Article pubs.acs.org/cm
Electronic Defect Formation in Fe-Doped BaZrO3 Studied by X‑Ray Absorption Spectroscopy Dong-Young Kim,† Shogo Miyoshi,† Takashi Tsuchiya,†,‡ and Shu Yamaguchi*,† †
Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
‡
S Supporting Information *
ABSTRACT: The electronic structure of Fe-doped BaZrO3 (BaZr1−xFexO3−δ) has been studied by X-ray absorption spectroscopy (XAS) to understand electronic defect formation reactions. The analysis of Fe L-edge XAS spectra suggests that the valence state and ground state of Fe under an oxidizing atmosphere are Fe3+ and 3d5L (L indicates a ligand-hole), respectively. In O K-edge XAS spectra, the formation of two pre-edge features by Fe doping is observed. One of these features is related to the unoccupied O2p−Fe3d hybridizing state, whereas the other is assigned to an acceptor level composed of an unoccupied nonbonding O2p state. The acceptor level splits into two fine structures of localized holes on O2− (O−) bridging FeO6 and FeO6 and the one bridging FeO6 and ZrO6. The intensity of the acceptor level is very sensitive to both Fe concentration and the partial pressure of oxygen gas for heat treatment, both of which modulate the acceptor concentration through defect chemical reaction. The present results reveal a surprising conclusion that the oxidation state of BaZr1−xFexO3−δ is governed by the valence state of the oxide ion between O2− and O− with the fixed valence state of the transition metal ion of Fe3+, and no signature of the formation of Fe4+ is observed. KEYWORDS: BaZrO3, X-ray absorption spectroscopy, O−, defect formation
1. INTRODUCTION Perovskite oxides exhibit a variety of unique properties from electronic, magnetic, and optical points of view. In particular, these oxides are widely studied as possible candidate materials for electrochemical energy conversion devices, such as cathode materials for solid oxide fuel cells, permeable membranes for gas separation, electrochemical catalysts, sensors, and so on, because the materials have superior electrochemical and defect chemical properties in a wide range of temperatures under various oxygen activities.1−3 In the perovskite ABO3 structure, BO6 octahedra mainly contribute to both electronic and ionic conductivities, which are important properties for such energyrelated applications. Both electronic and ionic conductivities in perovskite oxides for 3d to 5d transition metal elements as B-site cations with a relatively large band gap, such as BaTiO3, BaZrO3, and so on, are usually attributed to a hopping conduction along the B−O− B bonding.4 In the electronic conduction process by a small polaron hopping, extra holes or electrons, introduced either by chemical doping of the acceptor/donor or by intrinsic defect chemical formation reactions, are trapped to the dopant or its nearest-neighboring anions, and these trapped carriers have to jump to the neighboring site by a thermal activation process.5 The small polaron hopping conductivity is reportedly influenced significantly by a carrier formation mechanism due to the variation of the valence state (or redox state) of both the © 2013 American Chemical Society
matrix and dopant cation and an electronegativity of BO6 octahedra.6 Because the perovskite oxide of BaZrO3 with the ideal cubic symmetry (Pm3m), which possesses unusual versatility originated from chemical stability, is an insulator in the pristine form with almost no carriers in the ZrO6 matrix due to a wide band gap of about 5 eV,7 electronic and ionic conductivities are exclusively governed by carrier donation from extrinsic dopants or impurities. Recently, we reported electrical conductivity and related electrochemical properties of heavily Fe-doped BaZrO3,8−10 which exhibits excellent p-type conductivity with unique bond-percolation conductivity behavior. Now, we have accumulated information about the electrical properties of this material system, but the origins of hole formation and the bond-percolation phenomena are still not clear. The variation of the valence state of FeO6 octahedra is responsible for both electronic and ionic carrier formations as well as trap/ delocalization of the carriers. Also, an important contribution of FeO6 is the hole donation to the ZrO6 matrix as an acceptor. To seek the origin for the successful increase in density of holes and their conductivity upon the Fe doping, the electronic structure of BaZr1−xFexO3−δ is probed by X-ray absorption Received: July 15, 2013 Revised: December 10, 2013 Published: December 10, 2013 927
