Valence Band Structure and X-ray Spectra of Oxygen-Deficient

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Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeOx V. R. Galakhov,*,†,‡ E. Z. Kurmaev,† K. Kuepper,¶ M. Neumann,§ J. A. McLeod,| A. Moewes,| I. A. Leonidov,⊥ and V. L. Kozhevnikov⊥ Institute of Metal Physics, Russian Academy of Sciences s Ural DiVision, S. KoValeVskaya str. 18, 620990 Yekaterinburg, Russia, Ural State Mining UniVersity, Yekaterinburg, Russia, Department of Solid State Physics, UniVersity of Ulm, Albert-Einstein-Allee 11, 89069 Ulm (Donau), Germany, Fachbereich Physik, UniVersity of Osnabru¨ck, Barbarastrasse 7, D-49069 Osnabru¨ck, Germany, Department of Physics and Physics Engineering, UniVersity of Saskatchewan, 116 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada, and Institute of Solid State Chemistry, Russian Academy of Sciences s Ural DiVision, PerVomayskaya str. 91, 620990 Yekaterinburg, Russia ReceiVed: September 21, 2009; ReVised Manuscript ReceiVed: February 5, 2010

We present a study of the electronic structure of oxygen-deficient ferrites SrFeOx (x ) 2.46, 2.52, 2.68, and 2.82) by means of X-ray photoelectron, X-ray emission, and X-ray absorption spectroscopies. From the Fe 3s photoelectron splitting, the magnetic moments are estimated. It is found that the doped holes are localized in both Fe 3d and O 2p states. The valence band structure is analyzed. It is shown that the band gap decreases with the increase of oxygen concentration. Introduction Perovskite-type ferrites with iron in the rather unusual high oxidation state 4+ reveal a variety of interesting electronic properties, including metallic conductivity in the perovskite SrFeO3 and valence mixing for CaFeO3, where the ions Fe3+ and Fe5+ are present instead of Fe4+. The system SrFeO3-δ has been the subject of many investigations not only because SrFeO3 is one of the few oxide phases to contain tetravalent iron but also due to the structural features displayed by the oxygendeficient phases. A number of nonstoichiometric phases exist in the series SrFeO3-δ. These materials also show reasonably high electric conductivities and could be applicable as electrodes in fuel cell systems containing other perovskite-like materials. SrFeO3-δ has either the cubic perovskite structure where the oxygen deficiency δ varies between 0 and 0.5 or the rhombohedral brownmillerite SrFeO2.5+δ structure where the oxygen nonstoichiometry varies from -0.1 to 0.05.1,2 SrFeO3 is metallic and antiferomagnetic below 130 K.3,4 SrFeO3-δ is semiconducting.3,4 The electrical properties of oxygen-defective strontium ferrites SrFeOx have been analyzed, and it was found that the brownmillerite structure is characterized by mixed oxygen ionic and electronic conductivity.5 The band gap in the electron-hole conductor with the stoichiometric brownmillerite form was found to be about 2 eV.5 It was suggested that the extrastoichiometric oxygen in the brownmillerite leads to the appearance of acceptor states above the top of the valence band. There are a few spectroscopic investigations of stoichiometric SrFeO3, La1-xSrxFeO3, and SrFe1-xCoxO3.6-11 It was found that the doping holes on La1-xSrxFeO3 are of mixed Fe 3dsO 2p character.7 Formally, tetravalent Fe4+ ions (3d4) are in the high* To whom correspondence should be addressed. † Institute of Metal Physics, Russian Academy of Sciences s Ural Division. ‡ Ural State Mining University. ¶ University of Ulm. § University of Osnabru¨ck. | University of Saskatchewan. ⊥ Institute of Solid State Chemistry, Russian Academy of Sciences s Ural Division.

