Photon Factory Activity Report 2014 #32 (2015) B

BL-2A/2013S2-002

Electronic Structure of Nd1-xSrxFeO3 Probed by Soft-X-Ray Spectroscopy
Tohru Higuchi,1* Naoya Suzuki,1 Makoto Minohara,2 Masaki Kobayashi2 and Hiroshi Kumigashira2
1Department of Applied Physics, Tokyo University of Science, Tokyo 125-8585, Japan2
Photon Factory, KEK, Tsukuba 305-0801, JapanThe electronic structure of Nd1-xSrxFeO3 (x=0, 0.1 and 0.4) has been studied by soft-X-ray spectroscopy. The Nd1has the mixed valence states of Fe3+ and Fe2+. The intensity of hole-induced state at the bottom of theconduction band increases with increasing Sr concentration. The valence band is mainly composed of the O2p statehybridized with the Fe 3d state. The resonant photoemission spectra of Nd0.9Sr0.1FeO3 exhibit the eg-subband of Fe 3dstate at near EF, which is closely related to the electrical conductivity at room temperature.
xSrxFeO3

1 Introduction
Sr-doped NdFeO3 (Nd1-xSrxFeO3) with perovskite
structure is expected as a p-type thermoelectric material
due to its high chemical stability in air. The electricalconductivity for the oxygen gas partial pressure exhibits
hole-ion mixed conduction, which needs for electrode ofsolid state oxide fuel cell (SOFC), at high temperature
region [1]. The mixed conduction is useful for theperformance of SOFC. However, details of structural andelectrical properties have not been clarified far.
In this study, the authors have prepared the Nd1xSrxFeO3 bulk ceramics and probed their electronic
structures by X-ray absorption spectroscopy (XAS) andhigh-resolution photoemission spectroscopy (PES). The
Nd1-xSrxFeO3 has hole and oxygen vacancies in the crystal
lattice by Sr substitution. In this paper, the authors
discuss about the valence state of Fe, oxygen vacancies
and electronic structures of the valence band for Nd1xSrxFeO3 (x=0, 0.1, 0.2, and 0.4) in comparison withelectrical conductivity.
2 ExperimentNd1-xSrxFeO3 (x=0, 0.1, 0.2, and 0.4) bulk ceramics
were prepared by the solid-state reaction method. Nd2O3,SrCO3 and Fe2O3 powders were mixed using a wet ballmill. The mixture was pressed into a disk shape at 14 kNand sintered for 24 h at 1300°C. The crystallization of theprepared target was confirmed using X-ray diffraction(XRD) using CuKα.
The electronic structure was confirmed byphotoemission spectroscopy (PES) and X-ray absorption
spectroscopy (XAS), which was installed at the undulator
beamline BL-2A in Photon Factory, KEK. The XAS
spectra were measured by total-electron-yield mode. The
kinetic energy of photoelectron for PES was measured
using Gammadata-Scienta SES-2002.
The energy
resolutions of PES and XAS were 100 meV and 60 meV,
respectively, at hv=700 eV. The energy axis wascalibrated by measuring Au film evaporated onto themanipulator in all measurements.
3 Results and Discussion
Table 1 shows peak positions of (110), (112) and (202)for Sr concentration estimated from XRD patterns. The(110) and (112) peaks shifts to higher diffraction angle

side and (202) peak shifts to lower diffraction angle sidewith increasing Sr concentration. This result indicatesthat the lattice constant of a-axis increases and those of band c-axes decrease with Sr concentration.
Table 1 Peak positions of (110), (112) and (202) for theXRD patterns
x=0
x=0.1
x=0.2
x=0.4(110)22.78 °
22.86 °
22.88 °
22.90 °

(112)
32.50 °
32.54 °
32.58 °
32.64 °

(202)
40.36 °
40.32 °
40.26 °
40.20 °

Fig.1 Temperature dependence of electrical resistivityof Nd1-xSrxFeO3 (x=0, 0.1, 0.2, and 0.4).

