Article

The Local Structure of the BiS2 Layer in RE(O,F)BiS2 Determinedby In-Plane Polarized X-ray Absorption Measurements
G. M. Pugliese 1 , L. Tortora 1 , E. Paris 1,2 , T. Wakita 3 , K. Terashima 3,4 , A. Puri 5 , M. Nagao 6 , R. Higashinaka 7 ,T. D. Matsuda 7 , Y. Aoki 7 , T. Yokoya 3 , T. Mizokawa 8 and N. L. Saini 1, *
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Citation: Pugliese, G.M.; Tortora, L.;
Paris, E.; Wakita, T.; Terashima, K.;
Puri, A.; Nagao, M.; Higashinaka, R.;
Matsuda, T.D.; Aoki, Y.; et al. The
Local Structure of the BiS2 Layer inRE(O,F)BiS2 Determined by In-Plane
Polarized X-ray Absorption
Measurements. Physchem 2021, 1,
250–258. https://doi.org/10.3390/
physchem1030019
Academic Editors: Jacinto Sá andSergei Manzhos*

Department of Physics, Sapienza University of Rome, P. le Aldo Moro 2, 00185 Roma, Italy;
gianmarco.pugliese@uniroma1.it (G.M.P.); lorenzo.tortora@uniroma1.it (L.T.); eugenio.paris@psi.ch (E.P.);
Photon Science Division, Swiss Light Source, Paul Scherrer Institut, 5232 Villigen, Switzerland
Research Institute for Interdisciplinary Science (RIIS), Okayama University, Okayama 700-8530, Japan;
wakita@cc.okayama-u.ac.jp (T.W.); TERASHIMA.Kensei@nims.go.jp (K.T.); yokoya@cc.okayama-u.ac.jp (T.Y.)
National Institute for Materials Science, Sengen 1-2-1, Tsukuba 305-0047, Japan
CNR-IOM-OGG c/o ESRF—The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France;
puri@esrf.fr
Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu 400-8511, Japan;
mnagao@yamanashi.ac.jp
Department of Physics, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji 192-0397, Japan;
higashin@tmu.ac.jp (R.H.); tmatsuda@tmu.ac.jp (T.D.M.); aoki@tmu.ac.jp (Y.A.)
Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan; mizokawa@waseda.jp
Correspondence: naurang.saini@roma1.infn.it

Abstract: We have investigated the local structure of BiS2 -based layered materials by Bi L3 -edge
extended X-ray absorption fine structure (EXAFS) measurements performed on single crystal samples
with polarization of the X-ray beam parallel to the BiS2 plane. The results confirm highly instable nature of BiS2 layer, characterized by ferroelectric like distortions. The distortion amplitude, determinedby the separation between the two in-plane (Bi-S1) bonds, is found to be highest in LaO0.77 F0.23 BiS2
with ∆R∼0.26 Å and lowest in NdO0.71 F0.29 BiS2 with ∆R∼0.13 Å. Among the systems with intrinsic
doping, CeOBiS2 shows smaller distortion (∆R∼0.15 Å) than PrOBiS2 (∆R∼0.18 Å) while the highest
distortion appears for EuFBiS2 revealing ∆R∼0.22 Å. It appears that the distortion amplitude is controlled by the nature of the RE(O,F) spacer layer in the RE(O,F)BiS2 structure. The X-ray absorption
near edge structure (XANES) spectra, probing the local geometry, shows a spectral weight transfer
that evolves systematically with the distortion amplitude in the BiS2 -layer. The results provide aquantitative measurements of the local distortions in the instable BiS2 -layer with direct implicationon the physical properties of these materials.

Received: 12 October 2021
Accepted: 2 November 2021

Keywords: layered materials; local structure; X-ray absorption spectroscopy

Published: 10 November 2021

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1. Introduction
Layered materials have been a subject of intense research during these decades due
to possibility of control and manipulation of their physical properties to obtain desired
functions. The discovery of high Tc superconductivity in layered copper oxides [1] had been
one of the major breakthroughs and since then a large number of new layered materials
have been found to show unconventional superconductivity. The research in the field
was further fuelled by the discovery of superconductivity in the layered iron arsenides [2]resulting in several new layered systems. Among others, BiS2 -based systems [3,4] havereceived increasing attention during the last decade both for their superconducting andthermoelectric properties, leading to numerous theoretical and experimental studies tounderstand their functional properties [5–8]. The most studied BiS2 -based systems havebeen RE(O,F)BiS2 (RE = rare earth element) in which twin BiS2 -layer is alternated by aRE(O,F) spacer layer in the crystallographic unit cell. The characteristic transport properties

