Photon Factory Activity Report 2017 #35 (2018)

BL-2A/2015G577, 2017G557, 2015S2-005

Electronic states of layered perovskite oxyfluoride Sr2RuO3F2 thin films by X-ray
absorption spectroscopy and theoretical calculations
Akira CHIKAMATSU1,*, Yuji KURAUCHI1, Makoto MINOHARA2,
Hiroshi KUMIGASHIRA2, and Tetsuya HASEGAWA2
1
Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
2
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan
1 Introduction
The electronic properties of strontium ruthenates canbe readily altered by anion doping. For example,Sr3Ru5+2O7F2, which was obtained by inserting F– ions
into the (SrO)2 rocksalt blocks in double-layered
Sr3Ru4+2O7 [1], exhibits antiferromagnetism with G-typeordering of the Ru5+ spins. More recently, Sr2RuO3F2 wassynthesized in the form of epitaxial thin films by thetopotactic fluorination of single-layered Sr2RuO4
precursor films using polyvinylidene fluoride (PVDF) [2].
In this reaction, both fluorine insertion into the (SrO)2blocks and the formation of oxygen vacancies in the O2−
sites of the RuO6 octahedra take place simultaneously,
while the oxidation state of Ru remains at 4+ after
fluorination. The Sr2RuO3F2 thin film exhibits insulating
behavior, with a resistivity of ~4.1 × 10 Ω cm at 300 K,
which is five orders of magnitude higher than that of themetallic Sr2RuO4 film (~6.7 × 10–4 Ω cm). To unveil theorigin of the physical properties of Sr2RuO3F2,
spectroscopic approaches to investigate the electronic
states of Sr2RuO3F2 are highly desirable. In this study, weinvestigated the electronic structure of a Sr2RuO3F2 thinfilm by X-ray absorption spectroscopy (XAS) and density
functional theory (DFT)-based calculations.
2 Experiment
Precursor Sr2RuO4 and fluorinated Sr2RuO3F2 thinfilms were prepared on LaAlO3 (001) substrates by pulsed
laser deposition and successive topotactic fluorination
using PVDF. The thicknesses of the Sr2RuO4 andSr2RuO3F2 thin films were ~100 nm and ~135 nm,respectively. X-ray diffraction measurements confirmedthat both the thin films exhibited a K2NiF4-type structurewith no impurity phases. XAS measurements wereconducted using linear polarized light at the BL-2A
beamline of Photon Factory, KEK. The XAS spectra weremeasured at 300 K and vacuum pressure of ~3×10-8 Pa
using the total electron-yield method.
DFT calculations were performed using the VASP
package [3]. We employed the GGA+U method to treat
Coulomb repulsion between the Ru 4d electrons [4,5].
The valence electronic states were calculated with a plane
wave basis set and the projector augmented wave
approach [6,7]. We assumed an antiferromagnetic spinarrangement on Ru atoms, taking into consideration theexperimental observation that Sr2RuO3F2 exhibited nospontaneous magnetization [2].