spin state, and they are described by a sum 3d5L _ and 3d4 8 electronic configuration. L _ denotes a hole due to a charge transfer from ligand p to transition-metal 3d states. The _ configuration is larger than that of the occupation of the 3d5L 3d4 configuration.8 This means that SrFeO3 is in the negativecharge-transfer regime.8 From the combined valence band photoelectron and O 1s X-ray absorption spectra of La1-xSrxFeO3 epitaxial thin films, it was found that hole doping induces spectral weight transfer from below the Fermi level to above it across the gap in a highly nonrigid-band-like manner.9,10 In this paper, we have carried out X-ray photoelectron, O 1s and Fe 2p absorption and O KR X-ray emission measurements of the electronic structure of strontium ferrites SrFeOx with x ) 2.46, 2.52, 2.68, and 2.82. We have determined the band gaps and spin magnetic moments for SrFeOx from our spectroscopic data. We show that doped holes induced by oxygen are localized in Fe 3d and O 2p states. Experimental and Calculation Details The ferrites with nominal composition SrFeOx were prepared by heating under air atmosphere at 1250 °C the appropriate mixture of high-grade purity Fe2O3 and SrCO3.12 The powder was pressed into pellets under 1 kbar of uniaxial load and sintered at 1250 °C in air. The specimens with different values of oxygen content were prepared by annealing the obtained pellets at desired values of oxygen partial pressure and temperature in accordance with the equilibrium p(O2)-T-x diagram for SrFeOx.13,14 Independent control of oxygen content in the samples was carried out using thermogravimetric analysis in a reducing atmosphere of 30% H2/70%He. The uncertainty in x was about (0.02. Powder X-ray diffraction (λ ) 1.54178 Å) was used to confirm single-phase specimens. X-ray diffraction patterns for SrFeOx with x ) 2.46 and 2.52 were indexed with the orthorhombic brownmillerite-type structure. The oxygen vacancy ordered phases Sr4Fe4O11 and Sr8Fe8O23 with orthorhombic and tetragonal structures were measured for samples with x ) 2.68 and 2.82, respectively, in agreement with those found by Hodges et al.14 The lattice parameters for the

10.1021/jp909091s  2010 American Chemical Society Published on Web 03/02/2010

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TABLE 1: Structure and Lattice Parameters of SrFeOxa x ) 2.46 x ) 2.52 x ) 2.68 x ) 2.82

structure

a (Å)

b (Å)

c (Å)

O (B) O (B) O T

5.662(4) 5.636(5) 10.98(1) 10.944(5)

15.62(1) 15.59(2) 7.710(6)

5.520(2) 5.518(3) 5.480(3) 7.699(4)

a O: orthorhombic; B: brownmillerite; T: tetragonal; O (B): not resolved between the structures.

synthesized samples of SrFeOx measured by X-ray diffraction are shown in Table 1. Note that these samples contain a small amount of Si, which can be explained by the synthesis process. The initial mixture of Fe2O3 and SrCO3 and some intermediate products were ground together in a jasper mortar, which probably contained some Si residue. According to our estimation made by X-ray photoelectron spectroscopy methods, the ratio of Si/Fe in the SrFeOx samples is not more than 0.02. While there is no information concerning the possibility of Si entering a perovskite, a significant amount of Fe replacement is extremely unlikely since the ionic radii of Si4+ is quite different than that of Fe3+. Incorporating a significant amount of Fe replacement would cause the perovskite lattice to become highly distorted. Therefore, Si does not have a large effect on the oxygen content in the ferrite structure since the presence of Si cannot change the oxygen content by more than 0.01, which is within the uncertainty of the total oxygen content x. A small concentration of Si should therefore not affect the X-ray absorption, emission, or photoelectron spectra. The X-ray photoelectron spectra were measured using an ESCA spectrometer from Physical Electronics (PHI 5600 ci) using monochromatic Al KR radiation (Eexc ) 1486.6 eV). The spectrometer was calibrated using a Au foil as a reference sample (the binding energy of the Au f7/2 core level was 84.0 eV15). The energy resolution as determined at the Fermi level of the Au foil was approximately 0.4 eV. To get a surface free of contamination, the samples were fractured in situ. The Fe 2p and O 1s X-ray absorption spectra (XAS) and the oxygen KR [valence band (2p)f1s transition] X-ray emission spectra (XES) were recorded at the soft X-ray spectroscopy end station of the undulator-based beamline 8.0.1 at the Advanced Light Source (ALS), located at the Lawrence Berkeley National Lab.16 The oxygen KR X-ray spectra were measured at the excitation energy of 550 eV; the energy resolution was about 0.4 eV. The spectra were normalized to the number of incoming photons monitored by the photocurrent from a highly transparent clean gold mesh. The iron 2p and oxygen 1s absorption spectra were measured in the total electron yield mode (TEY). The density of state (DOS) calculations were performed within the spin-polarized full potential linear augmented plane wave method (FP-LAPW) as implemented in the WIEN2k code.17 For the exchange-correlation potential, we used the generalized gradient approximation (GGA) in the PerdewBurke-Ernzerhof variant.18 The Brillouin zone integrations were performed with a 10 × 10 × 10 special k-point grid, with min RMT Kmax ) 7 (the product of the smallest of the atomic sphere radii RMT and the plane wave cutoff parameter Kmax) used for the expansion of the basis set. The experimental values of the high-temperature lattice constants and the atomic positions were used.14,19-22 The atomic sphere radii were chosen to be RSr ) 2.4, RFe ) 1.8, and RO ) 1.6 au. The sphere radii were chosen in such a way that the spheres were nearly touching.

