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Journal of the Physical Society of Japan 81 (2012) 064709
DOI: 10.1143/JPSJ.81.064709

Valence Shift of Pr Ion from 3+ to 4+ in (Pr1 y Yy )0:7 Ca0:3 CoO3
Estimated by X-Ray Absorption Spectroscopy
Hiroyuki FUJISHIRO, Tomoyuki NAITO, Satoru OGAWA, Naoki YOSHIDA,
Kiyofumi NITTA1 , Jiri HEJTMÁNEK2 , Karel KNÍŽEK2 , and Zdenek JIRÁK2
Faculty of Engineering, Iwate University, Morioka 020-8551, Japan
Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan
2
Institute of Physics, ASCR, Cukrovarnická 10, 162 00 Prague 6, Czech Republic
1(Received January 12, 2012; accepted April 2, 2012; published online May 17, 2012)

The temperature dependence of the X-ray absorption near-edge structure (XANES) spectra at the Pr L3 edge wasmeasured for the (Pr1y Yy )0:7 Ca0:3 CoO3 samples (y ¼ 0:075 and 0.15), in which a peculiar metal–insulator (MI)transition and a spin-state (SS) transition took place simultaneously at a critical temperature TMI . The valence states ofpraseodymium ion were determined using the analyses of XANES spectra. The results suggest that the average valenceof the praseodymium ion increases below room temperature from the common value 3.0+, undergoes a steepestchange at TMI , and reaches ﬁnally 3.15+ and 3.27+ at 8 K for the y ¼ 0:075 and 0.15 samples, respectively. The ﬁnal
valences are consistent with those estimated by the entropy change using the Schottky peak due to the Pr4þ Kramers
doublet in the low-temperature speciﬁc heat shown in our previous paper [Phys. Rev. B 82 (2010) 165107].
KEYWORDS: Pr–Ca–Co–O, XANES, Pr valence shift, metal–insulator transition

1.

Introduction

The undoped perovskite cobaltites RECoO3 (RE: rareearth element and Y) show a temperature-induced spin-state
6 0(SS) transition of Co3þ ions from a low spin state (LS; t2geg )4 2
to a high spin state (HS; t2g eg ), followed by the formation
5 of the metallic state of the intermediate spin state (IS; t2g
 )1)
at higher temperatures. The transition indicates a small
diﬀerence in the electronic energy E between the crystalﬁeld splitting and Hund’s coupling energy.2,3) The doped
systems with a mixed Co3þ /Co4þ valence generally show astable metallic state, but a ﬁrst-order metal–insulator (MI)transition was observed on cooling some Pr-based cobaltites,
reported for the ﬁrst time in Pr0:5 Ca0:5 CoO3 at TMI ¼ 90 K
under ambient pressure by Tsubouchi and coworkers.4,5) The
electronic structure of this system was investigated byphotoemission spectroscopy soon after the ﬁrst report,
revealing that the electronic structure changes at TMI .6) The
mechanism of the transition was tentatively ascribed to a
5 
spin-state crossover from itinerant cobalt states t2g
 to an3þ 6 0ordered mixture of localized LS Co (t2g eg , S ¼ 0) and LS
5 0
Co4þ (t2geg , S ¼ 1=2) states. Shortly thereafter, the existence3þ
of Co /Co4þ ordering was questioned since the sametransition was evidenced in the less doped samples Pr1x Cax CoO3 (x ¼ 0:3) under high pressures,7) or in (Pr1y REy )1x Cax CoO3 (0:2  x  0:5) with a partial substitution of Pr bysmaller RE cations or Y under ambient pressure.8,9) It has
been found that MI transition accompanied by SS transition isconditioned not only by the presence of praseodymium ions,but also by a suitable structural distortion that depends on the
average ionic radius and size mismatch of perovskite A-siteions.9) Furthermore, the critical temperature TMI changes
depending on the applied magnetic ﬁeld, so that TMI
decreases with increasing magnetic ﬁeld and the metallic
state is ﬁnally stabilized.10,11) The applied ﬁeld eﬀect is thus
opposite to the pressure dependence of TMI , where TMI
increases with increasing applied pressure.7)E-mail: fujishiro@iwate-u.ac.jp

