Volume 4 | Number 12 | December 2020

MATERIALSCHEMISTRYFRONTIERS
Themed issue: In celebration of Prof. Fred Wudl’s 80th Birthday

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Pressure-induced valence transition in themixed-valence (Sm1/3Ca2/3)2.75C60 fulleride†
Naoya Yoshikane, a Keisuke Matsui, a Takeshi Nakagawa, bAnastasia G. V. Terzidou, c Yasuhiro Takabayashi, de Hitoshi Yamaoka, fNozomu Hiraoka,g Hirofumi Ishii,g John Arvanitidis c and Kosmas Prassides*ad(Sm1/3Ca2/3)2.75C60 is a member of the family of non-stoichiometric strongly-correlated rare-earth fullerides,(Sm1xCax)2.75C60 (0 r x r 1), in which an orthorhombic 2  2  2 supercell of the face-centred cubic(fcc) unit cell of stoichiometric A3C60 (A = alkali metal) fullerides is stabilized by the long-range ordering ofpartially-occupied metal sites. At ambient temperature and pressure, it is a mixed valence compound withan average Sm valence of +2.33(2) implying a formal charge of 5.78 for the C60 anions. Here we study its
electronic response to the application of pressure in the range 0–9 GPa. Synchrotron X-ray absorption
measurements in the high-resolution partial fluorescence yield mode (PFY-XAS) at ambient temperatureshow the onset of an abrupt strongly hysteretic (width B2.5 GPa) first-order reversible phase transition at
B4 GPa, accompanied by a drastic increase in the bulk Sm valence by B20% to +2.71(3). This is coincident
with the huge lattice contraction and concomitant insulator-to-metal transition encountered before forSm2.75C60 in the same pressure range and provides a possible explanation of the physical properties in termsof the strong coupling between the lattice and electronic degrees of freedom. The behaviour is reminiscentof the electronic and lattice response to pressure of highly correlated Kondo insulators like SmS and its
Received 14th September 2020,
Accepted 15th October 2020

ternary derivatives, Sm1xRxS (R = Ca, Y, etc.). However, a distinguishing feature of the fulleride systems is

DOI: 10.1039/d0qm00707b

derived by the t1g orbitals and can accept excess charge as the 4f-electron occupation number decreases.
The observed electronic response as a function of pressure is thus opening new possibilities for accessing

rsc.li/frontiers-materials

metallic fullerides at elevated pressures.

that the C60 anionic sublattice can act as an electron reservoir due to the availability of a close-lying band

1. Introduction
Rare-earth (RE) metal C60 fullerides constitute an intriguingclass of strongly-correlated molecular systems1 as they present
the possibility of strong coupling between two electronically
active sublattices, the anionic pp(C60) and cationic 4f,5d(RE)aDepartment of Materials Science, Graduate School of Engineering, Osaka
Prefecture University, Osaka 599-8531, Japan.
E-mail: k.prassides@mtr.osakafu-u.ac.jp; Tel: +81 72 254 6162bCenter for High-Pressure Science & Technology Advanced Research,
100094 Beijing, P. R. ChinacPhysics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki,
Greeced
WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University,
Sendai 980-8577, Japane
Department of Physical Science and Engineering, Nagoya Institute of Technology,
Nagoya 466-8555, Japanf
RIKEN SPring-8 Center, Sayo, Hyogo 679-5148, Japang
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan† Electronic supplementary information (ESI) available: Pressure dependence ofthe energies and intensities of the subpeaks used in the fitting of the experimental PFY-XAS spectra of Sm1/3Ca2/3C60. See DOI: 10.1039/d0qm00707b