Figure 1. Fe 2p X-ray absorption spectra of SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82. The arrows indicate features formed by Fe2+ ions.

Results and Discussion Fe 2p and O 1s X-ray Absorption Spectra. In transitionmetal oxides, the cation 2p X-ray absorption spectra are dominated by intra-atomic and short-range effects. In light of this, the Fe 2p X-ray absorption spectra correspond to the Fe 2p f Fe valence band (3d) transitions and are determined by the valence state of iron atoms. One displays the measured Fe 2p X-ray absorption spectra of SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82. The spectra are split by Fe 2p spin-orbit interactions that give rise to features labeled Fe 2p3/2 and Fe 2p1/2. The Fe 2p3/2 spectra exhibit two-peak structures. The spectrum of SrFeO2.52 is similar to that of LaFeO3 measured by Abbate et al.7 That means that the iron ions in SrFeO2.52 are in the 3+ valence state, as those in LaFeO3. The intensity of the peak at 708 eV of the spectrum for SrFeO2.46 is more intense than that for other ferrites. According to ref 23, the Fe 2p3/2 spectrum of Fe2+ ions in the octahedral Oh symmetry has a main peak at a lower photon energy than that of Fe3+ ions. Therefore, this peak in the low-energy part of the Fe 2p3/2 spectrum of SrFeO2.46 is evidence of Fe2+ ions in this compound. Furthermore, the Fe 2p spectrum of SrFeO2.46 exhibits a structure on the low-energy side of the 2p3/2 and 2p1/2 features, which are labeled by arrows in Figure 1. This structure can be explained by the Fe2+ component.24 It is obvious that a 3+ 2+ Fe0.08 O2.46 are not able to few percent of Fe2+ ions in SrFe0.92 give such intense peaks. XAS measurements in TEY mode are surface-sensitive to a few hundred Angstroms. We suggest that the surface of the sample is enriched by Fe2+ ions. One can see that the intensity of the peak at about 708.0 eV decreases with increasing of the hole concentration. Thus, these small changes of the Fe 2p X-ray absorption spectra indicate that the increase of the oxygen content in SrFeOx leads to the transition Fe3+ f Fe4+. In oxygen-defective ferrites, extra electron holes should be created either in the Fe 3d or in the O 2p states. The electronic configuration of the ground state of Fe ions in defective ferrites can be written as 3d5L _ + 3d4. A similar compound SrFeO3 is in the negative-charge-transfer regime, with holes in the O 2p level.8 To prove that the holes are formed both in oxygen 2p and in iron d states, we present measurements of O 1s X-ray absorption spectra. Figure 2 shows the O 1s X-ray absorption spectra of SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82. The O 1s X-ray

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Figure 2. O 1s X-ray absorption spectra of SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82. The features C, D, and E are discussed in the text.