An alternative explanation has been proposed by Knı́žek
et al. on the basis of electronic-structure calculations andsome experimental data for Pr0:5 Ca0:5 CoO3 such as thesigniﬁcant lattice contraction and decrease in Pr–O bondlengths that accompany MI transition.12) It is suggested, intheir paper, that the formal cobalt valence changes below
TMI from mixed-valence Co3:5þ toward pure Co3þ withstrong preference for the LS state, and that the praseodymium valence increases simultaneously from Pr3þ towardPr4þ . The SS transition and formation of an insulating state
are thus an analogy of the compositional transition fromthe ferromagnetic metal La0:5 Sr0:5 CoO3 to the diamagnetic
insulator LaCoO3 .13,14)
The idea of praseodymium valence shift was experimentally conﬁrmed in a study of the transport, magnetic, andthermal properties of (Pr1y Yy )0:7 Ca0:3 CoO3 (y ¼ 0{0:15)
by Hejtmánek et al.15) The crucial ﬁnding is the occurrenceof a Schottky peak at a low-temperature speciﬁc heat, which
results from a signiﬁcant population of Kramers Pr4þ ions.
Very recently, the valence transition of Pr has been probed in
Pr0:5 Ca0:5 CoO3 using Pr L3 and M4;5 edge X-ray absorption
spectroscopy (XAS) by Garcı́a-Munoz et al.16) They estimated that the valence of Pr ion shifted from 3.0+ to 3.15+
at 10 K. This seems to be a rather modest change in view of
the above-mentioned theoretical predictions and speciﬁcheat measurements in related (Pr1y Yy )0:7 Ca0:3 CoO3 .
In this work, we applied a similar XAS spectroscopy tothe analysis of (Pr1y Yy )0:7 Ca0:3 CoO3 (y ¼ 0:075 and 0.15),
the same samples as those used previously in the study of a
low-temperature speciﬁc heat.15) A detailed temperaturedependence of the XANES spectra around the Pr L3 edge
was measured using the bulk sensitive transmission method,and population of the Pr4þ state was derived from thespectra. In the calculation of average Pr valences, thedetected Pr4þ ions are taken as formally tetravalent,
although their electronic state is in fact a combination of
the ionic 4f 1 and oxygen-hole 4f 2 L conﬁgurations, making
the number of 4f electrons a non-integer. The Pr3þ ions are
essentially in an ionic 4f 2 electron conﬁguration.

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J. Phys. Soc. Jpn. 81 (2012) 064709

10

109

(Pr1-yYy)0.7Ca0.3CoO3

107

y=0
y=0.075
y=0.15

10

5

10

3

10

T

MI

-1

0

50

100

150
T (K)

200

250

300

Fig. 1. (Color online) Temperature dependence of the electrical resistivity of (Pr1y Yy )0:7 Ca0:3 CoO3 (y ¼ 0, 0.075 and 0.15).15)

120

(a)

T

MI

80

p

-1

-1

C (J mol K )10060(Pr1-yYy)0.7Ca0.3CoO340

y=0
y=0.075
y=0.15

20
0
0

50

100

150
T (K)

200

250

300

0.5

(b)
/T (J mol-1K-2)

3.1 Physical properties
The electrical resistivities and speciﬁc heats of the(Pr1y Yy )0:7 Ca0:3 CoO3 samples for y ¼ 0:075 and 0.15 arepresented in Figs. 1 and 2. (More detailed physical properties of the samples are reported in ref. 15.) The y ¼ 0
sample, which is metallic over the entire temperature range,is also shown for comparison. For the Y-substituted samples,
the resistivity exhibits a sharp jump below the transition
temperatures of TMI ¼ 64 and 132 K for y ¼ 0:075 and 0.15,
respectively, and increases further with decreasing temperature, pointing to a localized character of the low-temperaturephase. Note that magnetic susceptibility drops markedly at
the same temperature, which is a strong sign that cobalt ionstransform to LS states below TMI .15) MI transition was
further manifested by a pronounced peak in the speciﬁc
heat, as shown in Fig. 2(a). The transitions in the resistivity
and speciﬁc heat of the y ¼ 0:075 sample are very sharp,
suggesting a ﬁrst-order character, while those for the
y ¼ 0:15 sample are much broader.
The low-temperature speciﬁc heat of the samples is shownin the Cp =T vs T plot in Fig. 2(b), in which the lattice andlinear electronic terms of Cp as well as the nuclear heat