sublattices both of which are dominated by strong electronic
correlations.2,3 To date, metal fulleride research has been
dominated by work on alkali fullerides, especially those withstoichiometry A3C60 (A = alkali metal). These have emerged asarchetypal strongly correlated systems with superconducting
transition temperatures, Tc as high as 38 K 4 and upper criticalmagnetic fields, Hc2 in excess of 90 T.5 Nonetheless the electronic properties are entirely dominated by the narrow band
behaviour of the C60 anion sublattice – the supporting cation
sublattice is invariably electronically inactive and only plays acharge-balancing role as a structural spacer.6 A comparable but
exactly reverse situation is encountered in rare earth compounds which represent one of the most fascinating classesof strongly correlated systems that include Kondo insulatorsand heavy fermions.7,8 Here the properties are dominatedby the extreme narrow-band behaviour of the rare-earth
cation f-electron sublattice with the supporting anion sublattice
again playing a charge-balancing structural-spacer role. As
electronically active anion solids, the rare-earth fullerides canhave properties intrinsically distinct from comparable rareearth compounds with closed shell anions, and the emergenceThis journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2020

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Materials Chemistry FrontiersFig. 1 Temperature dependence of magnetic susceptibilities. Open red
circles show the total susceptibility of (Sm1/3Ca2/3)2.75C60, obtained after
subtracting the diamagnetic core contributions from the diﬀerence of thevalues measured at 4 and 2 T magnetic fields. The calculated magnetic
susceptibilities (blue dashed lines) of the free Sm2+ and Sm3+ ions are also
included. The susceptibility of Sm2+ (4f6, 7F0) is obtained by using van
Vleck’s formula which takes into account the presence of a J = 1 excitedstate at 420 K above the J = 0 ground state. For Sm3+ (4f5, 6H5/2), thesusceptibility is calculated by considering the sum of the corresponding
Curie and van Vleck contributions – in the latter case, the energy
difference between the J = 5/2 ground and J = 7/2 excited state is348 K. The green solid line through the experimental data is the weighted
average of the susceptibility calculated assuming 0.68 and 0.32 contributions from Sm2+ and Sm3+, respectively. The inset shows the building
block of the unit cell of the orthorhombic superstructure of thecompound, formed by doubling the shown subcell along all threelattice directions. Green and light-grey spheres correspond to offcentred octahedral and tetrahedral (randomly distributed samarium/
calcium) metal cations, respectively, while the dark-blue sphere represents
the partially-occupied tetrahedral site.

of novel phenomena such as co-existence of Kondo f-electron
behaviour with C60-based pp-electron superconductivity may be
anticipated.
Rare-earth fulleride phases are not widely available mainly
because of the synthetic diﬃculties in aﬀording phase-purematerials in a reproducible way. Of interest has been the
preparation of Eu6C60 and Eu9C70 fullerides that led to theisolation of molecular ferromagnets, which exhibit conducting
and giant magnetoresistive behaviour with significant
p(C60)-f,d(Eu) orbital interactions modulating the magnetic
exchange.9–11 However, most of the research work on rareearth fullerides has focused on materials with stoichiometry,RE2.75C60 (RE = Yb, Sm). These non-stoichiometric compounds
crystallize with a 2a  2a  2a superstructure of the facecentred cubic (fcc) unit cell of A3C60 alkali fullerides arisingfrom the long-rage ordered arrangement of partially occupied
tetrahedral interstices (Fig. 1 inset).12,13 The RE2.75C60 unit
cell comprises 32 C60 units and 88 RE atoms together with
8 RE partially occupied sites. Although early claims forsuperconductivity12,14 in this family proved erroneous,13,15,16
it was apparent from the presence of non-stoichiometry anddeviation from the ideal ‘‘RE3C60’’ stoichiometry that thesematerials may be endowed with interesting electronic properties

