Fe2p1/2 multiplet, and satellites at around 712 eV (ligandto-metal charge transfer LMCT) and 716 eV. Right panel: Comparison of simulated (thin black lines at bottom) and experimental (filled symbols connected by red line) Fe2p absorption spectra of the argon heated samples. The simulated spectra were obtained by atomic multiplet calculations.

spectra of our samples (iron L-edges). The degeneracy of the Fe (2p) core hole level is lifted by the spin−orbit coupling, resulting in the 2p3/2 and 2p1/2 multiplets (L3 and L2 absorption edges, respectively) at around 708 and 723 eV, separated by an exchange energy of Δ ≈ 13 eV. The octahedral crystal field lifts the degeneracy of the 2p3/2 level so that two levels with t2g and eg symmetry are created, as indicated by the two structures at around 707 and 708.5 eV. The same holds for the 2p1/2 level, with structures at around 720.5 and 723 eV. In line with previous conclusions on LSF,24 we find that the Fe ions in our samples remain essentially in the high spin 3d5 configuration (t2g3 eg2), and identify the spectrum of LSF50 in Figure 1 in ref 24 as the one that resembles our spectra most. Comparison of our normalized Fe spectra shows that the intense L3 peak (2p 1532



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Table 3. Relative Hole Concentrations τ from Simulation of Spectra Heated in Air (10Dq = 1.85 eV) and in Argon (10Dq = 1.70 eV)

LSF 50 LSFTi20 LSFTa10

calculated with δ = 0

calculated δ at 1173 K

10Dq = 1.85 eV

Fe4+/Fe3+

Fe4+/Fe3+ in air

Fe4+/Fe3+ in air

Fe4+/Fe3+ in argon

oxidation change

1 0.56 0.43

0.52 0.28 0.25

1 0.67 0.67

1 0.43 0.43

stable reduced in argon reduced in argon

prepared in air. Generally, the branching ratios of our samples are comparable with those of FeO, Fe2O3, and Fe3O4. In ref 24 the ratios of LSF50 are very close to those for LaFeO3. We subjected all Fe L-edge spectra to atomic multiplet simulations to verify the assessment of the Fe oxidation state and spin state.29 The right panel in Figure 3 shows the experimental and simulated spectra of the samples heated in argon. To obtain a good match between the experimental and simulated spectra, the crystal field splitting parameter 10Dq was set to 1.85 eV for the samples heated in air and 10Dq = 1.70 eV for those heated in argon. We find for all samples that Fe is in the high spin state. Absence of spin state transitions can help minimize thermal expansion.30 For the L-edge spectra of LSF50 heated in air and in argon, the multiplet simulation yields for both cases τ = 1 (Table 3), identical with the formal hole concentration τform = 1 (Table 2). We recall that LSF50 is stable upon heating in air and in argon. For LSFTi20 and LSFTa10 similar yields are obtained for the samples: τ = 0.76 after heating in air, and τ = 0.43 after heating in argon. Because the oxygen deficiency δgrav was determined in air for 1173 K3 (Table 2), a temperature where the Fe ion is getting reduced along with oxygen vacancy formation, the δ for ambient temperature should be noticeably smaller than those at 1173 K, that is, δ < 0.078. The fact that the relative concentration of Fe3+ and Fe4+ obtained by multiplet simulation (Table 3) fits better with those values that were calculated based on the assumption that the oxygen deficiency was δ = 0, is an indirect verification that the samples were virtually stoichiometric at ambient temperature. Figure 4 displays the oxygen (1s) absorption spectra of our samples. Oxygen (1s) spectroscopy probes the unoccupied electron states that contain oxygen character, that is, empty Fe 3d states. The spectra of our samples show systematic differences depending on (i) whether they were prepared in air or argon, and (ii) on their doping situation. The two prepeaks at 527.7 and 529.3 eV represent mixed states of O (2p) and Fe (3d) character, which originate from ligand-field splitting in the octahedral crystal environment (crystal field splitting plus hybridization) and hence form bands with eg and t2g symmetry.31,32 These structures represent the valence band and thus reflect features important for the transport properties of the material. The structures at 534.4 and 536 eV arise from O (2p) states covalently mixed with La (5d) and Sr (4d) bands. The structures at 540.0 and 541.8 eV are due to interactions of the oxygen with the Fe (4sp) and the La (6sp) bands. We make the general observation that the samples prepared in air have a noticeable spectral intensity at 527.7 eV, which corresponds to the eg level of the doped hole of the mixed O (2p)-Fe (3d) states. This hole structure is virtually absent in the samples prepared in argon. In contrast, the argon-prepared samples have a noticeable intensity at 529.3 eV, which corresponds to the t2g level at the O (2p)-Fe (3d) states.

10DQ = 1.70 eV

Figure 4. Comparison of the O2p (left panel) absorption spectra. The blue shaded region from 525 to 532 eV denotes the valence band region with predominantly Fe3d-O2p hybridized states. Blue dashed (red solid) lines denote spectra from samples synthesized in air (argon). Sequence of spectra is from top to bottom LSFTi20, LSFTa10, and LSF50. The green solid Gaussians at 527.7 eV are obtained by deconvolution of the spectra from the air heated samples (blue dashed spectra) and constitutes the hole doping state with spin up eg ↑ orbital symmetry, indicated by the first dashed thin vertical line.