11

101

schottky

Results and Discussion

ρ (mΩcm)

Experimental Procedure

Polycrystalline (Pr1y Yy )0:7 Ca0:3 CoO3 (y ¼ 0, 0.075, and0.15) samples were prepared by a solid-state reaction. The
detailed sample preparation procedures were described
elsewhere.15) For the XANES measurements, some of thesamples were pulverized, mixed with 3N boron nitride(BN) powder with proper molar ratios in order to optimize
absorption, and pelletized to be 6 mm in diameter and
0.5 mm in thickness.
The Pr L3 -edge XANES spectra of the samples were
measured at BL01B1 of SPring-8 in Japan. The beam was
monochromatized using a Si(111) double-crystal monochromator. The spectra were recorded in the transmission modewith the detectors of the ionization chambers and obtained atvarious temperatures from 8 to 300 K using a cryocooler.
The valence of all the Pr ions was supposed to be 3.0+ forthe samples at 300 K. This is a justiﬁed assumption for
T  TMI , also supported by the oxygen stoichiometry of thepresent samples fabricated under the ambient pressure, incontrast to common problem of oxygen deﬁciency in the
prototypical Pr0:5 Ca0:5 CoO3 system.8) To determine themixed Pr3þ /Pr4þ content at low temperatures, a comparative
measurement of Pr6 O11 was carried out.
The recorded XANES spectra were modeled by the sumof three Lorentzian functions and one arctangent function
representing the step like edge of continuum excitations.
One Lorentzian function (peak A: 5966 eV) shows an
excitation from 2p3=2 to 4f 2 5d , which represents Pr3þ ions.
The other Lorentzian functions (peak B2: 5969 eV andpeak B1: 5979 eV) show the excitations from 2p3=2 to
4f 2 L5d  and 4f 1 5d  , L being a ligand hole in the O 2p
orbital, both of which represent Pr4þ ions.17,18) The energy
diﬀerences between peaks A, B2, and B1 were ﬁxed
according to results of Hu et al.19) The curve ﬁttings were
performed in the energy range from 5944 to 5985 eV using
Athena software.20)
3.

H. FUJISHIRO et al.

0.4(Pr1-yYy)0.7Ca0.3CoO3

0.3

y=0
y=0.075
y=0.15

0.2
0.1

C

2.

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0
0

5

10
T (K)

15

20

Fig. 2. (Color online) (a) Temperature dependence of the speciﬁc heatof (Pr1y Yy )0:7 Ca0:3 CoO3 (y ¼ 0, 0.075 and 0.15).15) (b) Low-temperature
speciﬁc heat shown in the CSchottky =T vs T plot. The lattice and linear
electronic terms for Cp , as well as the nuclear heat anomaly at low
temperatures, were subtracted. (The full line shows the ﬁt of the Schottky
peak for y ¼ 0:15.)

anomaly at low temperatures were subtracted. The anomalous peak observed for the y ¼ 0:075 and 0.15 samples isunderstood as a Schottky peak associated with the grounddoublet of Kramers ion Pr4þ , split by the internal magnetic
ﬁeld existing in the samples. On the other hand, for the y ¼ 0
sample, there is no Schottky peak in the range of 1–2 K,

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normalized μ (E)(a) y=0.15
8K
90K
125K
132K
140K
200K
300K

2

5974

5976

5978
5980
E (eV)

5982

5984

1

(Pr1-yYy)0.7Ca0.3CoO3
0

5950

5960

5970
5980
E (eV)

5990

6000

3

(b) y=0.075
8K
30K
64K
70K
76K
132K
300K

2

5974

5976

5978
5980
E (eV)