absorption spectra are formed by transitions from the O 1s to unoccupied O conduction band (2p) states. The first features in the energy region from 530 to 533 eV correspond to the Fe conduction band (of d symmetry). The structure at about 538 eV relates to the Sr conduction band (of 4d -symmetry), and the feature at 545 eV is related to the Fe conduction band (of 4s and p symmetry).7 For LaFeO3, a compound with Fe3+ ions, Abbate et al.7 showed a splitting of the first peak in the O 1s absorption spectrum, which was interpreted as a difference between the energy levels of the t2g and eg orbitals induced by the crystal field. We do not observe a splitting of feature d of the spectra of SrFeO2.46 with the brownmillerite structure. Gloter et al.25 studied O 1s electron energy loss spectra of Ca2(AlxFe1-x)2O5 with the brownmillerite structure and assumed that either the t2g-eg splitting is small in brownmillerite or that the multiplicity of the oxygen sites in brownmillerite induces a blurring effect. The increase of oxygen concentration leads to the appearance of a prepeak C that grows below the absorption threshold of the SrFeO2.46 ferrite and dominates the spectrum of SrFeO2.82. The prepeak can be attributed to transitions to new states of O 2p character. A similar behavior of O 1s X-ray absorption spectra has been observed by Abbate et al.7 for the system La1-xSrxFeO3 going from LaFeO3 with Fe3+ ions to SrFeO3 with formally Fe4+ ions. This means that doping holes in SrFeOx have partial O 2p character. Therefore, the change in the oxygen content of SrFeOx leads to some changes of Fe 2p and O 1s X-ray absorption spectra, and the doped holes are localized in both Fe 3d and O 2p states. X-ray Photoelectron Fe 3s Splitting. The metal 3s level in transition-metal compounds is known to exhibit exchange splitting. The magnitude of this splitting can be estimated from

2 eff ∆E3s ) (2S + 1)G2(3s, 3d) ) (2S + 1)J3s,3d 5 where S is the local spin of the 3d electrons in the ground state and G2(3s,3d) is the Slater exchange integral.26 In addition to the exchange interaction, charge-transfer processes also play an important role. In Figure 3, we show a set of Fe 3s photoelectron spectra of the SrFeOx samples with x ) 2.46, 2.52, 2.68, and 2.82. The spectra exhibit the expected doublet A-B due to multiplet splitting, a well-known effect in transition metals which is a sensitive probe of the local spin moment of magnetic atoms.

Figure 3. Fe 3s X-ray photoelectron spectra of SrFeOx.

TABLE 2: Fe 3s Splittings (∆E3s), Local Spins of the 3d Electrons (Sv), and the Spin Magnetic Moments (m) of SrFeOx Estimated from the Fe 3s Spectra x

∆E3s (eV)

Sv

m (µB)

2.46 ( 0.02 2.52 ( 0.02 2.68 ( 0.02 2.82 ( 0.02

5.9 ( 0.1 5.9 ( 0.1 5.4 ( 0.1 5.3 ( 0.1

1.95 ( 0.05 1.95 ( 0.05 1.75 ( 0.05 1.70 ( 0.05

3.9 ( 0.1 3.9 ( 0.1 3.5 ( 0.1 3.4 ( 0.1

Feature C is a Si 2p signal due to a small impurity of Si in the SrFeOx samples, and feature D is a satellite formed by the charge-transfer process. In the case of the Fe3+ ions (SrFeO2.5), this feature reveals _ configuration of the photoemission final state. the Fe 3s13d6L The main peaks A and B are formed by the Fe 3s13d5 configuration. For the sample with x ) 2.46, the Fe 3s splitting is equal to 5.9 ( 0.1 eV. This oxide contains both Fe3+ (S ) 5/2) and 3+ 2+ Fe2+ ions (S ) 2), SrFe0.92 Fe0.08 O2.46. The Fe 3s splitting is expected to increase going from SrFeO2.46 to SrFe3+O2.50. The further increase in the oxygen content accompanied by appearance of Fe4+ ions leads to a decrease of the Fe 3s splitting; when x ) 2.82, the value of the Fe 3s splitting is thus given by ∆E3s ) 5.3 ( 0.1 eV. The values of ∆E3s of the SrFeOx samples are given in Table 2. In mixed-valence 3d oxides, transition-metal 3s spectra can in principle originate from either a linear combination of two doublets, corresponding to splitting of ions of different valence states, or an average between those spins, with a net valence spin Sv.27-29 In all cases, transition-metal 3s splitting in mixedvalence oxides should depend on the spin states of 3d electrons. This means that the Fe 3s spectra of defective ferrites SrFeOx are characterized by a valence spin Sv. The value of the Fe 3s splitting in CuFeO2 and LiFeO2 (Fe3+ ions) is 6.5 eV.30 Since the exchange interaction between the