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spectroscopy (XAS) for Fe L-edge and O K-edge. XAS is an excellent high-sensitivity tool to analyze the chemical environment of the specific elements, like chemical bonding nature, oxidation state, and so on.6,11,12
2. EXPERIMENTAL SECTION Powders of BaZr1−xFexO3−δ with various x values from 0 to 0.9 have been synthesized via a citrate process. Appropriate molar quantities of Ba(NO3)2 (Wako, Japan), ZrO(NO3)2·2H2O (Wako, Japan), and Fe(NO3)3·9H2O (Wako, Japan) are used as starting materials. The amounts of crystallization water in each nitrate have been determined by chemical titration and thermogravimetric analysis (TGA) prior to the synthesis. All the raw materials have been dissolved into distilled water to form a mixed aqueous solution, with citric acid added, whose pH is controlled at ∼6 by NH3 solution. The solutions are heated to remove water until the uniform solution turns into a gel. Then, the precursor gel is ignited for combustion to form ash, which is subsequently calcined at 1000 °C for 6 h. The calcined powders are pressed into pellets using a uniaxial pressing at 1000 kgf/cm2. The pellets are placed in a powder bed with the same composition to avoid possible reaction with stabilized zirconia crucible during the sintering at 1300−1600 °C for 10 h. In addition, a pure α-Fe2O3 pellet is also prepared by sintering at 1200 °C for 10 h as a reference sample for the XAS spectra of Fe L-edge absorption. X-ray diffraction analysis of powdered as-sintered BaZr1−xFexO3−δ (x = 0−0.9) is performed at room temperature using an X-ray diffractometer (40 kV, 200 mA, M18XHF, MacScience, Japan) equipped with a Ni filter using Cu Kα radiation for secondary phase identification. The lattice constant is calculated assuming a cubic symmetry (Pm3m). To observe the variation of electronic structure using XAS, the samples are heat-treated at 800 °C for 10 h under a flow of either pure O2 gas, Ar gas, or a 1 vol % H2−Ar gas mixture humidified by saturating water at 20 °C. Subsequently, the samples heat-treated in O2 and Ar gas are cooled to room temperature at a cooling rate of 1 °C/ min to obtain fully oxidized samples, whereas the samples heat-treated in a quartz firing tube under a flow of H2−Ar gas are quenched to room temperature using ice water to avoid oxidation and hydration during cooling. To prepare a fresh surface for measurements, the samples are cleaved just before installing the samples into the sample chamber of the spectrometers. The XAS measurements are performed using an undulator beamline BL-19B at the Photon Factory in High Energy Accelerator Research Organization (KEK), Tsukuba, Japan. O K-edge (∼535 eV) and Fe L-edge (∼710 eV) XAS spectra are collected in total fluorescence yield (TFY) mode using a photodiode (AXUV100, International Radiation Detectors Inc., Torrance, CA, USA) with an energy resolution of 0.1 eV under visible light shielding. All of the measurements are done at room temperature. The spectrum is taken several times to ensure its reproducibility and consistency, and the average spectra are used for further analyses. The incident photon energy is calibrated by measuring the 4f core level of Au film. As a routine work, all spectra are normalized to incident photon flux.
Figure 1. XRD patterns of the powdered BaZr1−xFexO3−δ (x = 0−0.3 and 0.8) sintered at 1300−1600 °C for 10 h in the air. Reference ICDD data for cubic BaZrO3 [PDF 06−0399], tetragonal BaFeO3−δ [PDF 23−1023], and cubic BaFeO2.88 [PDF 20−0127] are also plotted. All spectra are normalized by the maximum peak intensity. Small peaks in the reference ICDD data are shown by the magnified images in the insets.