5982

5984

1
(Pr1-yYy)0.7Ca0.3CoO3
0

5950

5960

5970
5980
E (eV)

3

5990

6000

3.8

(c) Pr6O11

Valence of Pr ion

3.2 XAS measurements
Figure 3(a) shows the temperature dependence of the
XANES spectra at the Pr L3 -edge for the y ¼ 0:15 sample.
The two main peaks situated at 5966 and 5979 eV (named
peaks A and B1) originate from the Pr 2p ! 5d transitions.
At 300 K, Pr3þ (4f 2 ) sites essentially contribute to thepeak A, with a little component at peak B1 caused
presumably by multiple scattering, which is commonly
introduced in the theoretical calculation of XANES.16) Whenthe temperature crosses the transition temperature TMI ¼
132 K, the shape of the XANES spectra changes markedly.
The intensity of peak B1 increases and, at the same time,the intensity of peak A decreases and a new component(peak B2 at 5969 eV) can be resolved on its high-energy
slope. The B1 and B2 peaks are manifestations of Pr4þstates, namely, of the conﬁgurations 4f 1 and 4f 2 L,
respectively.17–19) Similar behaviors are also observed forthe y ¼ 0:075 sample below TMI ¼ 64 K, as shown in
Fig. 3(b). These results suggest that the valence of the Pr
ions increases from 3+ toward 4+ below TMI , consistently
with what was reported for the Pr0:5 Ca0:5 CoO3 sample by
Garcı́a-Munoz et al.16) The temperature dependence of the
XANES spectra at the Pr L3 -edge was also measured for thePr0:7 Ca0:3 CoO3 sample (not shown), which was metallic
over the entire temperature range. There was no spectralchange down to 8 K, which suggested that the valence of the
Pr ion remained at 3.0+, as shown in the results of themetallic Pr0:55 Ca0:45 CoO3 sample.16)
In order to determine the valence shift quantitatively, the
XANES spectra were ﬁtted to a sum of three Lorentzianfunctions (peak A for Pr3þ and peaks B1 and B2 for Pr4þ )and one arctangent function representing the continuum
excitations, as indicated in x2. The Pr6 O11 spectrum shownin Fig. 3(c), in which no relevant spectroscopic changes
were detected at low temperatures, was regarded as astandard for Pr3:667þ . The spectra at 300 K for each of the(Pr1y Yy )0:7 Ca0:3 CoO3 samples served as standards of
Pr3:0þ . In the ﬁtting of the temperature-dependent spectra,

H. FUJISHIRO et al.

3normalized μ (E)because all praseodymium ions remained in the Pr3þ valencewith a nonmagnetic singlet ground state.21) Instead, one may
notice a broad maximum in Fig. 2(b) with a center at about
17 K, which is a sign of a closely-lying exited singlet stateat an energy of about 4 meV. The total entropy change,
which was obtained by the integration of CSchottky =T overT , was determined to be 0.61 and 0.98 J K1 mol1 forthe y ¼ 0:075 and 0.15 samples, respectively. Considering
the theoretical R ln 2 ¼ 5:77 J K1 mol1 , the number ofKramers ions amounts to 0.12 and 0.17 Pr4þ per f.u., whichcorresponds to average valences of praseodymium, i.e.,
3.17+ and 3.29+ for the y ¼ 0:075 and 0.15 samples,
respectively.
It should be noted that the original interpretation of the
low-temperature Schottky peak in the (Pr1y Yy )0:7 Ca0:3 CoO3 samples presumed a purely ionic state of Pr4þ ,particularly the free-ion multiplet 2 F5=2 split by a crystal
ﬁeld to Kramers doublets. The character of these states maychange signiﬁcantly in the covalent case, but the number ofPr4þ species determined in ref. 15 remains valid, since boththe ionic 4f 1 and oxygen-hole 4f 2 L states involve odd
number of electrons and display Kramers degeneracy.

