PHYSICAL REVIEW B 87, 155153 (2013)

Simultaneous valence shift of Pr and Tb ions at the spin-state transition in (Pr1− y Tb y )0.7 Ca0.3 CoO3
H. Fujishiro,1 T. Naito,1 D. Takeda,1 N. Yoshida,1 T. Watanabe,1 K. Nitta,2 J. Hejtmánek,3 K. Knı́žek,3 and Z. Jirák3
1

Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan
3
Institute of Physics, ASCR, Cukrovarnická 10, 162 00 Prague 6, Czech Republic(Received 11 December 2012; published 29 April 2013)2

Temperature dependence of the x-ray absorption near-edge structure (XANES) spectra at the Pr L3 and Tb L3
edges was measured for the (Pr1−y Tby )0.7 Ca0.3 CoO3 system, in which a sharp spin-state (SS) transition took place
at a critical temperature TSS . A small increase in the valence of the terbium ion was found below TSS , besides theenhancement of the praseodymium valence; the trivalent states, which are stable at room temperature, change to a
3 + /4 + ionic mixture at low temperatures. In particular for the y = 0.2 sample, the average valence determinedat 8 K amounts to 3.25 + and 3.03 + for the Pr and Tb ion, respectively. In analogous (Pr1−y REy )0.7 Ca0.3 CoO3
samples (RE = Sm and Eu), in which the SS transition also took place, no valence shift of the RE ion was detectedin the XANES spectra at the RE ion L3 edge. The role of the substituted RE ion for the Pr site on the SS transition
is discussed.
DOI: 10.1103/PhysRevB.87.155153

PACS number(s): 71.30.+h, 78.70.Dm, 75.30.Wx

I. INTRODUCTION

The perovskite cobaltites RECoO3 (RE = rare-earth element and Y) show a spin-state (SS) transition of Co3+ ions
6 0eg , S = 0) to a high spin statefrom a low spin state (LS; t2g
4 2(HS; t2g eg , S = 2) with increasing temperature, followed by
the formation of the metallic state of the intermediate spin state
5 ∗(IS; t2g
σ , S = 1) at higher temperatures.1 The temperatureinduced spin-state transition and its gradual course indicate asmall energy difference δE between the crystal-field splittingand Hund coupling energy.2,3 The hole-doped systems such
as La1−x Srx CoO3 generally show a temperature stable phasewith itinerant cobalt states (presumably the mixed IS Co3+ /LS
Co4+ or HS Co3+ /IS Co4+ configurations) and undergo a ferromagnetic ordering at low temperatures. Most interestingly,
some Pr-based cobaltites exhibit a pronounced first-ordertransition to a low-temperature phase of weakly paramagnetic
character and reduced conduction. It was revealed for the firsttime on Pr0.5 Ca0.5 CoO3 at TSS ∼ 90 K, and was documented,
in addition to the steplike resistivity jump, by concomitantanomalies in the magnetic susceptibility, heat capacity, andlattice dilatation.4,5 The change in the electronic structure wasconfirmed at TSS by the photoemission spectroscopy.6 The
mechanism of the transition was tentatively ascribed to a spinstate crossover from the itinerant cobalt states to an ordered
5 0eg , S = 1/2)
mixture of localized LS Co3+ and LS Co4+ (t2g
3+4+states. Thereafter, the existence of Co /Co ordering wasquestioned because similar transition was evidenced also in theless doped Pr1−x Cax CoO3 (x = 0.3) under high pressures,7 orin the (Pr1−y REy )1−x Cax CoO3 system (0.2  x  0.5) witha partial substitution of Pr by smaller RE cations such as
Sm, Eu, and Y under ambient pressure.7–9 This transition
appeared to be conditioned not only by the presence of both
Pr and Ca ions, but also by a larger structural distortion of the
CoO6 network, depending on the average ionic radius and size
mismatch of perovskite A-site ions.9 Furthermore, the criticaltemperature TSS was found to be depressed by the appliedmagnetic field.10,11
An alternative scenario explaining the nature of suchspecific transition was proposed on the basis of electronic
1098-0121/2013/87(15)/155153(7)structure calculations exploiting the temperature dependenceof the structural experimental data for Pr0.5 Ca0.5 CoO3 .12
It appeared that the formal cobalt valence should change
below TSS from mixed-valence Co3.5+ towards pure Co3.0+
with strong preference for the LS state and, concomitantly,
the praseodymium valence should increase simultaneously
from Pr3+ towards Pr4+ . The SS transition and electronic
localization was thus intuitively interpreted as an analogy of
the compositional transition from the ferromagnetic metal of
La0.5 Ca0.5 CoO3 to the diamagnetic insulator of LaCoO3 .13,14
Shortly afterwards, the theoretical hypothesis about the crucialrole of variable praseodymium valence was experimentallysupported by observation of a Schottky peak in the lowtemperature specific heat of (Pr1−y Yy )0.7 Ca0.3 CoO3 (y =0.075 and 0.15),15 which proved a stabilization of Kramers
Pr4+ ions in the low-temperature phase.
Recently, we directly confirmed the praseodymium valence
shift, realized in fact as a mixture of the Pr3+ and Pr4+
states, from the x-ray absorption near-edge structure (XANES)
spectra at the Pr L3 edge for the same (Pr1−y Yy )0.7 Ca0.3 CoO3
samples.16 It has been found that the average valence of thepraseodymium ion increases below room temperature from thecommon value 3.0 + , undergoes the steepest change at TSS ,and reaches finally 3.15 + and 3.27 + at 8 K for the y = 0.075
and 0.15 samples, respectively. Interestingly, these values are
consistent with those estimated by quantitative analysis of
the entropy associated with the Schottky peak dominating
the low-temperature specific heat. For Pr0.5 Ca0.5 CoO3 , similar
valence shifts from 3.0 + at 300 K to 3.15 + at 10 K
were obtained by other authors using XANES at Pr L3 andM4,5 edges.17 In addition, a change of cobalt spin states
and the valence shift of the Co ion have been evidenced
by the analysis of two complementary synchrotron x-ray
spectroscopic techniques.18 Very recently, the spin state of
the Co3+ and Co4+ ions was estimated above and below TSS
by soft x-ray absorption spectroscopy at Co L2,3 and O 1s
edges in (Pr0.7 Sm0.3 )0.7 Ca0.3 CoO3 .19
In the present work, we apply XANES spectroscopy measurements to the analysis of (Pr1−y REy )0.7 Ca0.3 CoO3 systems(RE = Tb, Sm, and Eu), in which the SS transition between