reference to the ICDD data, [PDF 23−1023] and [PDF 20− 0127], respectively. The small peaks in the reference ICDD data are shown in the inset figures. Details of phase relation for BaZr1−xFexO3−δ are discussed in the Supporting Information. In addition, the phase stability under a reducing atmosphere for samples with the composition x = 0.1−0.3 is discussed in the Supporting Information. The position of the peaks shifts to a higher angle side with the increase of Fe concentration, because the ionic radius of Fe3+ (0.645 Å) is smaller than Zr4+ (0.72 Å) in the octahedral coordination. The XRD analysis results suggest that BaZr1−xFexO3−δ forms solid solutions of BaZrO3 with a Zr-rich cubic phase, and a two-phase mixture of Fe-rich tetragonal and cubic phase. Further details of phase relations will be reported elsewhere.10 3.2. X-Ray Absorption Spectroscopy (XAS) Analysis. 3.2.1. Fe L-Edge XAS Spectra. The 3d ground state configuration and the valence state of the transition metal cation can be deduced from peak shapes and energy shifts of the transition metal cation in the L-edge XAS spectrum.6,11−13 Figure 2 shows the comparison of Fe L-edge XAS spectra for BaZr1−xFexO3−δ with x = 0.1−0.3 and 0.8 annealed in an oxidizing atmosphere. The spectrum of α-Fe2O3, in which the Fe3+ ion occupies octahedral sites coordinated by O2− similar to the perovskite structure, is also plotted at the bottom of Figure 2 as a reference Fe3+ spectrum. The Fe L-edge XAS spectra are composed of two main absorption features of L3-edge and L2edge, separated by a spin−orbit splitting due to core-hole and 3d-electron interaction. In order to identify the valence state of Fe in BaZr1−xFexO3−δ, the interpretation is focused on the L3edge spectra. The main absorption edge of the Fe L3-edge of BaZr1−xFexO3−δ and α-Fe2O3 is the same at around 710 eV, whose value shows a good agreement with the reported ones
3. RESULTS AND DISCUSSION 3.1. Phase Analysis. Figure 1 shows the XRD patterns of powdered BaZr1−xFexO3−δ (x = 0−0.3 and 0.8) sintered at 1300−1600 °C for 10 h in the air. The XRD patterns of samples from x = 0 to 0.3 are indexed with a cubic (Pm3m) symmetry in reference to the ICDD data for BaZrO3 [PDF 060399]. All of the patterns do not exhibit any trace of secondary phases except for the sample with x = 0.8. The XRD peaks of the x = 0.8 sample, which looks like a diffraction from a single phase, exhibit asymmetric features with a peak tail to higher angles, indicating that the crystal symmetry is lowered from cubic. The XRD pattern of the x = 0.8 sample might be indexed with a combined pattern of tetragonal structure of BaFeO3‑δ (P4/mmm) and cubic structure of BaFeO2.88 (Pm3m) in 928
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attributed to the charge transfer satellite due to the increase of covalency in the 3d ground state configuration described below. The substitution of Fe3+ for Zr4+ generates holes to maintain the electroneutrality conditions, if the aliovalent charge of Fe3+ in reference to matrix Zr4+ is not compensated by ionic defects, such as oxygen vacancy (VO••) or protonic defects of OHO•. Accordingly, these holes are localized to the O2p orbital when the 3d−3d coulomb interaction energy, interpreted as barrier energy for the electron transfer from one transition metal cation to the neighboring transition metal cation, is higher than the ligand to metal charge transfer energy, corresponding to barrier energy for the electron to be transferred from the ligand to the transition metal cation.5,6 Then, a different 3d ground state configuration of 3d5L becomes stable. A similar interpretation is proposed by Abbate et al.,6 who provided the relationship between the broadening on Fe L3-edge XAS spectra and the change of the ground state configuration in La1−xSrxFeO3. Figure 3 shows the effect of crystal field symmetry and charge transfer on Fe3+ L-edge XAS spectra calculated with 10Dq = 1.8
Figure 2. (a) Fe L-edge XAS spectra of BaZr1−xFexO3−δ. (b) Magnification of L3-edge XAS spectra as a function of x in BaZr1−xFexO3−δ heat-treated at 800 °C under dry O2. Fe L-edge XAS spectrum of α-Fe2O3 is also shown as a reference. Figure 3. Calculated Fe L-edge XAS spectra using CTM4XAS software for different crystal field symmetry (Oh and D4h) with a charge transfer effect and 10Dq = 1.8 eV. CT means the charge transfer.