normalized μ (E)J. Phys. Soc. Jpn. 81 (2012) 064709

2

3.6
Pr 6O11
3.4
3.2
3
0.1

exp
fit

standard sample at 300K
y=0.075
y=0.15
0.2

0.3

0.4

0.5

0.6

0.7

0.8

I /I

B1 A

A

1
B2

B1
arctan

0

5950

5960

5970

5980

5990

6000

E (eV)
Fig. 3. (Color online) Temperature dependence of the XANES spectraat the Pr L3 -edge for the (a) y ¼ 0:15 and (b) 0.075 samples. The inset
of each ﬁgure shows the magniﬁcation of the spectra related to Pr4þ ions.
(c) XANES spectra for Pr6 O11 at 300 K, for which the ﬁtting were
performed (see text). The inset shows the calibration line for thedetermination of the valence of the Pr ion used in the study.

the intensity ratio of B1 to B2 was ﬁxed to be unity. The
valence of Pr ions for the samples in the course of
temperature was simply deduced from the intensity ratio
IB1 =IA of the B1 spectral peak to the A spectral peak22) using
the calibration line in the inset of Fig. 3(c). The validity ofthis procedure is based on the fact that the Pr3þ and Pr4þvalence states in Pr6 O11 are manifested in XANES spectraby single and double peaks, respectively (see also ref. 23),
which points to the same ionic 4f 2 and covalent 4f 1 =4f 2 L
conﬁgurations as those of the praseodymium ions in thepresent cobaltites.

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J. Phys. Soc. Jpn. 81 (2012) 064709

2.5

3.4
y=0.15

measuredarctanLorentz1 (A)Lorentz2 (B2)
Lorentz3 (B1)
simulation

T=8 K

1.5

(Pr1-yYy)0.7Ca0.3CoO3Valence of Pr ion

normalized μ (E)2.0

H. FUJISHIRO et al.

1.0

3.3 3.27+
y=0.075
y=0.15

TMI

3.2

3.15+

3.1

0.5
3.0
0

5950

5960

5970

5980

5990

0

6000

E (eV)

50

100

150
T (K)

200

250

300

Fig. 4. (Color online) Example of the ﬁtting of the XANES spectrum forthe y ¼ 0:15 sample at 8 K. For the ﬁtting, one arctangent function and three
Lorentzian functions are used (see text).

Fig. 5. (Color online) Temperature dependence of the valence of Pr ionsin the samples estimated using the XANES spectra and curve ﬁtting.

Figure 4 shows an example of the ﬁtting curve of the
XANES spectrum for the y ¼ 0:15 sample at 8 K, where thevalence change achieves a maximum. It is seen that the
XANES spectrum can be well ﬁtted within the energy range
including the related peaks of Pr3þ and Pr4þ . The B2component is also resolved and appears notable so thatthe resemblance with the Pr3þ /Pr4þ mixture in Pr6 O11 is
obvious. The temperature course is shown in Fig. 5. The
average valence changes gradually on cooling from 300 K,
with the steepest increase at TMI , and reaches ﬁnal values of
3.15+ and 3.27+ at 8 K for the y ¼ 0:075 and 0.15 samples,
respectively. The ambiguity of the estimated valence values
is 0:03, which results from the arbitrariness of theparameters in the arctangent and Lorentzian functions used.
We also deduced the average valence of Pr ions from theintensity ratio ðIB1 þ IB2 Þ=ðIA þ IB1 þ IB2 Þ of the B1, B2,
and A spectral peaks using another calibration line, in which
all the contributions of 4f 1 , 4f 2 , and 4f 2 L were considered tobe the same as those adopted by Yamaoka et al.17) Under
this condition, the average valences of Pr ions were 3.15+
and 3.31+ for the y ¼ 0:075 and 0.15 samples, which arenearly the same as that decided from the intensity ratio
IB1 =IA . Note that the estimated valences of the Pr ions are invery good agreement with that determined from the lowtemperature speciﬁc-heat experiments, i.e., 3.17+ and 3.29+
for the y ¼ 0:075 and 0.15 samples, respectively.15) The
gradual increase in the average valence of the Pr ions at
T > TMI on cooling from 300 K also suggests that thevalence of the Co ion must evolve from 3.3+ to a lower
value in order to maintain the charge neutrality. This means
that, with decreasing temperature, part of the LS-Co4þ
changes to IS-Co3þ still within the metallic phase, and thenthe IS-Co3þ states transform to LS-Co3þ below TMI . The
gradual charge transfer between the praseodymium andcobalt subsystem is in striking contrast to the abrupt (ﬁrstorder) character of the physical properties such as resistivity
and susceptibility, especially at y ¼ 0:075.
The valences of the Pr ions of 3.15+ and 3.27+ for the
y ¼ 0:075 and 0.15 samples, respectively, estimated fromthe intensity ratio IB1 =IA in XANES spectra, correspondto 0.097 and 0.161 Pr4þ per f.u. and is indicative of asigniﬁcant change in doping level in the cobalt subsystem(Co4þ content) in the low-temperature insulating state. For
y ¼ 0:075, the common valence distribution in the metallic
state (T  TMI ), i.e., (Pr0:925 3þ Y0:075 3þ )0:7 Ca0:3 2þ Co0:7 3þ Co0:3 4þ O3 2 , changes to (Pr0:786 3þ Pr0:139 4þ Y0:075 3þ )0:7 Ca0:3 2þ Co0:797 3þ Co0:203 4þ O3 2 (T  TMI ). For y ¼ 0:15,
the (Pr0:85 3þ Y0:15 3þ )0:7 Ca0:3 2þ Co0:7 3þ Co0:3 4þ O3 2 in themetallic state changes to (Pr0:620 3þ Pr0:230 4þ Y0:15 3þ )0:7 Ca0:3 2þ Co0:861 3þ Co0:139 4þ O3 2 in the low-temperature insulating state. The electron transfer between the praseodymium and cobalt subsystems thus acts as the driving force
for the stabilization of the LS-Co3þ states in the lowtemperature insulating state. Discussions of the relation
between the valence shift and the transition temperature TMI ,and of the eﬀect of the Ca ions on the transition are in
progress.
4.