Physchem 2021, 1, 250–258. https://doi.org/10.3390/physchem1030019

https://www.mdpi.com/journal/physchemPhyschem 2021, 1

251of these materials are generally controlled by manipulating the electronic structure andcrystal structure in different ways [5–7]. The parent REOBiS2 is semiconducting andsubstitution of O2− by F1− in the REO layer introduces electrons in the BiS2 -layer that
leads to superconductivity at low temperature with transition temperature dependingon the nature of the spacer layer as well on the growth conditions [5–7]. The highest
superconducting transition temperature Tc ∼10.5 K has been found in high pressure grown
LaO1− x Fx BiS2 system.
Beyond electron doping by substitution in the REO spacer layer, the superconductivity in these materials has been manipulated by physical and chemical pressure. For
example, isovalent substitutions in the REO-layer by RE3+ and in the BiS2 -layer by Se2−
(at of S2− ) have been exploited to optimise the transition temperature and the nature of
the superconducting properties of these materials through the chemical pressure [9]. The
isovalent substitution of S2− by Se2− in undoped LaOBiS2 has also been used to manipulate
the thermoelectric characteristics resulting a quality factor zT as high as ∼0.36 at 650 K
in LaOBiSSe [10,11]. Besides, it has been found that the mixed valent rare earth ion in
the REO spacer layer can be a source of electron doping in the BiS2 layer. This is known
as self-doping, that is of particular interest since no external substitution is required andthe self-doped system is free from any substitutional disorder. Indeed, self-doped superconductivity in EuFBiS2 is known to occur due to mixed valence of Eu (Eu2+ /Eu3+ ) [12].
Similarly, CeOBiS2 has been found to show superconductivity due to self-doping introduced by the Ce3+ /Ce4+ mixed valence [13]. The examples also include other BiS2 -based
superconductors in which RE mixed valence provides the self-doping [14–18] including in
PrOBiS2 [19].
One of the peculiar properties of the BiS2 -based superconductors is their structural
instability. Indeed, BiS2 -layer in these materials has been found to be intrinsically instable [20–22] in the structure driven by the local Bi3+ defect chemistry and hence their
physical properties can be easily manipulated by chemical and/or physical pressures.
Experimental techniques sensitive to the local structure have revealed ferroelectric likedistortions of BiS2 -layer to be prevalent that varies from system to system [23–26]. Furthermore, the local structure studies have also revealed the axial sulfur position with respect to
Bi to play an important role in the physical properties [26,27]. Incidentally, all these local
structural studies have been performed on polycrystalline samples and considering thehighly susceptible nature of the BiS2 -layer under physical and/or chemical pressures aquantitative estimation of the local distortions can be challenging as it may depend on thesample morphology and growth conditions.
Here, we have measured the local structure of a series of RE(O,F)BiS2 (RE = La,
Eu, Pr, Ce, Nd) using single crystal samples to quantify the local distortions in the instable BiS2 -layer. For the purpose, we have studied two samples (LaO0.77 F0.23 BiS2 andNdO0.71 F0.29 BiS2 ) in which the doping is introduced by substitution while three samples
are self-doped (EuFBiS2 , CeOBiS2 and PrOBiS2 ). The samples were chosen on the basis of
their superconducting transition temperature at optimum doping and their crystal quality.
The optimum doping in the LaO1− x Fx BiS2 and NdO1− x Fx BiS2 corresponds to x∼0.25. The
crystal quality was the factor for slightly different doping contents in the two systems but
such a small difference hardly affects the transition temperature and the local structure. We
have exploited Bi L3 -edge extended X-ray absorption fine structure measurement (EXAFS)
with polarization of the X-ray beam parallel to the BiS2 -layer. The study has been carriedout at low temperature (∼20 K) on five different systems confirming that the BiS2 -layerof these materials is characterized by ferroelectric or quasi-ferroelectric like distortions ofwhich magnitude depends on the nature of the RE(O,F) spacer layer. The single crystalof LaO0.77 F0.23 BiS2 reveals largest distortion with ∆R∼0.26 Å, while NdO0.71 F0.29 BiS2 ischaracterized by the smallest distortion (∆R∼0.13 Å). Among self-doped systems, EuFBiS2
with EuF-spacer layer reveals distortion amplitude ∆R∼0.22 Å, substantially larger than the
one in CeOBiS2 (∆R∼0.15 Å) and PrOBiS2 (∆R∼0.18 Å) with REO-spacer layer. The X-ray
absorption near edge structure (XANES) spectra reveal a differing spectral weight that de-Physchem 2021, 1