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associated with mixed valence states of the RE ions, i.e. that the
average RE valence should be greater than +2. Indeed, magnetic
susceptibility data clearly indicated deviation of the RE valencefrom +2.13,15
As a result of these considerations, an important finding wasthen that both Sm2.75C60 and Yb2.75C60 were discovered to
exhibit on cooling isosymmetric phase transitions that areaccompanied by giant negative thermal expansion (NTE).13,15
The origin of this isotropic lattice response was ascribed to atemperature-induced valence transition of the RE ions from+(2 + e) towards +(2 + e 0 ) with e 0 o e and this interpretation was
supported by synchrotron X-ray absorption measurements.1
This eﬀect is immediately reminiscent of the valence orconfiguration transitions established in typical RE-based
Kondo insulators such as SmS.17,18 The connection becomeseven more prominent when the electronic properties areprobed by the application of external pressure. For instance,
SmS undergoes a pressure-induced abrupt catastrophic transition from a semiconducting black to a metallic gold phase thatwas rationalised phenomenologically by a valence fluctuation(4fn5d0 2 4fn15d1) model. In a similar fashion, Sm2.75C60
undergoes a pressure-induced first-order phase transition,
which is accompanied by a huge lattice collapse of B6% at
B4 GPa and a change in colour to gold, implying a drasticchange in the conducting properties.19 Complementary Raman
spectroscopic measurements in the same pressure range confirm the occurrence of the transition and support the emergence of metallic behaviour.20 The analogy with the behaviourof the Sm chalcogenides makes the possibility that Sm2.75C60
undergoes a similar pressure-induced valence transition towards
a state with an increased average Sm valence +(2 + e00 ) with e00 4 ehighly likely.
Isovalent or aliovalent cation substitutions have been extensively used in the past to control the electronic configurationsof the rare-earth ions and tune the rare-earth exchange interactions, the Fermi level position and the lattice constant(chemical pressure) in mixed valence Kondo insulators such as
SmS.21 The ternary phase fields Sm–R–S (R = Y, Ce, Nd, Ca, etc.)
are recently drawing much attention in order to bring theinsulator-to-metal transition to ambient pressure and utilize
the huge NTE that accompanies the valence transition bytuning its operational temperature range.22–24 In the process,it is of particular importance to clarify the differences on thevalence transition between controlling size (chemical or physical pressure) and doping (electronic structure).
In a similar fashion, we have initiated the study of theternary fulleride phases, (Sm1xCax)2.75C60 (0 r x r 1) in an
attempt to mimic chemically the eﬀect of physical pressure on
the valence transition of Sm2.75C60 by taking advantage of therobust divalent character of the smaller valence-precise Ca2+
cations.25 We found that the (Sm1xCax)2.75C60 (0 r x r 1)
family of materials obeys Vegard’s law with the Sm and Ca
metal ions randomly distributed in the available octahedraland tetrahedral interstitial sites.25 They also adopt the same
orthorhombic superstructure (space group Pcab) of the facecentred cubic unit cell of A3C60 (A = alkali metal) fullerides

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Materials Chemistry Frontiersarising from the long-range ordering of tetrahedral partiallyoccupied vacancies (Fig. 1 inset) consistent with the stoichiometryof the materials.25 Moreover X-ray absorption spectroscopy measurements in the partial fluorescence yield mode (PFY-XAS) at theSm-L3 edge have authenticated the mixed valence state of Sm
across the whole series of (Sm1xCax)2.75C60 compositions. In
particular, it turns out that, at ambient conditions, chemicalpressure sensitively tunes the 4f electron occupation number withthe average Sm valence, n increasing quasi-continuously by 410%
as x increases from 0 (for Sm2.75C60, n = +2.07(3)) to 2/3 (for(Sm1/3Ca2/3)2.75C60, n = +2.33(2)). At the same time, the C60
anion charge remains nearly invariant at E5.78.25
To date, there has been no direct study of the rare-earth
electronic states in rare-earth fullerides at elevated pressures.
Synchrotron X-ray powder diﬀraction19 and Raman spectroscopy20
agree on the occurrence of a hysteretic phase transition forSm2.75C60 in the vicinity of 4 GPa with dramatic changes in bothlattice size and electronic/conducting properties. Here, we directly
address the issue of the evolution of the Sm electronic states withapplied pressure by means of PFY-XAS spectroscopy at theSm-L3 edge. We chose to investigate the calcium co-doped(Sm1/3Ca2/3)2.75C60 fulleride as this was found to incorporate

Research Article
Sm at the highest average valence, n known for these systems at
ambient pressure at +2.33(2).25 We find that n initially increases
monotonically to B4 GPa whereupon an abrupt phase transformation sets in – this is accompanied by a rapid increase in
n to +2.71(3) at B6.5 GPa. The electronic changes are reversible
upon the release of pressure and are characterized by a large
hysteresis, signature of the first-order nature of the transformation. They provide a possible rationalization of the pressureinduced drastic changes in structural and optical properties ofrare-earth fullerides that may be coupled with the extreme
fragility of the rare-earth valence.