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50–140 K was obtained.8,9 The choice of substituting RE ionswas motivated by the fact that terbium ion is known to adopt
the mixed valence state Tb3+ /Tb4+ in several compounds.20
The Sm and Eu ions are known to exist also in various valence
states. As an example, in the Sm-based filled skutterudite,
SmOs4 Sb12 , Sm ions possess a mixed valence state between
Sm2+ and Sm3+ , which can be detected by XANES.21 In
the Eu-contained niobates such as Eu2 Nb5 O9 and EuNbO3 ,a mixed valence state between Eu2+ and Eu3+ was detectedby photoelectron spectroscopy.22 In the present study, we
thus show a detailed temperature dependence of the XANES
spectra around the RE L3 edges measured using the bulk
sensitive transmission method, and derive the RE ion statesfrom the spectra.
II. EXPERIMENTAL

Polycrystalline samples (Pr1−y Tby )0.7 Ca0.3 CoO3 (y = 0,0.1, 0.2), (Pr1−y Smy )0.7 Ca0.3 CoO3 (y = 0.2, 0.3, 0.4), and(Pr1−y Euy )0.7 Ca0.3 CoO3 (y = 0.15, 0.2, 0.3) were fabricatedby a solid-state reaction. Raw powders of Pr6 O11 , Tb4 O7 ,Sm2 O3 , Eu2 O3 , Co3 O4 , and CaCO3 were weighted with proper
molar ratios and ground using an agate mortar and pestle
for 1 h. Mixed powders were calcined at 1000 ◦ C for 24 hin air. Then they were pulverized, ground, and pressed into
pellets of 20 mm diameter and 4 mm thickness. Pellets weresintered at 1200 ◦ C for 24 h in 0.1 MPa flowing oxygengas. The measured relative densities of each sample weregreater than 90%. Powder x-ray diffraction patterns weretaken for each sample using Cu Kα radiation; the samples
were confirmed to have a single-phase orthoperovskite Pbnm
structure. The oxygen stoichiometry was checked on the y =0 sample (Pr0.7 Ca0.3 CoO3 ) using a high-resolution neutron
diffraction, and practically ideal oxygen content 2.99 ± 0.01
has been found. For the XANES measurements, a small
amount of the samples were pulverized, mixed with boron
nitride powder (99.9%) with proper molar ratios in order to
optimize absorption, and pelletized 6 mm in diameter and
0.5 mm in thickness.
Each XANES spectrum of the samples was 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 at various temperatures from 8 to 300 K using a
cryocooler. The measurements were performed upon theheating run. The energy resolution was within 1.5 eVaround E = 8 keV.
To determine the mixed Pr3+ /Pr4+ contents at low temperatures for (Pr1−y Tby )0.7 Ca0.3 CoO3 , the calibration line wasobtained using XANES spectra of Pr6 O11 as Pr3.667+ and(Pr1−y Tby )0.7 Ca0.3 CoO3 as Pr3.0+ measured at 300 K, whichwere performed similarly to the previous paper.16 The recorded
XANES spectra were modeled by the sum of three Lorentzianfunctions and one arctangent function representing the steplikeedge of continuum excitations. One Lorentzian function (peakA: 5966 eV) shows an excitation from 2p3/2 to 4f 2 5d ∗ , which
represents Pr3.0+ ions. The other Lorentzian functions (peakB2: 5969 eV and peak B1: 5979 eV) show the excitations