252

pends on the distortion amplitude. We find that the local distortions amplitude measured
on single crystal samples is systematically lower than the one reported on polycrystalline
samples. The results provide a quantitative measurements of the local distortions in the
instable BiS2 -layer of these materials.
2. Materials and Methods
Single crystals samples of LaO0.77 F0.23 BiS2 , NdO0.71 F0.29 BiS2 and self-doped EuFBiS2 ,CeOBiS2 and PrOBiS2 were grown by CsCl-based flux method [28–30]. A single-crystal
X-ray analysis was performed revealing the average crystal structure to have the tetragonal
space group P4/nmm with lattice parameters shown in Table 1. The samples were wellcharacterized for their physical properties prior to the X-ray absorption measurements
at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The samples studied were all superconducting (except EuFBiS2 ) showing zero resitivity below
the transition temperature Tc shown in Table 1. Details on sample synthesis, growthand characterization are reported elsewhere [28–30]. Bi L3 -edge (13418 eV) X-ray absorption experiments were carried out in the normal incidence geometry in which the X-ray
polarization is parallel to the ab-plane of the flat single crystal samples. Four samples(LaO0.77 F0.23 BiS2 , NdO0.71 F0.29 BiS2 , EuFBiS2 and CeOBiS2 ) were measured at the beamline
BM23 while PrOBiS2 was measured at BM08 using similar experimental set-up obtaining
the X-ray absorption spectra in the fluorescence mode. In-plane polarized X-ray beam
was monochromatized by a double crystal Si(111) monochromator and a 13-element Ge
detector system was used for the fluorescence detection. A continuous flow He cryostat
was used for the sample cooling and the measurements were carried out at 20 K with thesample temperature control within ±1 K during the measurements. Four to five absorptionscans were obtained on each sample for the spectral reproducibility. Standard procedure
based on the spline fits [31,32] was used to extract EXAFS oscillations from the measured
X-ray absorption spectra exploiting ATHENA software [33].
Table 1. Lattice parameters of the tetragonal unit cell (Space group P4/nmm) of different compounds.
The transition temperatures (Tc ) and in-plane distortion amplitudes (∆R) are also included. ∆Rsc represents the separation between the two peaks in the pair distribution function derived in the present
work while ∆R p corresponds to the distortion amplitudes reported on polycrystalline samples.

LaO0.77 F0.23 BiS2
EuFBiS2
NdO0.71 F0.29 BiS2
CeOBiS2
PrOBiS2

Tc (K)

a (Å)

c (Å)

∆Rsc (Å)∆R p (Å)

2.5
5.1
1.3
3.5

4.057
4.051
3.996
3.984
4.010

13.54713.56013.46413.49013.824

0.26
0.22
0.13
0.15
0.18

0.40
0.26
0.18
0.19
0.21

3. Results and Discussion
Figure 1 shows the Bi L3 -edge EXAFS oscillations extracted from the in-plane polarized
X-ray absorption spectra measured at 20 K on the single crystal samples of LaO0.77 F0.23 BiS2 ,EuFBiS2 , PrOBiS2 , CeOBiS2 and NdO0.71 F0.29 BiS2 . The EXAFS oscillations are multiplied by
k2 to enhance the spectral amplitude at higher k. The differing EXAFS oscillations indicatethat the local structure has strong rare-earth dependence in RE(O/F)BiS2 . The crystal unit
cell of RE(O/F)BiS2 is also included for a ready reference. Large differences in the EXAFS
oscillations indicate that the local structure evolves significantly with the RE(O/F)-layer.
The oscillations for k ≤ 8 Å−1 are largely damped in LaO0.77 F0.23 BiS2 and EuFBiS2 due tolarger distortions involving nearest neighbors sulfur atoms around the photoabsorbing
Bi (i.e., Bi-S1 bonds). Incidentally, self-doped systems (EuFBiS2 , PrOBiS2 , CeOBiS2 ) show
relatively damped oscillations for k ≥ 10 Å−1 , likely to be related with mixed valence of
the RE atoms since the higher k-oscillations are sensitive to the heavier atoms.