2. Results and discussion(Sm1/3Ca2/3)2.75C60 is a member of the (Sm1xCax)2.75C60(0 r x r 1) family of materials with x = 2/3. Fig. 1 shows themeasured magnetic susceptibility, w vs. temperature data for(Sm1/3Ca2/3)2.75C60, along with those calculated for the free
Sm2+ and Sm3+ ions. For Sm2+ (4f6, 7F0), there is only a van
Vleck contribution to the magnetic susceptibility, originatingfrom the mixing of the J = 0 ground and the J = 1 excited state. On
the other hand, the calculated susceptibility of Sm3+ (4f5, 6H5/2) isFig. 2 Representative ambient-temperature Sm-L3 edge PFY-XAS spectra of (Sm1/3Ca2/3)2.75C60 at various pressures shown in linear scale, recorded
upon pressure (a) increase and (b) decrease. The spectrum recorded at ambient conditions is also included in both panels.

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Materials Chemistry Frontiers

derived by summing the corresponding Curie and van Vleck(mixing of the J = 5/2 ground and J = 7/2 excited states)
contributions.13 In mixed valence Sm-based compounds, anestimate of the average samarium valence can be derived fromthe magnetic susceptibility data at suﬃciently high temperatures
as a linear combination of the contributions of the two distinct
Sm2+ and Sm3+ valence states.26 As it can be inferred from Fig. 1,
the experimental values of the susceptibility of (Sm1/3Ca2/3)2.75C60
at high temperatures fall between those calculated for thetwo ionic states, in accord with the mixed valence character ofsamarium in this material. The experimental data can be reproduced well down to B50 K (below which valence transitions are
expected)13,15 assuming 0.68 and 0.32 contributions from Sm2+
and Sm3+, respectively (solid line through the data). These values
lead to an estimate of the average samarium valence, n of +2.32
in excellent agreement with that obtained directly by PFY-XAS
measurements at ambient conditions (+2.33(2)) for the same
composition.25
The PFY-XAS spectra of (Sm1/3Ca2/3)2.75C60 at the Sm-L3 edge
at various pressures are displayed in Fig. 2 together with thatmeasured under ambient conditions. We note that, since only
one quantum transition (an intermediate state belonging to ashallow level) is selected in this X-ray spectroscopic technique,the lifetime broadening of the spectra (mainly originating fromthe deep core-hole in the final state) is suppressed, leading to amuch better resolution than typical XAS measurements.27
Compared to the ambient pressure spectrum in which thesample was contained in a glass capillary,25 the sample chamber of the Be gasket in the present high pressure experiments issignificantly more transparent to the synchrotron X-ray beam in
the incident photon energy range used. This introduces
the danger of causing radiation damage to the soft fulleride
solids by X-ray illumination in the course of the measurements.
Therefore, in order to avoid the occurrence of sample degradation, the incident X-ray beam power was reduced by a factor ofB30 compared to that of the ambient pressure experiments
by introducing appropriately thick aluminium foil filters. This
unavoidably led to a worsening in the signal-to-noise ratio
compared to that achieved at ambient pressure (Fig. 2).25 Nonetheless, good data statistics and improved spectral resolution
still allow us to follow the pressure evolution of the average
samarium valence with suﬃcient accuracy.
The appearance of two peaks separated by B8 eV in the
spectra shown in Fig. 2 and corresponding to the 2p - 5ddipolar excitations for divalent (lower energy peak) andtrivalent (higher energy peak) samarium, unambiguously

Fig. 3 Representative fits (solid lines through the experimental data) of the PFY-XAS spectra (open circles) of (Sm1/3Ca2/3)2.75C60 recorded upon pressure
increase (P = 0.42, 4.16 and 8.58 GPa, upstroke) and release (P = 0.51 GPa, downstroke). The two main peaks, corresponding to the 2p - 5d dipolar
excitations for Sm2+ and Sm3+, are represented as blue and red shaded areas, respectively. Solid green lines represent the arctangent functions used todescribe the corresponding transitions to the continuum, while the solid blue line at the bottom is the underlying baseline. Dashed lines mark the Voigt
components employed in the fitting procedure of the experimental data.