3 Results and Discussion
Figure 1 shows the partial density of states (PDOS) ofRu dxy, dyz + dzx, d3z2-r2, and dx2-y2 bands in Sr2RuO4 andSr2RuO3F2. Ru4+ oxides with octahedral geometry areusually a low-spin configuration (xy, yz, zx)↑↑↑↓, such asSrRuO3 and Ca2RuO4 [8,9], or non-spin-polarized (xy, yz,zx)4, such as Sr2RuO4 and RuO2 [10,11]. In thesecompounds, the fourth d electron occupies not a eg band
but a t2g band due to the large crystal field splitting
between the t2g and eg orbitals. Figure 1(a) shows PDOS
for Sr2RuO4. It depicts the (t2g)4 configuration as reported
in Ref. [12]. However, PDOS for Sr2RuO3F2 (Fig. 1(b))
suggests the Ru4+ exhibited a high-spin configuration(xy)↑(yz, zx)↑↑(3z2-r2)↑. This anomalous occupation of the(3z2-r2)↑ orbital would be due to the change in crystal
field around Ru atoms.
In order to examine the conduction band structure of
the Sr2RuO4 and Sr2RuO3F2 thin films, we measured theO 1s XAS spectra using polarized light. In thismeasurement, the O 1s electrons are excited into
unoccupied 2p orbitals hybridized with Ru 4d orbitals.
Due to the dipole selection rule, the direction of the 2p
orbitals to which the 1s electrons are transferred depends
on the angle between the incident beam and the surface
normal (θinc) [13]. When θinc is zero, 1s electrons aretransferred to the 2px or 2py orbital. The final state
gradually changes to 2pz as θinc increases up to 90º.
Therefore, the position of the XAS peak and its θincdependence indicate the energy position of the Ru 4dstates and the direction of the O 2p orbitals (px, py, and pz),respectively. Figure 2 exhibits the XAS spectra measured
at θinc = 0º, 30º, and 60º. The angular dependence of the O
1s XAS spectra could be clearly observed for both theSr2RuO4 and Sr2RuO3F2 films.
The results obtained for the Sr2RuO4 film are in good
agreement with those reported previously on a Sr2RuO4(001) single crystal prepared by in situ cleaving at 160 K[13]. The peak at 528.2 eV (structure A) decreased withincreasing θinc, while the peak at 529.2 eV (structure B)
considerably increased with θinc. According to Ref. [13],structure A is assigned to the 2px and 2py states of theapical oxygen hybridized with Ru 4dyz and 4dzx andstructure B is a combination of the 2px, 2py, and 2pz statesof the in-plane oxygen hybridized with the Ru 4d t2g
orbitals. The weight of A from the apical oxygen wasmuch smaller than that of B from the in-plane oxygen,reflecting twice smaller amount of the apical oxygen thanin-plane one. The structures C and D at ~531 eV andPhoton Factory Activity Report 2017 #35 (2018)533.1 eV are derived from Ru 4d with eg symmetry (d3z2-r2and dx2-y2) hybridized with both apical and in-plane O 2pσstates.

Fig. 1: PDOS for the Ru-4d electrons in (a) Sr2RuO4 and(b) Sr2RuO3F2. The lines in positive and negative regions
denote the PDOS of majority and minority of spinelectrons, respectively.
On the other hand, as seen in Fig. 2(b), the behavior of
the O 1s spectra of the Sr2RuO3F2 film was different fromthat of the Sr2RuO4 film. The peaks at 528.6 eV and 532.0
eV (structures E and H) were observed at θinc = 0º. With
increasing θinc, the intensities of E and H slightly
decreased and prominent peaks at 530.2 eV and 531.2 eV(structures F and G) evolved. Such θinc dependence
suggests that the peaks E and H arise from the Ru 4d
orbitals hybridized with the 2px, 2py, and 2pz orbitals of
the in-plane oxygen atoms and F and G arise from the 4dorbitals hybridized with the 2pz orbitals of the in-plane
oxygen. The weak peaks from the apical oxygen are
possibly hidden by the intense peaks from the in-plane
oxygen. The 4d orbitals responsible for F and G are
hybridized with the 2pz orbital in-plane oxygen only andthus they can be assigned to the Ru 4dyz and Ru 4dzx
orbitals. The Ru 4d orbitals related to the structures E andH are hybridized with all the 2px, 2py, and 2pz orbitals ofthe in-plane oxygen. The calculated PDOS for the Ru 4dstates shown in Fig. 2(b) suggests that the conduction
band minima, corresponding to peak E, are derived from4dyz, 4dzx, and 4dx2-y2 and that the highest 4d band (peakH) is attributable to 4dx2-y2. It should be noted that 4dx2-y2
can be hybridized with not only 2px and 2py but also the2pz orbital of the in-plane oxygen because the Ru atom isdisplaced toward the apical oxygen and the Oin-plane-RuOin-plane bond angle is not 180º.

Fig. 2: O 1s XAS spectra of (a) Sr2RuO4 and (b)Sr2RuO3F2 films. The spectra are shown at θinc = 0º, 30º,and 60º; θinc is the angle between the incident beam andthe surface normal.
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* chikamatsu@chem.s.u-tokyo.ac.jp