Physchem 2021, 1

253

O/F

2

CeOBiS

S1

2

S1

O/F
O/F

S2

Bi

O/F

RE

S1

RE

S1

2

k χ(k)

Nd(O,F)BiSREREPrOBiS

2

Bi

Bi

S1

Bi

Bi

EuFBiS

2

La(O,F)BiS

4

2

6

8

10

12

14

-1

k (Å )

Figure 1. EXAFS oscillations of RE(O/F)BiS2 single crystals samples (RE = La, Eu, Pr, Ce, Nd)
measured at Bi L3 -edge with X-ray polarization parallel to the crystallographic ab-plane. The
oscillations are weighed by k2 to amplify the signal at higher k-values and vertically shifted for abetter visualization. The crystal unit cell of RE(O/F)BiS2 is also shown.

The Fourier Transform (FT) magnitude of the k2 -weighted EXAFS oscillations is shownin Figure 2, providing real-space information on the local atomic distribution around thephotoabsorbing Bi. In the RE(O/F)BiS2 structure, the Bi atom is coordinated with one
axial sulfur atom (S2) and one sulfur of the twin BiS2 layer, however, their contributions
are minimum in the EXAFS measured with polarization parallel to the crystallographic
ab-plane. Therefore, the nearest neighbor contribution, at 2–3 Å, is mainly coming fromthe four in-plane sulfur atoms (S1) since the polarization vector of the X-ray beam ê k abplane [32,34]. The contribution appears as peak structures in the FT magnitude at ∼2.5 Å.
The peak structure ∼4 Å is mainly due to the Bi-Bi bonds within the plane, representing
the in-plane lattice parameter in RE(O/F)BiS2 structure. Apparently the Bi-Bi peak shifts
and follows the in-plane lattice behavior in the tetragonal RE(O/F)BiS2 structure. The
lattice parameters of the studied crystals are summarized in Table 1 [28–30,35]. The distantpeaks beyond ∼4 Å include contribution of Bi-RE and diagonal Bi-Bi distances including
also multiple scattering contributions.
Here the aim is to quantify the distortions in the BiS2 -layer and for the purpose thecontribution of Bi-S1 was Fourier filtered in the R-window (1.5–3.5 Å) shown as shadowed
region in Figure 2. The shapes of filtered EXAFS oscillations, shown in Figure 3 (inset),
are significantly different between them indicating that the local Bi-S1 distribution in
RE(O/F)BiS2 evolves largely with RE(O/F)-spacer layer. We have evaluated the pair
distribution function (PDF) of the Bi-S1 bonds from the analysis of the filtered EXAFS. The
EXCURVE 9.275 code (with calculated phase shift functions and backscattering amplitudes)
was used for extracting the PDF [36] starting from a model containing a distribution ofharmonic oscillators describing the Bi-S pairs. The final Bi-S1 distribution was merged in a
two peak function for all the samples (Figure 3) confirming that BiS2 -layer in these materials
is characterized by a ferroelectric or quasi ferroelectric-like distortion [20–22]. Similar Bi-S1
distribution was obtained by constrained EXAFS model fits in which residual contributionof axial Bi-S2 bonds was included together with the in-plane Bi-S1 bonds respecting theeffective number of near neighbors expected from the polarization dependence of L3 -edge
EXAFS [32,34]. The corresponding theoretical signals are included as solid lines with thefiltered EXAFS as insets. It is worth mentioning that such a distortion corresponds toatomic displacement within the BiS2 -layer. On the basis of first principles calculations a
spontaneous in-plane ferroelectric polarization has been suggested [37].

Physchem 2021, 1

254

Bi-S1
Bi-Bi

2

| FT (k χ (k))|

Nd(O,F)BiS

2

CeOBiS

2

PrOBiS

2

EuFBiS

2

La(O,F)BiS

1

2

2

3

4

5

6

7

8

R (Å)
Figure 2. Fourier transforms (FT) magnitudes of Bi L3 -edge k2 -weighted EXAFS oscillations for
RE(O/F)BiS2 at 20 K. The FTs are performed using a Gaussian window in the k-range of 3–14.5 Å−1 .