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TABLE 3: XPS Core-Level Binding Energies and Energies of Maxima of O Kr X-ray Emission Spectra (in eV) of SrFeOxa Fe 2p3/2 XPS Fe 3p XPS Sr 4s XPS Sr 3d XPS O 1s XPS O KR XES O 2p (XES-XPS)

x ) 2.46

x ) 2.52

x ) 2.68

x ) 2.82

709.0 54.45 36.5 131.7 528.2 525.7 2.5

709.2 54.4 36.4 131.7 528.2 525.95 2.25

709.2 54.3 36.0 131.4 527.95 526.0 1.85

709.2 54.4 36.1 131.4 527.9 526.0 1.8

a The errors in determination of the binding energies and the energies of maxima of O KR X-ray emission spectra are (0.1 eV. The positions of the O 2p spectra estimated from the combination of O KR X-ray emission and O 1s X-ray photoelectron spectra are determined with errors of (0.2 eV.

Fe 3s and 3d levels is mainly localized on the Fe atom, we can eff is nearly reasonably assume that the exchange integral J3s,3d constant for these Fe oxides. The value of the spin magnetic moment for CuFeO2 is 4.4 µB.31 eff Taking J3s,3d to be 1.2 eV, we can estimate effective spins (Sv) and spin magnetic moments of 3d electrons (m). For SrFeO2.46, Sv ) 1.95 ( 0.05 and m ) 3.9 ( 0.1 µB. For SrFeO2.82, these values are 1.7 ( 0.05 and 3.4 ( 0.1 µB, respectively. The results are summarized in Table 2. For comparison, magnetic moments of SrFeO2.5 and SrFeO2.75 are 3.9220 and 3.55 µB,32 respectively. Core-Level X-ray Photoelectron Spectra and Chemical Potential Shift. The Fe 2p, Fe 3p, O 1s, Sr 3d, and Sr 4s corelevel binding energies are presented in Table 3. The Fe 2p and Fe 3p binding energies were observed to be nearly constant, whereas O 1s, Sr 3d, and Sr 4s binding energies decreased with increasing oxygen concentration. It is known that the shift ∆EB of the core-level binding energy measured from the chemical potential µ is determined by the formula33-35

∆EB ) ∆µ + K∆Q + ∆VM - ∆ER where ∆µ is the change in the chemical potential, K is the Coulomb coupling constant between the valence and core electrons, ∆Q is the change in the number of valence electrons on the atom considered, ∆VM is the change in the Madelung potential, and ∆ER is the change in the core-hole screening. The ∆ER term is known to be neglected from the main origin of the core-level shifts in transition-metal oxides.34 A similar shift of the O 1s, Sr 3d, and Sr 4s core levels indicates that the change in the Madelung potential is small because it would shift core levels of anions and cations in different directions.9,10 Therefore, we can describe the core-level shifts as a measure of ∆µ in SrFeOx. We suggest that the Fe 2p and 3p chemical shifts (due to the decrease in formal valence of Fe when shifting from x ) 2.46 to 2.82) are compensated for by the shift in chemical potential. Therefore, the increase of the oxygen concentration leads to a shift toward higher chemical potential µ. This conclusion is confirmed by valence band XPS and O 1s XAS measurements. Valence Band X-ray Photoelectron and O 1s Absorption Spectra and Band Structure Calculation. Figure 4 shows the combined valence band X-ray photoelectron (XPS), O KR emission (XES), and O 1s absorption (XAS) spectra of SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82. The XES and XAS spectra are brought to the common energy scale using the O 1s binding energies obtained from our X-ray photoelectron

Figure 4. Valence band X-ray photoelectron (XPS), O KR X-ray emission (XES), and O 1s absorption (XAS) spectra of SrFeOx. The XES and XAS spectra are given on a common energy scale based on the binding energy of the O 1s levels. Second derivatives of the XES and XAS spectra are shown.