for LaFeO36 and α-Fe2O314, both of which have the same valence state of Fe3+. This result suggests that the valence state of Fe in BaZr1−xFexO3−δ is Fe3+ under an oxidizing atmosphere. It is known that the Fe L3-edge spectrum splits into t2g and eg states due to a crystal-field splitting effect, which is sensitive to the 3d ground state configuration.6,12,15 The Fe L3-edge XAS spectrum of α-Fe2O3 (Figure 2) exhibits a sharp peak splitting into multiplets (t2g and eg), and the energy difference between t2g and eg states (crystal field splitting, 10Dq) is about 1.8 eV. This experimental result agrees well with reported values for the Fe L-edge XAS spectrum of LaFeO3,6 which has a high spin 3d5 (t2g3eg2) ground state, indicating that the 3d ground state configuration of Fe3+ in α-Fe2O3 is mainly composed of the high-spin 3d5. The 10Dq value of Fe L3-edge XAS spectra for BaZr1−xFexO3−δ is also observed as being about 1.8 eV, which is the same with the one for α-Fe2O3. The Fe L3-edge XAS spectra of BaZr1−xFexO3−δ show a broadened feature, which is different from the α-Fe2O3 spectrum. This broadening is wellcharacterized by the reduction of feature A and the increase in features B and C with the increase of Fe concentration as shown in Figure 2b. First, feature A can be attributed to the variation of local Fe3+ symmetry. α-Fe2O3 exhibits a cubic octahedral crystal field symmetry of Oh (6A1g),14 showing a sharp peak splitting. This local symmetry may be slightly reduced in BaZr1−xFexO3−δ, because of the smaller ionic radius of Fe3+ than that of Zr4+ in the host ZrO6 octahedra, which introduces local distortion around Fe3+.16 Second, features B and C, which appear in the high photon energy region, can be
eV using the CTM4XAS software17 by the de Groot group. The L3-edge spectra simulated by Oh symmetry in spectra a and b in Figure 3 exhibit a sharp peak splitting into multiplets, whereas this feature becomes broadened upon the reduction of symmetry to tetragonal crystal field symmetry of D4h as shown in spectra c and d in Figure 3. The measured Fe L3-edge XAS spectra of BaZr1−xFexO3−δ are similar to the calculated one with D4h crystal field symmetry. In addition, the calculation that takes into consideration the charge transfer caused by the ligand-hole effect gives a much improved description for the broadened feature in the high photon energy region of L3-edge under D4h symmetry (Figure 3d). The results suggest that the local crystal field symmetry around doped Fe is reduced from cubic to tetragonal, and a new 3d ground state configuration of Fe3+, 3d5L, is formed by Fe doping in BaZrO3. 3.2.2. O K-Edge XAS Spectra. The O K-edge XAS spectra are related to O2p unoccupied states hybridizing with various metal cations’ orbitals.6,18,19 Hence, it is possible to evaluate the overall electronic structure of materials by the spectral analysis of O K-edge XAS. Figure 4 shows the O K-edge XAS spectra for BaZr1−xFexO3−δ annealed under oxidizing conditions at 800 °C. The O K-edge XAS spectrum for undoped BaZrO3 is composed of energy bands for unoccupied antibonding states 929
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Figure 4. O K-edge XAS spectra of BaZr1−xFexO3−δ heat-treated at 800 °C under an oxidizing atmosphere (O2). Features A, B, and C characterize the effect of Fe doping.
Figure 5. (a) O K-edge XAS spectra of BaZr1−xFexO3−δ (x = 0.3) heattreated at the different PO2’s of ∼1 × 10−17, ∼1 × 10−3, and 1 atm. Broken lines indicate the energy states of (A) acceptor level and (B) O2p level hybridizing with Fe3d. (b−d) Fe L-edge XAS spectra of BaZr1−xFexO3−δ (x = 0.1−0.3) oxidized or reduced samples.
of Zr4d and 5s and Ba5d and 6s orbitals hybridizing with O2p.20 The features at ∼533 and ∼537 eV are t2g and eg states of the Zr4d−O2p hybridizing orbital, respectively. The feature at ∼536 eV is associated with Ba5d, whereas the feature at ∼540 eV corresponds to Ba6s. The intensity values of ∼525 eV and ∼540 eV are used as references for the normalization of whole spectra of BaZr1−xFexO3−δ, due to the least chance of spectral overlap as well as a constant density of state (DOS) of Ba6s (∼540 eV) at a fixed Ba content in BaZr1−xFexO3−δ. The substitution of Fe3+ for Zr4+ in the cubic BaZr1−xFexO3−δ phase causes three remarkable changes in the O K-edge XAS spectra. For undoped BaZrO3, there are no pre-edge features but a strong feature C, which corresponds to the O2p state hybridizing with the Zr4d antibonding orbital. On the other hand, pre-edge features A and B appear upon Fe doping, and the intensity shows a positive Fe concentration dependence. In addition, the intensity of feature C decreases with increasing Fe concentration due to the decrease in Zr4d DOS, and the increase of Fe3d DOS. To confirm the origin of these pre-edge features A and B, we compared in Figure 5a the O K-edge XAS spectra of BaZr1−xFexO3−δ (x = 0.3) heat-treated at various oxygen partial pressures (PO2) ranging from 1 × 10−17 to 1 atm. The intensity of feature A for the lower energy part of the pre-edge region, which is significantly attenuated in reducing atmosphere (1 × 10−17 atm), is very sensitive to the variation of PO2 and steadily increases with increasing PO2. Because the feature is very sensitive to the redox reaction, it is reasonable to attribute the feature to the acceptor level (hole energy states) on the O2p orbital due to oxygen nonstoichiometry: A portion of VO•• introduced for charge compensation of doped Fe3+ is occupied by oxygen to form the acceptor level above the valence band maximum (VBM). On the other hand, feature B does not show any PO2 dependence, indicating that the origin is the inherent energy state formed by Fe doping. Therefore, this feature (B) is attributed to an antibonding state of O2p hybridizing with the Fe3d orbital. The origin of this feature is similarly explained in the previous studies for O K-edge XAS spectra of La1−xSrxFeO36 and α-Fe2O3,14 which suggests that the nature of Fe3+ in the octahedral site of BaZr1−xFexO3−δ is identical to those of Fe3+ in the same octahedral coordination in α-Fe2O3 and LaFeO3.