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Materials Chemistry Frontiers
evidences the mixed valence character of the samarium ions in(Sm1/3Ca2/3)2.75C60 at both ambient and elevated pressures.28 At
ambient and low pressures, the higher intensity of the Sm2+
peak compared to that of the Sm3+ one qualitatively indicatesthat the average samarium valence, n is closer to +2 rather than
+3. This situation is gradually altered with increasing pressure
as the Sm3+ peak gains in intensity at the expense of that of
Sm2+, signalling a drastic decrease in the 4f electron occupationof the samarium ions induced by pressure. The reverse valencetransition (Sm3+ - Sm2+) occurs upon pressure release. The
transformation is reversible as the relative intensity of the two
peaks in the spectrum recorded at 0.51 GPa upon depressurizationis comparable to that in the spectra measured at ambientpressure and at 0.42 GPa at the start of the present measurements (Fig. 2a and b).
In order to extract quantitative information about the average valence, n of samarium in (Sm1/3Ca2/3)2.75C60 and its
pressure dependence, the data were fitted by appropriate functions to account for the contributions of the Sm2+ and Sm3+
peaks together with that of the background. The fits of
the PFY-XAS spectra at selected pressures recorded during
the upstroke (P = 0.42, 4.16 and 8.58 GPa) and downstroke(P = 0.51 GPa) cycles are shown in Fig. 3. The spectral contribution attributed to Sm2+ was fitted by the sum of three Voigt
functions (one main and two satellites) for the main peak andan arctangent-like function to account for the backgroundoriginating from the corresponding transitions to the continuum. The need to use more than one Voigt function tosatisfactorily reproduce each white peak originates from thecrystal-field splitting of the Sm 5d band.24,29 A similar fitting
strategy was used for the Sm3+ spectral contribution except that
it was found necessary to include only two Voigt functions to
describe the corresponding main peak. In the lower pressure
range of the experiments, an additional Voigt function was
included in order to account for the small spectral uplift above
6730 eV (underlying baseline in Fig. 3). Fig. S1 and S2 (ESI†)
collect together the pressure dependence of the energies andintensities of the individual subpeaks of the Sm2+ and Sm3+
components employed in the fits of the experimental PFY-XAS
data. The average Sm valence, n can be derived then from therelative integrated intensities of the divalent, I(+2) and thetrivalent, I(+3) Sm PFY-XAS peaks (corresponding shaded areasin Fig. 3) by the expression: n = 2 + {I(+3)/[I(+2) + I(+3)]}.
The fit of the PFY-XAS spectrum at 0.42 GPa yields a value forthe average samarium valence, n = +2.31(3), similar to thatobtained from the corresponding measurements on the same
composition at ambient pressure (n = +2.33(2)).25 Such an
agreement clearly demonstrates the reproducibility of ourfindings and confirms the successful loading of the highly
air-/moisture-sensitive sample into the diamond anvil cell(DAC) for the high pressure experiments, completely avoiding
any degradation. At the same time, the fit of the spectrum
recorded at the highest pressure of our experiments (8.58 GPa),
where the trivalent samarium peak clearly dominates, provides
a value, n = +2.71(3) for the average Sm valence – this corresponds to an increase of the Sm valence by 0.4 induced by

Research Article

Fig. 4 Pressure dependence of the average samarium valence and C60
charge in (Sm1/3Ca2/3)2.75C60. Open (solid) circles represent data obtained
upon pressure increase (decrease), while the open square is the ambientpressure datum. Dashed lines through the data are guides to the eye.

