Photon Factory Activity Report 2019 #37 (2020)

BL-2A/2017G557, 2018S2-004, 2019G544

Electronic structures of perovskite-type strontium chromium oxyfluoride thin films
Akira CHIKAMATSU,1,* Takahiro MARUYAMA,1 Tsukasa KATAYAMA,1 Yu SU,2Yoshihiro TSUJIMOTO,2,3 Kazunari YAMAURA,2,3 Miho KITAMURA,4 Koji HORIBA,4Hiroshi KUMIGASHIRA,4,5 and Tetsuya HASEGAWA1
1Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Research Center for Functional Materials, NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
3Graduate School of Chemical Sciences and Engineering, Hokkaido University, North 13 West 8, Kita-ku,
Sapporo 060-0808, Japan
4
Institute of Materials Structure Science, KEK, 1-1 Oho, Tsukuba, Ibaraki 305–0801, Japan
5
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku,
Sendai 980–8577, Japan
1 Introduction
Perovskite-type chromium oxides exhibit unique
physical properties due to the strong Cr 3d–O 2p
interaction. A typical example is SrCrO3 containing Cr4+(3d2), which exhibits antiferromagnetic metallic behaviors
because its transport property lies in the crossover region
between itinerant and localized carriers [1,2]. The physicalproperties of perovskite-type chromium oxides can bereadily modified through topotactic fluorination because
insertion or substitution of F- ions provides additionalholes or electrons to the Cr ions, respectively [3,4]. In thisstudy, we investigated the electronic structures ofstrontium chromium oxyfluoride thin films by X-ray
photoemission (XPS) and absorption spectroscopies(XAS), and discussed them in comparison with those ofSrCrO2.8 and SrCrO3 thin films.
2 Experiment
Precursor SrCrO2.8 thin films were deposited on LaAlO3(001) substrates by pulsed-laser deposition. The obtained
SrCrO2.8 thin films were further subjected to fluorination
using polyvinylidene fluoride at 250 °C for 12 h in Ar gas
flow. Oxidized films were prepared by annealing theSrCrO2.8 precursor thin films at 250 °C for 2 h in air. The
typical thickness of the films was ~50 nm.
Crystal structures of the films were investigated by Xray diffraction, and chemical compositions were evaluatedby energy dispersive X-ray analysis. Cr 2p and valenceband XPS spectra were measured at 300 K using a VGSCIENTA SES-2002 electron-energy analyzer with anenergy resolution of 300 meV at a photon energy of 1200
eV. The Fermi level of the samples was set as that of an insitu evaporated gold foil which was in electrical contact
with the sample. The O K-edge XAS spectra weremeasured at 300 K by the total-electron-yield method.
3 Results and Discussion
In order to examine the changes of the valence numbersof Cr by the oxidation and fluorination of the Sr2IrO4 thinfilms, we performed Cr L-edge XAS measurements, asshown in Fig. 1. It is known that the shape of Cr L-edge
XAS spectrum of Cr4+ is very different from that of Cr3+
and that the valence number of Cr can be roughly estimatedfrom the linear combination of Cr4+ and Cr3+ XAS spectra[5]. As seen in Fig. 1, the positions of the highest L3 and L2
peaks (578.7 eV and 587 eV) of the oxidized film werealmost the same as those of the SrCrO3 reference,indicating that the oxidized film had Cr4+. Thus, wedetermined the chemical formula of the oxidized film asSrCrO3. Note that the peak at ~582 eV was not observed in
the oxidized film, which may be due to a change in Cr-O
bond lengths associated with lattice strain and/or degradedcrystallinity. On the other hand, the valence numbers of Crin both precursor and fluorinated films were estimated tobe 3.6 ± 0.1 by linearly combining the spectra of theoxidized film (Cr4+) and LaCrO3 reference [6] (Cr3+).
Assuming x + y = 3 in SrCrOxFy, the chemical compositionof the fluorinated film can be estimated as SrCrO2.6F0.4.
Note that our fluorinated film had a higher fluorine content
than polycrystalline bulk sample, SrCrO2.8F0.2 [4],probably reflecting a higher reactivity of thin films thanbulk because of larger surface areas and smaller volumes.