One can make several observations from the PDF shown in Figure 3. The largest
distortion amplitude, defined by the separation between the two peaks (∆R) of the distribution, is found to be for La(O/F)BiS2 (see, e.g., the vertical dotted lines in Figure 3).
The distribution is quasi ferroelectric-like with differing weight of the two Bi-S1 bonds.
Incidentally, the distortion amplitude of ∆R∼0.26 Å is smaller than the one found inpolycrystalline samples reporting ∆R∼0.4 Å by neutron pair distribution function (PDF)
analysis [23] and by EXAFS analysis [24]. On the other hand Nd(O/F)BiS2 reveals smallest
distortion amplitude (∆R∼0.13 Å) with the distortion being quasi ferroelectric-like. Again,
the distortion amplitude is significantly lower than the one reported by neutron PDF on
polycrystalline sample showing ∆R to be ∼0.18 Å [25]. Among the self-doped BiS2 systems, EuFBiS2 shows almost equal probability of the two bonds indicating ferroelectric-likedistortion. The distortion amplitude for EuFBiS2 (∆R∼0.22 Å) is lower than what has
been reported in polycrystalline sample of the same system (∆R∼0.26 Å) measured by
EXAFS [26]. The self-doped system PrOBiS2 also appears with ferroelectric-like distortion
although the amplitude (∆R∼0.18 Å) is lower than the one in EuFBiS2 . Similarly, the distribution in CeOBiS2 is quasi ferroelectric-like, that is smallest among the self-doped systems(∆R∼0.15 Å). Again, the distortion amplitude is lower than the one found in polycrystalline
samples CeOBiS2 (∆R∼0.19 Å). This seems plausible due to the fact that the ground stateof these materials is highly susceptible to the external conditions and one can expect themorphology of the system to have substantial effect on the distortion amplitude as well. It
should be mentioned that the mean Bi-S1 distance, estimated by the weighted average of
the PDF, is consistent with the one measured by diffraction experiments [28–30,35]. It isalso worth noting that the distortion evolves with c-axis of the system and hence the axial
coupling may have some important role in these materials.
Apparently, there is no direct correlation between the superconducting transition
temperature and the Bi-S1 distortion amplitude in the studied samples. However, if wecompare La(O/F)BiS2 and Nd(O/F)BiS2 , the less distorted system tends to have higher
Tc . Indeed, La(O/F)BiS2 , showing highest distortion amplitude, is superconducting withTc ∼2.5 K while Nd(O/F)BiS2 with lowest distortion amplitude shows Tc ∼5.1 K. However,Physchem 2021, 1

255

among the self-doped systems, PrOBiS2 is superconducting at Tc ∼3.5 K with distortion
amplitude larger than that in CeOBiS2 showing superconductivity at lower temperature (Tc ∼1.3 K). Unlike the above, the self-doped EuFBiS2 with EuF spacer layer is nonsuperconducting down to 0.15 K and reveals substantial distortion in compare to otherself-doped systems with REO spacer layer (PrOBiS2 and CeOBiS2 ). Therefore, although
the Tc is affected by in-plane distortions, a direct relationship can not be argued. Here, it
is worth mentioning that in the self-doped systems, if the carriers provided by the mixed
valence of the rare-earth ions are trapped by the out-of-plane Bi 6pz orbitals, the in-plane
Bi 6px,y orbitals are less populated and the strong in-plane distortion can still survive
with superconductivity.
Nd(O,F)BiS

2k χ(k)

24

6

8

10 12 14-1k (Å )22

k χ(k)

CeOBiS4

6

8

10 12 14-1k (Å )PrOBiS

2

PDF(R)k χ(k)

24

6

8

10 12 14-1k (Å )
EuFBiS2k χ(k)

24

6

8

10 12 14-1k (Å )22

k χ(k)

La(O,F)BiS

4

2.6

2.8

3

R (Å)

3.2

6

8

10 12 14-1k (Å )
3.4

3.6

Figure 3. Bi-S1 bonds distribution functions obtained by in-plane polarized EXAFS measured
on single crystal samples of RE(O/F)BiS2 . The insets show the Fourier filtered EXAFS oscillations(symbols) compared with the theoretical EXAFS (solid line) corresponding to the plotted distributions.