Figure 5b−d shows the Fe L-edge XAS spectra of BaZr1−xFexO3−δ (x = 0.1−0.3) for both the oxidized and reduced samples. No significant peak shape changes are observed by the variation of PO2, indicating that the valence state of Fe under oxidizing and reducing atmospheres is unchanged with keeping the same oxidation state of Fe3+. Broadening of the Fe L-edge XAS spectrum of x = 0.3 (Figure 5d) is attributed to the increased covalency as already discussed in Figures 2 and 3. No formation of new features by the introduction of holes is observed in the Fe L3-edge XAS spectra, whereas pre-edge feature A, assigned as the acceptor level, is observed in the O K-edge XAS spectra just below O2p−Fe3d hybridizing level. Because the holes formed by Fe doping are localized to the O2p orbital, the variation of Fe valence state from 3+ to neither 4+ (under oxidizing atmosphere) nor 2+ (under reducing atmosphere) is observed. These results are a clear evidence that the acceptor state is composed of the nonbonding state of the O2p orbital as discussed in the preceding section. Figure 6 shows simulation results of Fe L-edge XAS spectra for various valence states, calculated with 10Dq = 1.8 eV under the D4h symmetry. If the valence state of Fe is +2, the spectrum shifts to the lower photon energy side from the Fe3+ spectrum accompanied with a large change in peak shape. Further, if the valence state of Fe is +4, the spectrum shifts to higher photon energy with an intensive change in spectrum shape and peak ratios. These simulated results support the present conclusion that the valence state of Fe in BaZr1−xFexO3−δ under oxidizing and reducing atmospheres is always maintained at +3. Therefore, the concentration of holes on the O2p orbital exclusively governs the redox state and maintains the charge neutrality as mentioned above. Figure 7a shows the intensity of feature A for BaZr1−xFexO3−δ as a function of Fe concentration annealed at 800 °C under oxidizing and reducing atmospheres to estimate the variation of hole concentration. The result for the sample with x = 0.8 annealed under the reducing atmosphere is not 930
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conditions, because almost all the holes induced by Fe doping are replaced by VO•• to maintain the charge neutrality condition of [Fe′Zr] = 2[VO••] through the following reaction, expressed using Kröger−Vink notation. × OO + 2h• =
1 O2 (g) + V •• O 2
(1)
Figure 7b shows the intensity of feature B for BaZr1−xFexO3−δ as a function of Fe concentration annealed at 800 °C under oxidizing and reducing atmospheres to estimate the variation of DOS for the O2p−Fe3d hybridizing orbital. The intensity of feature B does not show a linear increment under an oxidizing atmosphere, due to the overlapping of features A and B. Under a reducing atmosphere, feature B shows an almost linear increment with increasing Fe concentration, due to less of an overlap effect from feature A than the one under oxidizing conditions, because of the reduction of hole concentration on the O2p orbital. Figure 8a shows the magnified view of feature A in O K-edge XAS spectra for x = 0.3 and 0.8 samples with schematic peak deconvolution. Feature A splits into two peaks marked by A-I and A-II, suggesting the presence of two different energy states of acceptor level on the O2p orbital. These two peaks become apparent with the increase of Fe concentration. The A-II peak heavily overlaps with feature B, corresponding to the O2p− Fe3d hybridizing level, resulting in a significant broadening. In addition, these two acceptor energy states are much clearly observed by the O K-edge analysis using a combined technique of XAS and resonant X-ray emission spectroscopy (RXES)21 and are attributed to the presence of two possible hole sites of oxide ions in different configurations. In the following discussion, we assume that the aliovalency of Fe3+ to Zr4+ is charge-compensated by holes ([Fe′Zr] = p) to exclude the effect of VO••. Figure 8b shows the schematic illustration for two arrangements of B-site cations and oxide ions, corresponding to feature A-I and A-II on a two-dimensional lattice. A-I indicates the oxide ion with a hole bridging between the FeO6 and ZrO6 octahedron forming Fe−O−Zr bonds in the {(FeO6)·h•}− (ZrO6) configuration, and A-II is the oxide ion with a hole bridging between two FeO6 octahedra to form Fe−O−Fe bonds in {(FeO6)·h•}−{(FeO6)·h•} configuration. Figure 8c shows the variation of probability for possible configurations of (i) FeO6−ZrO6, (ii) FeO6−FeO6, and (iii) ZrO6−ZrO6, under the assumption that FeO6 is randomly distributed. The concentration of i is higher than ii at x = 0.3, while the concentration of ii is higher than i at x = 0.8. In Figure 8a, peak A-II is greatly intensified with the increase