from 2p3/2 to 4f 2 L5d ∗ and to 4f 1 5d ∗ , L being a ligand holein the O 2p orbital, both of which represent Pr4+ ions.23,24
The energy differences between peaks A, B2, and B1 were
fixed according to results of Hu et al.25 The curve fittings were
performed in the energy range from 5944 to 5985 eV using
ATHENA software.26
To estimate the mixed Tb3+ /Tb4+ contents, a similar
procedure was applied. A comparative measurement of Tb4 O7and (Pr1−y Tby )0.7 Ca0.3 CoO3 was carried out at 300 K as
standards of Tb3.5+ and Tb3.0+ , respectively. The recorded
XANES spectra for Tb L3 line were modeled by the sum of
two Lorentzian functions [peak C: 7518 eV for Tb3+ (4f 8 )and peak D: 7528 eV for Tb4+ (4f 7 )] and one arctangent
function. For simplicity, the peak related to the ligand hole
state 4f 8 L was omitted because of its weaker intensity asreported by Dexpert et al.27 For (Pr0.7 Sm0.3 )0.7 Ca0.3 CoO3 , and(Pr0.85 Eu0.15 )0.7 Ca0.3 CoO3 samples, the XANES spectra were
measured around Pr L3 (5966 eV), Sm L3 (6720 eV), and Eu
L3 (6981 eV) edges, respectively.
Electrical resistivity ρ(T ) was measured on the heating
run using a four-probe method with a parallelepiped sample cut from the pellet with a typical current density of
0.01 A/cm2 . Magnetic susceptibility χ (T ) was measured
under the magnetic field of 0.1 T after field cooling using
a superconducting quantum interference device (SQUID)
magnetometer (Quantum Design MPMS-XL).
III. RESULTS AND DISCUSSION
A. Magnetic and electrical propertiesin (Pr1− y Tb y )0.7 Ca0.3 CoO3

The
magnetic susceptibility χ (T ) of the(Pr1−y Tby )0.7 Ca0.3 CoO3 (y = 0, 0.1, 0.2) samples ispresented in Fig. 1(a). For the Tb free sample y = 0, χ (T )steeply increases to large values at low temperatures inaccordance with the ferromagnetic ground state of thiscompound (TC ∼ 55 K). The Tb-substituted samples exhibit amarked drop of magnetic susceptibility, which is a strong signthat cobalt ions transform to LS states at low temperature. A
sharp transition can be seen at 75 K for the y = 0.1 sample.
For the y = 0.2 sample, a broad transition was observed at
145 K. These characteristics of the Tb-substituted samples
are the same as those observed for (Pr1−y Yy )0.7 Ca0.3 CoO3
and other (Pr1−y REy )0.7 Ca0.3 CoO3 systems.8,9 It is worth
mentioning that, in distinction to Y and Sm substituted
samples, the low-temperature susceptibility increases withy, which should be related to large moment of Tb ion
independently on its trivalent or tetravalent state. (Tb3+ with
4f 8 configuration is a specific case of non-Kramers ion.
Although its eigenstates are nonmagnetic in the low-symmetry
crystal field of Pbnm perovskite structure, the ground and firstexcited singlets are nearly degenerate, forming a magnetic
quasidoublet.28 )The electrical resistivity ρ(T ) of the same samples
is presented in Fig. 1(b). The resistivity measured on
the y = 0 sample is only weakly temperature dependent,
without any sign of phase transition below 300 K. On
the other hand, the Tb-substituted samples show features typical for the conductivity change similarly to
Pr0.5 Ca0.5 CoO3 .