X-ray absorption near edge structure (XANES) probes higher order atomic correlation
function, thus highly sensitive and useful probe of the local geometry and the valence
electronic states [31,32]. The Bi L3 -edge XANES spectra measured at 20 K on single crystal
samples of RE(O/F)BiS2 are shown in Figure 4. The spectra are normalized with respect tothe atomic absorption estimated by standard procedure based on a linear fit to the EXAFS
region. Here, the spectrum of PrOBiS2 is not included for the comparison consideringPhyschem 2021, 1

256

different experimental set-up and spectral resolution. The spectra show different spectralfeatures and the nearest edge peak features are denoted by A and B. These features are
related to the dipole allowed transition from Bi 2p to the unoccupied Bi 6d states in thecontinuum, admixed with the unoccupied Bi 6p states [31,32]. The latter are also hybridized
with Bi 6s and S 3p states due to Bi3+ defect chemistry [38,39]. Therefore these peaks are
sensitive probe of the local geometry around the photoabsorbing Bi.

A

B

Normalized Absorption (arb.units)

Nd(O/F)BiS

2

CeOBiS

2

EuFBiS

2

La(O/F)BiS

2

0.008

A
B

A B A

I -I /I +I

B

0.006

13,435

13,445

0.004
0.002

13,455

0.10

0.20

0.30

ΔR (Å)

13,40013,42013,44013,46013,48013,500

Photon Energy (eV)Figure 4. X-ray absorption near edge structure (XANES) spectra of RE(O/F)BiS2 single crystals
measured at the Bi L3 -edge. The nearest edge features are denoted by A and B. The left inset shows azoom-over the features A and B. The right inset shows normalized intensity difference plotted as afunction of the distortion amplitude in the BiS2 -layer. The error bars represent maximum uncertainty
estimated by analyzing different scans for each sample.

The differences in the XANES features can be seen in the inset displaying a zoom-overthe peaks A and B. Apparently, the peak A moves towards higher energy from La(O/F)BiS2
to Nd(O/F)BiS2 while the peak B tending to shift towards lower energy with an exchange ofthe spectral weight. Here, the spectral weight transfer is estimated by normalized intensity
difference between the two peak features (I A − IB /I A + IB ) and plotted as a function of the
distortion amplitude determined by EXAFS (Figure 3) and shown in the inset (right) of
Figure 4. The intensity difference shows a gradual decrease as a function of the distortionin the BiS2 -layer. Although, due to complexity of the distortions in these materials, it isdifficult to establish a quantitative correlation between the distortions and the spectral
weight transfer in the XANES spectra, the observed change in the local geometry can betaken as a marker for the local distortions.
4. Conclusions
In summary, the local distortions in RE(O/F)BiS2 have been determined by Bi L3 edge EXAFS measured on single crystal samples with polarization of the X-ray to beparallel to the BiS2 layer. The BiS2 layer is characterized by a two peak distributionfunction confirming ferroelectric-like nature of the local distortions in these materials. The
distortion is largest for La(O/F)BiS2 and shows a substantial change with the RE(O/F)spacer layer appearing smallest in the case of Nd(O/F)BiS2 . The self-doped EuFBiS2
shows a significantly larger distortion than other self-doped systems, namely PrOBiS2 andCeOBiS2 . The distortion amplitude, measured on single crystal samples using polarized
EXAFS, is systematically lower than the one found in polycrystalline samples. This is likely
to be due to highly susceptible nature of BiS2 -based materials. Apparently, we do notPhyschem 2021, 1

257

observe any quantitative correlation between the local distortions in the instable BiS2 -layer
and the superconductivity. It is likely that in-plane distortions are responsible for the charge
fluctuations in the BiS2 -layer with axial displacements having a crucial role in controllingthe electronic transport properties of these materials.
Author Contributions: Conceptualization, N.L.S., T.M., T.Y. and Y.A.; methodology, E.P., T.W., A.P.,
K.T., R.H., T.D.M., M.N.; formal analysis, G.M.P. and L.T.; writing—original draft preparation, G.M.P.
and N.L.S. All authors have read and agreed to the published version of the manuscript.
Funding: The work is partially supported by the Sapienza University of Rome and JSPS/MEXT
KAKENHI (Grants No. 15H03693, 19H01839, 18KK0076 and 20H05882).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on reasonable requestfrom the corresponding author.
Acknowledgments: The authors thank the ESRF staff for the support during the experimental run.
We acknowledge technical assistance from Joe Kajitan, Takuya Asano, and Naoki Yamamoto forsample preparation.
Conflicts of Interest: The authors declare no conflict of interest.

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