of Fe concentration, which suggests that peak A-II corresponds to the holes in O2p of Fe−O−Fe bonding in the chained FeO6 octahedra ii, and peak A-I corresponds to the holes in the Fe−O−Zr bonding in the FeO6−ZrO6 octahedral configuration i. Holes are easily delocalized along the network of FeO6 connected chains, while holes are strongly localized on the bridging oxide ion between FeO6 and ZrO6 in the isolated FeO6 octahedral configuration. The possibility of finding holes on (ZrO6)−(ZrO6) is low due to the large activation energy required to delocalize holes by thermal activation from the strong Fe3+−(O2−·h•) bonding that acts as a hole trap. All these discussions are consistent with the occurrence of bond percolation in this material system.9 3.3. Defect Formation and Electronic Band Structure. The present results suggest that the redox state of BaZr1−xFexO3−δ is governed by the variable valence state of
Figure 6. Fe L-edge XAS spectra simulated by CTM4XAS for various valence states with 10Dq = 1.8 eV without the charge transfer effect under D4h symmetry.
Figure 7. Comparison of the intensity for (a) feature A and (b) feature B in O K-edge XAS spectra of BaZr1−xFexO3−δ heat-treated at 800 °C under an oxidizing or reducing atmosphere as a function of Fe concentration.
plotted, because of the decomposition by reduction. The intensity of feature A increases almost linearly with the increase of Fe concentration under an oxidizing atmosphere, indicating that the hole concentration is proportional to Fe concentration at a fixed PO2 and annealing temperature. On the other hand, no Fe concentration dependence is observed under reducing 931
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Figure 8. (a) Magnification of feature A in the O K-edge XAS spectrum of x = 0.3 and 0.8 annealed under an oxidizing atmosphere, (b) two possible hole configurations indicated by A-I and A-II, corresponding to holes on the isolated FeO6 octahedron and holes in chained FeO6 octahedra, respectively, and (c) variation of probability for each configuration: (i) Fe−O−Zr, (ii) Fe−O−Fe, and (iii) Zr−O−Zr with the increase of Fe concentration.
oxide ion (O2− to O−), or in other words, the formation of ligand hole of 3d5L, instead of variation of the Fe valence state from the +3 (3d5) to the +4 (3d4) state. Figure 9 shows the
conductivity is dominated by the partial hole conductivity under oxidizing conditions, due to much higher mobility of the holes in comparison with that of VO••, whereas the partial electronic conductivity dominates under reducing conditions. From the Brouwer diagram, although free electrons and electrons trapped by Fe′Zr (Fe3+) to form Fe″Zr (Fe2+) are expected to become major defect species under extremely reducing conditions, the Fe2+ state is not clearly identified under the present experimental conditions, possibly because of the instability of Fe2+ to fit into a small cavity of the octahedra. Meanwhile, the ionic radius of Fe2+ (under octahedral conditions) is slightly larger (0.78 Å) than those of Zr4+ (0.72 Å). Further discussion will be reported separately.10 In previous reports, the hole formation mechanism of La1−xSrxMO3 (M = Ti, Mn, Fe, and so on) has been well studied. The hole formation of these perovskite oxides is classified by the difference in two energy states of d−d Coulombic interaction energy (U) of the M cation and ligand to metal charge transfer energy (Δ). When U < Δ, holes are formed on the d orbital of the M cation,5,22 as reported for the La1−xSrxTiO3 system,23 in which holes are formed on the Ti3d orbital by Sr doping into the LaTiO3 matrix, corresponding to oxidation of the Ti ion from +3 (3d5) to +4 (3d4), or alternatively interpreted as an easier variation of the M-cation oxidation state due to the strong stability of O2−. When U > Δ, holes are formed on the oxygen site (on O2p) as in La1−xSrxFeO3 upon the substitution of La for Sr up to x = 0.5, which is interpreted as the oxidation of an oxide ion to a peroxide ion (O− (O2−·h•)) instead of an Fe ion (3+ (3d5)/4+ (3d4)).6 Similar behaviors are observed in La2NiO424 and La2CuO425. When U ≈ Δ, holes are formed on the mixed energy state of the d orbital of the M cation and the O2p orbital as in the case of the La1−xSrxMnO3 system,6 in which holes are introduced to the O2p−Mn3d hybridizing orbital upon the substitution of La for Sr. In the present BaZr1−xFexO3−δ system, U corresponds to the energy separation between the unoccupied Fe3d state observed as the pre-edge feature of the conduction band minimum (CBM) and occupied Fe3d state composing VBM, and Δ corresponds to the energy separation between the O− state and
Figure 9. Defect chemical model for the BaZr1−xFexO3−δ system estimated from the electrical conductivity and the oxygen nonstoichiometry.