measurements (see Table 3). Although there is an unknown shift of O 1s XAS spectra because of the core-level potential, the set of the valence band X-ray photoelectron, O KR emission, and O 1s absorption spectra allow one to monitor changes in the occupied and unoccupied electronic states caused by oxygen defects. The XPS valence band spectra provide information about the total DOS distribution. For the Al KR excitation, the cross section ratio multiplied by the related oxygen and iron concentrations σ(O 2p)c(O)/σ(Fe 3d)c(Fe) is equal to 0 0.13:1.36 Consequently, the main contribution to the XPS valence band spectrum in the presented energy region stems from the Fe 3d states. The valence band region of the XPS spectra of all studied ferrites reveals two features labeled A and B. According to band structure calculations of SrFeO3 and SrFeO2.875,37,38 the peak A should be related to Fe 3d(t2gv) states, and the feature B should be ascribed to the Fe 3d-O 2p bonding states. The position of the feature B for all compounds is nearly constant: 5.1 ( 0.1 eV. The energy of feature A tends to decrease in energy with increasing oxygen concentration (from 3.0 eV for SrFeO2.46 to 2.4 eV for SrFeO2.82). These shifts are in agreement with the core-level shifts. The energy area near the Fermi level (from 0 to about 2 eV) can be ascribed to the Fe 3d(eg) states. This interpretation is similar to that of the XPS and XAS studies of La1-xSrxFeO3.9,10 The electric dipole-allowed 2p f 1s transition of the oxygen KR X-ray emission spectra directly probes occupied oxygen 2p states and indirectly probes the Fe-O bonding since the 2p orbitals of oxygen ligand are involved in the bonding configuration with the Fe metal ions. The main maxima a of the O KR X-ray emission spectra in the binding energy scale reside at 2.5 and 1.8 eV for SrFeO2.46 and SrFeO2.82, respectively. This peak should be attributed to the O 2p states π-bonded to the metal 3d(t2g) states. Feature b results from mixing of Fe 3d (eg) states with O 2pσ bonding states.

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Galakhov et al. Sr8Fe8O23 together with the valence band XPS spectra of SrFeO2.46, SrFeO2.68, and SrFeO2.82. The XPS spectra are presented in the energy scale relative to the valence band tops estmated from second derivatives of the XES and XAS spectra (see Figure 4). The calculated valence DOS shown in Figure 5 is in good agreement with the measured XES and XPS spectra; however, all of the calculations give metallic solutions, whereas experiment reveals that the materials are insulators. This is not surprising, however, since it is well-known that GGA methods underestimate the band gap.41 To match experiment, the GGA+U approximation, with an Hubbard-like term U added to the potential,42 is a better choice. However, the U parameter must be optimized to match experimental data, and the calculation is no longer truly ab initio. Conclusions

Figure 5. Calculated valence total and partial densities of states for Sr2Fe2O5, Sr4Fe4O11, and Sr8Fe8O23. For comparison, the XPS spectra of SrFeO2.46, SrFeO2.68, and SrFeO2.82 are shown.

It is possible to estimate band gaps (Eg) in ferrites SrFeOx using our X-ray emission and X-ray absorption results. The estimation of the band gap from the measured O K-edge XES and XAS spectra is achieved by a method based on the second derivative of the measured spectra.39 The second derivatives of the XES and XAS spectra for all of the samples are plotted on a common energy scale, and the distances between the highestenergy peaks of the XES derivatives and the lowest-energy peaks of the XAS derivatives are determined. The results are presented in Figure 4. The band gap for the ferrite SrFeO2.46 is estimated to be Eg ) 2.7 ( 0.2 eV. Note, the electric conductivity measurements of strontium ferrites5 show a band gap of about 2 eV in the near-stoichiometric brownmillerite phase. According to ref 5, increasing the concentration of oxygen in SrFeOx should lead to the appearance of acceptor states above the top of the valence band. Our X-ray spectral studies of SrFeOx confirm this suggestion. By going from x ) 2.46 to 2.82, the first maximum of the XPS spectra, A, shifts toward lower binding energy. Moreover, acceptor-like states (feature C) appear for ferrites with x > 2.50. The energy distance between the top of the valence band and this acceptor band for the ferrites SrFeO2.52, SrFeO2.68, and SrFeO2.82 are found to be 1.5 ( 0.2, 1.5 ( 0.2, and 1.0 ( 0.2 eV, respectively. The electrical measurements of ref 5 estimated this distance as 0.1 eV. Note, calculations of Sr4Fe4O11 (SrFeO2.75) give a band gap of 0.13 eV.40 The trend in the dependence of band gap on the composition agrees well with the results of electric conductivity experiments and band structure calculations. Figure 5 shows the calculated total and Fe 3d, O 2p, and Sr 4d partial densities of states for Sr2Fe2O5, Sr4Fe4O11, and