pressure. Comparable changes associated with the pressureinduced transition from the semiconducting to the metallic
phase have been reported for the Sm monochalcogenides,
namely B0.8 for SmS, B0.6 for SmSe and B0.5 for SmTe.24
The pressure dependence of n in (Sm1/3Ca2/3)2.75C60, as
obtained from the fits of all the high pressure data recorded
during the upstroke and downstroke cycles, is illustrated
in Fig. 4. In the beginning, the valence increases slowly
and continuously from +2.31(3) to +2.49(3) as the pressure
approaches B4 GPa. This is followed first by a pressure regime(B4–6.5 GPa) whereupon the average valence rapidly increases
reaching a value of +2.71(3) and then by a plateau where nremains invariant up to B9 GPa, the highest pressure reachedin the present measurements. On the other hand, uponpressure release, the average samarium valence is robust overa broad pressure range decreasing only very slightly to a valueof +2.64(3) between 9 and 3 GPa. However, this is followed
next by a rapid drop to +2.32(3) at 0.51 GPa (the lowest
pressure achieved during depressurization), a value essentially
identical to that measured at the beginning of the experimentat 0.42 GPa. The data confirm the reversible nature of thepressure-induced electronic changes and exclude any extraneous origin of the phase transformation as well as any sample
degradation due to X-ray illumination. The large hysteretic
behaviour – pressure width of the hysteresis loop is B2.5 GPa –of the reversible Sm2+ 2 Sm3+ valence change (Fig. 4) is consistent
with the transition being first-order in nature analogous to what isestablished for SmS.30 Noticeably however, when compared with
Sm2.75C60, the transition here appears to be more sluggish and thehysteretic loop significantly broader.
In (Sm1/3Ca2/3)2.75C60, the abrupt Sm valence transition
occurs over the range of 4 to 6.5 GPa upon pressurization. As
both the lattice collapse and the insulator-to-metal transition
evidenced for the binary Sm2.75C60 fulleride also occur over acomparable pressure range of 4–5 GPa,19,20 the present results
provide direct evidence for the strong coupling between theThis journal is © The Royal Society of Chemistry and the Chinese Chemical Society 2020

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Materials Chemistry Frontierscharge and lattice degrees of freedom – both structural andconductivity changes are driven by changes in the electronic
configuration possibly mediated by the C60 anionic sublattice.
The latter has the potential to act as a charge reservoir
accepting electrons from the Sm 4f/5d bands in the course of
the valence transition. The availability of the t1g-derived C60
band, which can begin to be populated when the Sm valence
exceeds values of 2.5–2.6 (Fig. 4) and the t1u-derived C60 band isfull, appears to be a crucial ingredient to allowing the valencechange to proceed. Such an availability contrasts with the cases
in which the anion sublattice comprises closed-shell anions
like the chalcogenides.
Finally, given that the valence transition of samarium atelevated pressure is towards the generation of the magnetic
Sm3+ ion with a 4f5:6H5/2 configuration also opens up the
possibility of coupling to the magnetic degrees of freedom
and the emergence of long-range magnetic ordering, in analogy
with the Sm monochalcogenides in the high pressure golden
phase.31

3. Conclusions
In summary, we have employed synchrotron X-ray spectroscopic (PFY-XAS) measurements to investigate the electronic
properties of the mixed valence fulleride, (Sm1/3Ca2/3)2.75C60 atelevated pressures. The observed pressure response of theaverage Sm valence, n is similar to that of strongly correlated
Kondo insulators such as SmS with a sharp decrease in the
4f-electron occupation number occurring abruptly above
B4 GPa. The resulting B20% increase in n to +2.71(3) coincides with the occurrence of a concomitant lattice collapse andthe appearance of metallic behaviour in the same pressure
range in the parent mixed valence fulleride, Sm2.75C60 providing a possible explanation of the observed properties, that may
involve the strong coupling between lattice and electronic(valence) degrees of freedom. The availability of the t1gderived band of C60 appears crucial in being able to accommodate the excess charge generated by the pressure-induced
Sm2+ - Sm3+ transition after the t1u-derived band becomesfully occupied at n E +2.55 and drive the system to the metallic
state. Therefore, given that the C60 sublattice can support
superconductivity, the possibility emerges that mixed valence
rare-earth fullerides can become superconducting at higher C60
doping levels achieved by further increase in the average rareearth valence at elevated pressures, opening new prospects inthis research field. Finally, the valence transition is reversible
and of first-order as evidenced by its highly hysteretic behaviour together with the complete recovery of the electronic
properties upon pressure release.