Brouwer diagram to summarize the defect chemical model of BaZr1−xFexO3−δ estimated from the electrical conductivity and oxygen nonstoichiometery analysis.8 The charge compensation to the introduction of an acceptor dopant of Fe3+ is dominantly made either by oxygen vacancies or holes as [Fe′Zr] = 2[VO••] + p with maintaining a fixed valence state of Fe3+. Under oxidizing conditions, to compensate for doped Fe3+ ions on the Zr4+ site, Fe′Zr holes are formed on the O2p orbital to satisfy the charge neutrality condition of [Fe′Zr] = p. In such a situation, the concentration of minority defects of [VO••] is proportional to PO2−1/2. Under reducing conditions, the concentrations of the predominant defect of Fe′Zr and VO•• maintain the electroneutrality conditions of [Fe′Zr] = 2[VO••], and the hole concentration, p, is proportional to PO21/4. At the same time, electron concentration, n, increases with reducing of PO2, with PO2−1/4 dependence deduced from the intrinsic ionization reaction of 0 = h• + e′. The total electrical 932
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DOS of the acceptor level is very sensitive to both Fe concentration and oxygen activity, due to the variation of the predominant defect from holes at a high PO2 to oxygen vacancies at a lower PO2.
the occupied Fe3d orbital composing VBM. Apparently, the present analysis suggests U > Δ; in other words, the Fe3d occupied state is located below the unoccupied O2p level as observed in the present study, resulting in the stable formation of an acceptor level composed of an unoccupied O2p orbital. To summarize the discussion of the hole formation mechanism above, a schematic illustration of the band origin is proposed in Figure 10, which indicates a schematic
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ASSOCIATED CONTENT
S Supporting Information *
Phase analysis of BaZr1−xFexO3−δ and chemical stability of BaZr1−xFexO3−δ. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The present work has been supported by a Grant-in-Aid for Scientific Research (A) (23246112) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The authors appreciate financial support for one of the authors (D.Y.K.) from Global Center of Excellence for Mechanical Systems Innovation (GMSI), of School of Engineering, The University of Tokyo. The XAS experiments have been performed under the approval of the Photon Factory Program Advisory Committee (Proposal 2010G613).
Figure 10. Electronic band structure of BaZr1−xFexO3−δ under oxidizing atmosphere.
illustration of the electronic band structure for the BaZr1−xFexO3−δ system under an oxidizing atmosphere, based on the present results. The occupied nonbonding O2p energy states and the O2p−Zr4d hybridizing orbital compose the valence band of undoped BaZrO3. When Fe is doped into BaZrO3, occupied nonbonding O2p energy states and the O2p−Fe3d hybridizing orbital are introduced just below the native valence band of undoped BaZrO3. In addition, the acceptor levels (O−) with two different hole energy states with delocalized and localized configurations are formed above VBM, which correspond to an unoccupied nonbonding O2p energy state. These energy states of holes observed in XAS spectra include strong core−hole interaction and shift to the higher energy side. In addition, the t2g and eg modes of unoccupied antibonding states composed of the O2p−Fe3d hybridizing orbital is formed below the native CBM composed of the O2p−Zr4d orbital.