Compared with the Fe spectra, the oxygen spectra are richer in structure, and they also show clearer differences depending on the doping situation, and on whether they were prepared in argon or in air. All spectra were normalized to unity intensity at 560 eV. The structure and variation of the two pre-peaks are a sufficient basis for the discussion of the correlation of the transport and electronic properties. We begin with the two LSF50 samples (Figure 4, lower spectra). The remarkable difference between the LSF50 air (dashed blue spectrum) and LSF50 argon (solid red spectrum) sample is that the former has a pronounced pre-peak at 527.7 eV, whereas the latter has a less pronounced pre-peak which is found at 529.3 eV. We have highlighted the pre-peak at 527.7 eV, which is indicative of hole doping, by deconvolution of the spectrum and plotted this as a single Gaussian (green solid peak denoted with eg ↑ spin up in Figure 4). Closer inspection of the spectra reveals that the LSF air sample has a slight shoulder at 529.3 eV, and the LSF sample prepared in argon has a slight shoulder at 527.7 eV. We therefore identify these peaks as arising from transitions to new mixed states of O (2p) and Fe (3d) character, separated into t2g and eg bands by ligand-field splitting, which includes the crystal field and the hybrid1533



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ization.24,29 In their LSF series,24 this peak grows as a function of the Sr hole doping. Wadati et al.32 find that the spectral weight of this structure is created inside the band gap and proportional to the doping parameter x, indicating that doped holes go into this structure at 527.7 eV. This peak is pronounced for our air-synthesized sample, but very small for the argon-synthesized sample. Wadati et al.32 support that doped holes go into states primarily of O (2p) character, and not Fe (3d) character.24 In the samples LSFTa10, the eg peak of the sample heated in air is as large as the t2g peak, as is shown by the equal height of the blue dashed spectrum at 527.7 and 528.5 eV. Whereas the t2g peak in the corresponding sample heated in argon clearly supersedes the corresponding eg peak, as is shown by the solid red spectrum at the same energies in Figure 4. The single green Gaussian peak denotes the hole state and was obtained by deconvolution of the blue dashed spectrum for LSFTa10 heated in air. In LSFTi20 air, the eg and t2g peak are also of comparable magnitude, whereas the sample heated in argon shows a larger t2g peak, and a vanishing eg contribution. To quantitatively account for the relative contributions of eg and t2g for the samples, and to correlate them with the conductivity data, the areas under the peaks for the samples heated in air were determined and their ratios are plotted versus the formal hole concentration τ, Figure 5.33 We include also data points

of the conducting eg bands, which require octahedral coordination.12 Wu et al. and de Groot et al.24,31 have elaborated on the effects that cause the relative intensities of these pre-peaks in transition metal oxides, including the number of 3d holes on Fe, hybridization effects, exchange interaction, and material nonstoichiometry, for instance. We recall that the actual relative Fe4+ content in LSF50 and LSF60 was determined by other groups as 40%,12,13 whereas the formal Fe4+ content would have to be 50%. On the basis of our gravimetric data, we find 34.4% Fe4+ in LSF50, or τ = 0.52.

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CONCLUSIONS Out of the investigated samples, La1/2Sr1/2FeO3 heated in air has the smallest lattice volume but the largest isothermal expansion. Δl/l0 scales linearly with the oxygen deficiency, and for the samples heated in air, the conductivity decreases upon volume increase. Our NEXAFS data and simulation show that the Fe in all samples remains in the high spin 3d5 configuration, and those samples prepared in argon have the iron in a relatively larger amount of Fe3+. The samples prepared in air have a larger hole concentration and a lower net spin value, confirming the perception that Fe4+ as the hole doping ion controls the conductivity. The multiplet simulation suggests that the samples were virtually stoichiometric at ambient temperature. Comparison of the relative spectral weight of the hole doping peak in the oxygen NEXAFS spectra with the conductivity and the formal hole concentration shows that with increasing hole concentration we find a larger ratio between eg and t2g symmetry bands in the hybridized O2p-Fe3d states. Those likely manage the interplay between oxygen deficiency and isothermal expansion in the nonsubstituted La1/2Sr1/2FeO3 . Substitution of Fe by Ta and Ti partially depletes these states and narrows this interplay.

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AUTHOR INFORMATION

Corresponding Author

*Phone +41 (0) 58 765 4850. Fax +41 (0) 58 765 4150. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the European Commission (MIRG # CT2006-042095 and Real-SOFC # SES6-CT-2003-502612) and the Swiss National Science Foundation (SNF # 200021-116688 and # 200021-100674/1) are acknowledged. During preparation of this manuscript, A.B. had financial support through Indo-Swiss grant ISJRP 138 864 and by EU FP7 project SOFCLife with contract number 256694. The Science and Technology Facilities Council of the U.K. is acknowledged for granting and funding the beamtime at the Synchrotron Radiation Source under project # 47093. We are grateful to Dr. P. Holtappels (Empa) for making this project possible.

Figure 5. Correlation of conductivity (open cirlces) and relative spectral weight (filled symbols) of the hole doping peak as a function of the formal (bottom axis) and experimental (top axis) electron hole concentration. Filled squares are from ref 34. The dotted straight line is a guide to the eye highlighting the general trend between electron hole concentration and conductivity.

that are determined from data in ref 34. It is obvious that with increasing hole concentration we find a larger ratio between eg and t2g symmetry bands in the mixed O (2p)−Fe (3d) states. We also have to consider that the LSF50 unit cell is expanding slightly upon addition of Ti and Ta, likely because the ionic radii of Ti4+ and Ta5+ are larger than the radius of Fe4+.19 Fe4+ with 3d4 configuration assumes octahedral d2sp3 coordination, whereas Fe3+ with 3d5 configuration assumes square/planar dsp2 coordination. The depletion of Fe4+ with increasing Ta or Ti substitution, and the resulting enrichment with Fe3+, causes rearrangement from octahedral to planar coordination, which consequently manifests itself in a depletion

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2Fe-Oxide in Oxidative and - American

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