Fig. 1: Cr L-edge XAS spectra of the SrCrO2.8 precursor
film (blue line), the oxidized film annealed at 250 °C in air(green line), and the film fluorinated with PVDF at 250 °C(red line). The reference spectrum of the LaCrO3 (Cr3+) [6]is also shown in the figure.

Photon Factory Activity Report 2019 #37 (2020)

The electronic structures of the valence and conduction
bands were probed by synchrotron XPS and O K-edge
XAS measurements. Fig. 2(a) shows the valence-band
XPS spectra of the SrCrO2.8, SrCrO3, and SrCrO2.6F0.4
films. The valence-band spectrum of the SrCrO2.8 film
mainly consists of three structures, labeled as B, C, and D.
According to previous density functional theory (DFT)
calculations and XPS measurements [2], the structures B,C, and D can be assigned as incoherent parts of the Cr 3dt2g, O 2p non-bonding, and O 2p bonding bands,respectively. In the SrCrO3 film, the structures C and D are
rigidly shifted to lower binding energies by 0.25 eV,thereby representing the chemical-potential shiftassociated with electron doping. Near the Fermi level (EF),as shown in the inset of Fig. 2(a), the structure B was
suppressed and a coherent part of Cr 3d t2g (structure A)
evolved owing to the strong electron correlation effects,
similar to other strongly correlated metallic transition
metal oxides, such as SrVO3, La1-xSrxMnO3, and Ca1xSrxRuO3 [7-9]. On the other hand, in the SrCrO2.6F0.4 film,
a gap opened at EF and the structure B was enhanced,
which was consistent with the insulating behavior of theSrCrO2.6F0.4 film. The positions of the structures C and D
were the same as those of the SrCrO2.8 film, which wasconsistent with the XAS results, showing that SrCrO2.8 andSrCrO2.6F0.4 films had the same Cr valence numbers. Inaddition, a new spectral structure E appeared around thebinding energy of 7−10 eV. Considering higher
electronegativity of fluorine than oxygen, the structure E is
assignable to the F 2p bonding states.
Fig. 2(b) shows the O K-edge XAS spectra of theSrCrO2.8, SrCrO3, and SrCrO2.6F0.4 films measured using
linearly polarized light, where the angle between theincident beam and the surface normal (θinc) was set at 60°.
The O K-edge XAS spectra correspond to transitions into
unoccupied O 2p states hybridized with Cr 3d and Sr 4d.
As seen in Fig. 2(b), the peaks at 527–534 eV and those at534–540 eV can be assigned to the Cr 3d-derived states(structures F, G, H, and I) and Sr 4d-derived states,
respectively, by comparing the O K-edge XAS spectrum ofthe SrCrO3 film [6]. Referring to the reported DFT theory
calculations of SrCrO3 and CrO2 [2,10], we assigned thestructure F to excitations of the unoccupied t2g ↑ band, thestructures G and H to the combination of unoccupied eg ↑and t2g ↓, and the structure I to the unoccupied eg ↓ band.
The structure F of the SrCrO2.6F0.4 film is located at 528.3
eV, which is 0.3 eV higher than that of the SrCrO3 film.
This suggests spectral weight transfer from the coherent
part to the incoherent part with fluorination due to strong
electron correlation effects. Moreover, the peak intensityof the structure F of the SrCrO2.6F0.4 film was smaller thanthat of the SrCrO3 film. This is mainly due to the decreasein Cr valence number, as seen in the case of La1-xSrxCrO3
thin films [6]. On the other hand, the SrCrO2.6F0.4 film
showed a larger intensity than the SrCrO2.8 film, although
the Cr valence numbers were the same. This may be due tolocal distortion of Cr-O bonds, because XAS using linearlypolarized light is sensitive to the angle between theincident beam and the axes of O 2p orbitals due to thedipole selection rule [11].

Fig. 2: (a) Valence-band XPS spectra of the SrCrO2.8 (blueline), SrCrO3 (green line), and SrCrO2.6F0.4 films (red line).
The inset of (a) shows the valence-band spectra close to EF,
along with the spectrum from Au foil for energy calibration
purposes (gold line). (b) O K-edge XAS spectra of theSrCrO2.8 (blue line), SrCrO3 (green line), and SrCrO2.6F0.4films (red line) measured at θinc = 60°.
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* chikamatsu@chem.s.u-tokyo.ac.jp