4. Experimental section
The (Sm1/3Ca2/3)2.75C60 polycrystalline sample used in this work
was prepared, chemically analyzed and structurally characterized, as reported elsewhere.25 Due to the extreme sensitivity of3526 | Mater. Chem. Front., 2020, 4, 3521--3528

the sample to air and moisture, all manipulations were undertaken in argon-filled gloveboxes (H2O/O2 o 0.1 ppm).
Ambient-pressure magnetic measurements were performed
on a ca. 20 mg sample, sealed in a thin-walled quartz ampoulewith a Quantum Design SQUID MPMS5 magnetometer. Temperature dependent magnetic susceptibility data were collected
at 2 and 4 T under field-cooled protocols at temperatures
between 1.8 and 300 K. The total susceptibilities were obtained
after subtracting the diamagnetic core contributions from thediﬀerence of the values measured at applied magnetic fields of
4 and 2 T.
PFY-XAS spectra of (Sm1/3Ca2/3)2.75C60 at the Sm-L3 edge
were collected at room temperature at the Taiwan BL12XU
beamline, SPring-8, Japan, in a 901 geometry. For these experiments, the emitted photon energy was fixed around the maximum of the La1 emission line at B5636 eV, while the incident
photon energy was scanned across the Sm-L3 edge.32 For every
individual spectrum, four diﬀerent scans in the energy range
6735–6705 eV, divided into 90 energy steps with a measurement
time of 5 s each, were averaged. The spectral intensity was
normalized to the intensity recorded by a monitor situatedbefore the DAC in which the sample was loaded. High pressure
was generated by means of a 4-pin type DAC with a culet size of
700 mm, utilizing a 30 mm thick Be gasket with a 250 mm
diameter hole at the centre. A pellet fragment of the sample
was carefully placed in the sample chamber of the Be gasket,
inside an Ar gas atmosphere glovebox in order to avoid sample
oxidation. Daphne 7373 oil, which solidifies at 2.2 GPa at room
temperature33 was used as the pressure transmitting medium,
while the pressure was calibrated by means of the ruby fluorescence method.34

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
We thank SPring-8 for access to synchrotron X-ray facilities
and Dr A Machida for the use of the glovebox at the facility.
AGVT acknowledges the support by the Hellenic Foundation
for Research and Innovation (HFRI) under the HFRI PhD
Fellowship grant (Fellowship Number: 707). This work was
financially supported by Grants-in-Aid for Scientific Research(JSPS KAKENHI Grant Numbers JP18H04303, JP18K18724, andJP19H04590) by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan, by the Murata Foundation and by the KAKENHI Specific Support Operation of Osaka
Prefecture University. The experiments at the SPring-8 Taiwan
beamline BL12XU were performed under SPring-8 Proposal
No. 2017B4268, 2018B4252 and 2019A4264, corresponding to
NSRRC Proposal No. 2018-1-058-1, 2018-1-058-3 and 2018-1058-5, respectively.

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Notes and references
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2 Y. Takabayashi and K. Prassides, Unconventional high-Tc
superconductivity in fullerides, Philos. Trans. R. Soc., A,
2016, 374, 20150320.
3 P. Wachter, Intermediate valence and heavy fermions, in
Handbook on the physics and chemistry of rare earths, ed.
K. A. Gschneidner, L. Eyring, G. H. Lander and G. R.
Chopin, Elsevier Science B. V., Amsterdam, 1994, vol. 19,
pp. 177–388.
4 Y. Takabayashi, A. Y. Ganin, P. Jeglic, D. Arcon, T. Takano,
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