Figure 1 shows the temperature dependence ofelectrical resistivity of Nd1-xSrxFeO3.
The NdFeO3
exhibits insulator-like behavior. The resistivities of x=0.1
and 0.2 decrease with decreasing temperature between
400 and 1000°C. However, those increase below 400°C.
On the other hand, the resistivity of x=0.4 decreases withdecreasing temperature, which corresponds to the metallicbehavior. The resistivity of Nd1-xSrxFeO3 is lower thanthat of La1-xSrxFeO3.
Figure 2 shows the Fe 2p XAS spectra of Nd1-xSrxFeO3.
Each spectrum consists of two parts derived from thespin-orbit split of L3 (2p3/2) and L2 (2p1/2) states. They arePhoton(2015)BBPhoton Factory
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Fig. 2 O 1s XAS spectra of Nd1-xSrxFeO3 (x=0, 0.1, 0.2,and 0.4).

Fig. 4 Fe 2p resonant PES spectra of Nd0.9Sr0.1FeO3
measured at various excitation energies of Fig. 3.

O 2p state and Fe 3d (t2g) state. The on.2 spectrum
corresponding to eg-resonance exhibits strong resonanceeffect at peak α at near the Fermi level (EF). This result
indicates that the eg-subband of Fe 3d state is closely
related to the electrical conductivity of Nd0.9Sr0.1FeO3.

Fig. 3 Fe 2p XAS spectra of Nd1-xSrxFeO3 (x=0, 0.1,0.2, and 0.4).

further split into the t2g and eg states due to the octahedral
ligand field.
The crystal-field splitting (10Dq)
corresponding to the energy separation between t2g and eg
states is 1.5 eV. The spectral shape and peak position are
similar to those of La1-xSrxFeO3, which was calculated
assuming a high-spin [t2g]3[eg]2 ground state [2]. The
intensity of t2g-subband increases with Sr concentration,
indicating that the doped Sr ions are doped as hole.
Figure 3 shows the O 1s-XAS spectra of Nd1-xSrxFeO3.
The O 1s XAS of Fe oxide corresponds to the transition
from O 1s core level to unoccupied O 2p state hybridized
with Fe 3d state. The conduction band is manly
composed of Fe 3d state. Hole or oxygen vacancies
induced state (h) is observed at the bottom of theconduction band. The intensity of the structure increases
with Sr concentration.
Figure 4 shows the resonant-PES spectra of
Nd0.9Sr0.1FeO3 measured at various excitation energies.
The off-resonance spectrum reflects the O 2p states
without Fe 3d state. The intensity of on.1 spectrum
corresponding to t2g-resonance is enhanced in the valenceband region, indicating the hybridization effect between

4 SummaryWe have studied the electrical resistivities andelectronic structures of Nd1-xSrxFeO3. The lattice constantof a-axis increases and those of b- and c-axes decrease
with Sr concentration. The Fe has the mixed valence
states of Fe2+ and Fe3+. The hole-induced state with Sr
doping is observed at the bottom of the conduction band.
The valence band is mainly composed of t2g- and egsubbands of Fe 3d states hybridized with O 2p states. The
electrical conductivity of Nd0.9Sr0.1FeO3 is achieved bythe eg-subbnad at EF.
References[1] K. Kobayashi, S. Yamaguchi, T. Higuchi, S. Shin, T.
Tsukamoto and T. Tsunoda, Electrochemistry 72(2004) 870.
[2] H. Wadati, A. Chikamatsu, M. Takizawa, R.
Hashimoto, H. Kumigashira, T. Yoshida, T. Mizokawa,
A. Fujimori, M. Oshima, M. Lippmaa, M. Kawasaki,and H. Koinuma, Phys. Rev. B 74 (2006) 115114.
Research Achievements
1. T. Higuchi, T. Owaku, Y. Iida, E. Sakai, M. Kobayashi
and H. Kumigashira; Solid State Ionics 270 (2014) 1.
2. S. Yamaguchi, Y. Tasaki, M. Kobayashi, K. Horiba, H.
Kumigashira, and T. Higuchi; Jpn. J. Appl. Phys. 54(2015) 06FJ04.
3. Y. Shimazu, T. Okumura, T. Tsuchiya, A. Shimada, K.
Tanabe, M. Kobayashi, K. Horiba, H. Kumigashira andT. Higuchi; J. Phys. Soc. Jpn. 84 (2015) 064701.
* higuchi@rs.kagu.tus.ac.jp