Conclusion

The temperature dependence of the X-ray absorption nearedge spectra (XANES) at the Pr L3 -edge was measured fortwo (Pr1y Yy )0:7 Ca0:3 CoO3 samples (y ¼ 0:075 and 0.15), in
which a peculiar metal–insulator (MI) transition and a spinstate (SS) transition took place simultaneously at criticaltemperatures TMI ¼ 64 and 132 K, respectively. The existence of two distinct praseodymium species was evidenced: the Pr3þ valence state of the ionic 4f 2 conﬁgurationand the formal Pr4þ valence state of highly covalentcharacter with admixed 4f 1 and oxygen-hole 4f 2 L conﬁgurations. The average valences of the praseodymium ion
were estimated by analyzing the XANES spectra. The
important experimental results and conclusions are summarized as follows.
(1) The room-temperature XANES experiment spectra
show a dominant single-peaked feature A (5966 eV),characteristic of the presence of the pure Pr3þ valence
state. On cooling across the transition temperature TMI , anincreasing population of Pr4þ states becomes evident fromthe fast increase in the intensity of peak B1 (5979 eV) in the
spectrum. At the same time, the intensity of peak A due toPr3þ decreases.
(2) The results obtained are indicative of a rather gradualcharge transfer between the praseodymium and cobalt sites
in the perovskite structure, which precedes the spin-state

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transition of cobalt ions that occurs abruptly at a welldeﬁned TMI .
(3) Below TMI , the population of Pr4þ ions continues toincrease, reaching saturation at low temperatures. Using thequantitative analysis of the XANES spectra, the averagevalence of Pr ions at 8 K is determined to be 3.15+ and3.27+ for the y ¼ 0:075 and 0.15 samples, respectively.
These valences are in good agreement with those obtained
from the entropy change using the Schottky peak due to thePr4þ Kramers doublet in the low-temperature speciﬁc heat,i.e., 3.17+ and 3.29+.
Acknowledgments
The synchrotron radiation experiments were performedat the BL01B1 of SPring-8 with approval of Japan Synchrotron Radiation Research Institute (JASRI) (Proposal
No. 2011A1060). Part of the work was performed under the
ﬁnancial support of the Grant Agency of the Czech Republic
within the Project No. 204/11/0713.

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