We presented a detailed X-ray spectroscopic study of the electronic structure of defective strontium ferrites SrFeOx with x ) 2.46, 2.52, 2.68, and 2.82. We found that the increase of the oxygen concentration leads to a shift toward higher chemical potential µ. From the Fe 3s photoelectron splitting, we estimated spin magnetic moments of the Fe atoms. For SrFeO2.46, SrFeO2.52, SrFeO2.68, and SrFeO2.82, the estimated spin magnetic moments were 3.9 ( 0.1, 3.9 ( 0.1, 3.5 ( 0.1, and 3.4 ( 0.1 µB, respectively. Our results pointed to the possibility to use X-ray photoelectron 3s spectra for estimation of local spin magnetic moments localized on 3d elements. Using Fe 2p and O 1s X-ray absorption spectra, we have established that doped holes are localized in both Fe 3d and O 2p states. We estimated band gaps for oxygen nonstoichiometric SrFeOx and found that the band gap decreases with the increase of oxygen concentration. Acknowledgment. This work is supported by the Russian Foundation for Basic Research (Grants Nos 07-02-00540 and 08-03-99071) and by the Research Council of President of the Russian Federation (Project NSH-3572.2010.2). Part of the work has been performed at the ALS, which is supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. A.M. gratefully acknowledges the National Sciences and Engineering Research Council and the Canada Research Chair program. K.K. is grateful for financial support by the Ph.D. Program of the Federal State of Lower Saxony, Germany. References and Notes (1) Takeda, Y.; Kanno, K.; Takada, T.; Yamamoto, O.; Takano, M.; Nakayama, N.; Bando, Y. J. Solid State Chem. 1986, 63, 237–249. (2) Mizusaki, J.; Okayasu, M.; Yamauchi, S.; Fuek, K. J. Solid State Chem. 1992, 99, 166–172. (3) Gallagher, P. K.; McChesney, J. B.; Buchanan, D. N. E. J. Chem. Phys. 1964, 41, 2429–2434. (4) MacChesney, J. B.; Sherwood, R. C.; Potter, J. F. J. Chem. Phys. 1965, 43, 1907–1913. (5) Kozhevnikov, V. L.; Leonidov, I. A.; Patrakeev, M. V.; Mitberg, E. B.; Poeppelmeier, K. P. J. Solid State Chem. 2000, 158, 320–326. (6) Bocquet, A. E.; Fujimori, A.; Mizokawa, T.; Saitoh, T.; Namatame, H.; Suga, S.; Kimizuka, N.; Takeda, Y.; Takano, M. Phys. ReV. B 1992, 45, 1561–1570. (7) Abbate, M.; de Groot, F. M. F.; Fuggle, J. C.; Fujimori, A.; Strebel, O.; Lopez, F.; Domke, M.; Kaindl, G.; Sawatzky, G. A.; Takano, M.; Takeda, Y.; Eisaki, H.; Uchida, S. Phys. ReV. B 1992, 46, 4511–4519. (8) Abbate, M.; Zampieri, G.; Okamoto, J.; Fujimori, A.; Kawasaki, S.; Takano, M. Phys. ReV. B 2002, 65, 165120. (9) Wadati, H.; Kobayashi, D.; Kumigashira, H.; Okazaki, K.; Mizokawa, T.; Fujimori, A.; Horiba, K.; Oshima, M.; Hamada, N.; Lippmaa, M.; Kawasaki, M.; Koinuma, H. Phys. ReV. B 2005, 65, 035108. (10) Wadati, H.; Kobayashi, D.; Chikamatsu, A.; Hashimoto, R.; Takizawa, M.; Horiba, K.; Kumigashira, H.; Mizokawa, T.; Fujimori, A.; Oshima, M.; Lippmaa, M.; Kawasaki, M.; Koinuma, H. J. Electron Spectrosc. Relat. Phenom. 2005, 144s147, 877–880.

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