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REFERENCES
(1) Kuklja, M. M.; Kotomin, E. A.; Merkle, R.; Mastrikov, Yu. A.; Maier, J. Phys. Chem. Chem. Phys. 2013, 15, 5443. (2) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. J. Membr. Sci. 2008, 320, 13. (3) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Nat. Chem. 2011, 3, 546. (4) Konysheva, E.; Irvine, J. T. S. Chem. Mater. 2011, 23, 1841. (5) Cox, P. A. The Electronic Structure and Chemistry of Solids; Oxford Science Publications: Oxford, U.K., 1987; Chapter 6. (6) 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. Phys. Rev. B 1992, 46 (8), 4511. (7) Terki, R.; Feraoun, H.; Bertrand, G.; Aourag, H. Phys. Status Solidi B 2005, 242 (5), 1054. (8) Kim, D.; Miyoshi, S.; Tsuchiya, T.; Yamaguchi, S. ECS Trans. 2012, 45 (1), 161. (9) Kim, D.; Miyoshi, S.; Tsuchiya, T.; Yamaguchi, S. Solid State Ionics (Proc. SSI-19) 2013, Submitted. (10) Kim. D.; Miyoshi, S.; Tsuchiya, T.; Yamaguchi, S. 2013, to be submitted. (11) Van der Laan, G.; Kirkman, I. W. J. Phys.: Condens. Matter 1992, 4, 4189. (12) Tanaka, I.; Mizojuchi, T.; Yamamoto, T. J. Am. Ceram. Soc. 2005, 88 (8), 2013. (13) De Groot, F. M. F. J. Electron Spectrosc. Relat. Phenom. 1994, 67, 529. (14) De Groot, F. M. F.; Glatzel, P.; Bergmann, U.; van Aken, P. A.; Barrea, R. A.; Klemme, S.; Havecker, M.; Knop-Gericke, A.; Heijboer, W. M.; Weckhuysen, B. M. J. Phys. Chem. B. 2005, 109, 20751. (15) Miedema, P. S.; De Groot, F. M. F. J. Electron Spectrosc. Relat. Phenom. 2013, 187, 32. (16) Yoon, J.; Kim, D.; Oyama, Y.; Miyoshi, S.; Yamaguchi, S. 2013, to be submitted. (17) Stavitski, E.; De Groot, F. M. F. Micron 2010, 41, 687. (18) De Groot, F. M. F.; Grioni, M.; Fuggle, J. C.; Ghijsen, J.; Sawatzky, G. A.; Petersen, H. Phys. Rev. B 1989, 40 (8), 5715.
4. CONCLUSIONS The electronic structure of BaZr1−xFexO3−δ with various compositions annealed under oxidizing and reducing atmospheres has been investigated using XAS measurement. The Fe L-edge XAS spectra of BaZr1−xFexO3−δ annealed under oxidizing atmospheres exhibit Fe3+ with a 3d5L ground state rather than the 3d5 ground state caused by the increased bond covalency of the hybridizing orbital between Fe3d and O2p. The formation of pre-edge features on the O K-edge XAS spectra by Fe doping and their dependence on Fe concentration and oxygen activity leads to the conclusion that the lowest energy pre-edge feature corresponds to the acceptor level on O2p (O−), and the other feature corresponds to the unoccupied O2p state hybridizing with Fe3d, respectively. In addition, the acceptor level splits into two possible configurations of delocalized holes on the oxide ion of Fe−O−Fe in chained FeO6 octahedra and those on Fe−O−Zr bonding in the FeO6−ZrO6 octahedral configuration. Note that the oxidation state of BaZr1−xFexO3−δ is governed by the redox reaction of O2−/O−, whereas that of Fe3+ ions is unchanged. 933
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Article
(19) Grioni, M.; Czyzyk, M. T.; De Groot, F. M. F.; Fuggle, J. C.; Watts, B. E. Phys. Rev. B 1989, 39 (8), 4886. (20) Lupina, G.; Seifarth, O.; Dudek, P.; Kozlowski, G.; Dabrowski, J.; Thieme, H. J.; Lippert, G.; Schroeder, T.; Mussig, H. J. Phys. Status Solidi B 2011, 248 (2), 323. (21) Kim, D.; Tsuchiya, T.; Miyoshi, S.; Yamaguchi, S. 2013, to be submitted. (22) Zaanen, J.; Sawatzky, G. A.; Allen, J. W. Phys. Rev. Lett. 1985, 55 (4), 418. (23) Abbate, M.; De Groot, F. M. F.; Fuggle, J. C.; Fujimori, A.; Tokura, Y.; Fujishima, Y.; Strebel, O.; Domke, M.; Kaindl, G.; van Elp, J.; Thole, B. T.; Sawatzky, G. A.; Sacchi, M.; Tsuda, N. Phys. Rev. B 1991, 44 (11), 5419. (24) Kuiper, P.; van Elp, J.; Sawatzky, G. A.; Fujimori, A.; Hosoya, S.; de Leeuw, D. M. Phys. Rev. B 1991, 44 (9), 4570. (25) Fujimori, A.; Takayama-Muromachi, E.; Uchida, Y.; Okai, B. Phys. Rev. B 1987, 35 (16), 8814.
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