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FIG. 1. (Color online) Temperature dependence of the (a) magnetic susceptibility χ (T ) and (b) electrical resistivity ρ(T ) forthe (Pr1−y Tby )0.7 Ca0.3 CoO3 samples (y = 0, 0.1, and 0.2). The
measurements were performed on the heating runs.
B. XANES measurements of Pr L 3 and Tb L 3 edges

Figure 2(a) shows the temperature dependence of
the XANES spectra at the Pr L3 edge for the(Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 sample. Two main peaks situated at
5966 and 5979 eV (named peaks A and B1) originate fromthe Pr 2p → 5d transitions. At 300 K, the Pr3+ (4f 2 ) sites
essentially contribute to the peak A, with a little component at
peak B2, which is caused presumably by multiple scattering,commonly treated in the theoretical simulations of XANES.16
At temperatures close to TSS ∼ 145 K, the shape of the XANES
spectra changes markedly; the intensity of peak B1 increases
notably and shifts to lower energy side, while the intensity ofpeak A decreases. At the same time, a new component (peak B2
at 5969 eV) can be resolved on its high-energy slope. Since theB1 and B2 peaks are manifestations of Pr4+ states, originatingin particular of the configurations 4f 1 and 4f 2 L,23–25 theobserved changes confirm that the average valence of the
Pr ions in the (Pr1−y Tby )0.7 Ca0.3 CoO3 increases from 3 +toward 4 + below TSS , consistently with those reported for(Pr1−y Yy )0.7 Ca0.3 CoO3 by us16 or for Pr0.5 Ca0.5 CoO3 by
Garcı́a-Muñoz et al.17 In Fig. 2(a), the first oscillation around6000 eV can be seen in the XANES spectra, where thebroad peak shifts to the high-energy side with decreasing
temperature. These results qualitatively suggest that the Pr-O
distance shortens with decreasing temperature. The detailed
analysis must be performed using extended x-ray absorption
fine structure (EXAFS).

FIG. 2. (Color online) Temperature dependence of the XANES
spectra at the (a) Pr L3 edge and (b) Tb L3 edge for the(Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 sample. Insets show the magnification
around the 4f 1 and 4f 7 spectra related with the Pr4+ and Tb4+ ions,
respectively.

Figure 2(b) shows the temperature dependence of
the XANES spectra at the Tb L3 edge for the same
(Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 sample. Two main peaks situatedat 7518 and 7529 eV (named peaks C and D) originate
from the Tb 2p → 5d transitions. The spectrum at 300 K is
characteristic for the Tb3+ (4f 8 ) valence, contributing to themain peak C. The form of XANES spectra is changing around
TSS ∼ 145 K. There is a small drop of the intensity of peakC, which is a fingerprint of Tb3+ , and at the same time, peak
D increases slightly and shifts to lower energy. The peak D
is a manifestation of Tb4+ states, namely, of the configuration
4f 7 .27,29 The observed behavior thus suggests that the valenceof the Tb ions increases below TSS , simultaneously with thevalence shift of Pr ions. In Fig. 2(b), a slight peak shift of
the first oscillation around 7552 eV can be also observed withdecreasing temperature.
C. Valence shifts of the Pr and Tb ions

In order to determine the valence shift of Pr ion quantitatively, the XANES spectra were fitted to a sum of three
Lorentzian functions (peak A for Pr3+ and peaks B1 and B2
for Pr4+ ) and one arctangent function, as indicated in Ref. 16.

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FIG. 3. (Color online) (a) Temperature dependence of the
XANES spectra at the Pr L3 edge for Pr0.7 Ca0.3 CoO3 . (b) The
XANES spectrum for Tb4 O7 at 300 K, fitted by a sum of two
Lorentzian functions (peak C for Tb3+ and peak D for Tb4+ ) andone arctangent function. The inset shows the calibration lines todetermine the valence of the Tb ion in the (Pr1−y Tby )0.7 Ca0.3 CoO3
samples (y = 0.1, 0.2).

Then the valence of Pr ions in the Tb-substituted samples wasdeduced from the intensity ratio IB1 /IA of the B1 spectral
peak to the A spectral peak using the calibration line thattook into account the actual IB1 /IA values for Pr3.0+ and thatfor Pr3.667+ (not shown). Figure 3(a) shows the temperaturedependence of the XANES spectra at the Pr L3 edge for purePr0.7 Ca0.3 CoO3 with ferromagnetic metallic ground state (seeFig. 1). Apart of the main peak A, there is again a small bumpat around 5980 eV, originating in the multiple scattering as
mentioned above. The same feature has been observed foranother metallic compound Pr0.55 Ca0.45 CoO3 ,17 and seems
thus to be a general manifestation of trivalent praseodymium
in the mixed Co3+ /Co4+ systems. Importantly, no spectralchange of the Pr L3 edge is observed down to 8 K, whichsuggests that the Pr3+ valence in Pr0.7 Ca0.3 CoO3 remains
temperature independent.
Figure 3(b) shows the XANES spectrum for Tb4 O7 at
300 K, which was fitted to a sum of two Lorentzian functions(peak C for Tb3+ and peak D for Tb4+ ) and one arctangent
function. The measurement was also performed down to lowtemperatures and since no relevant spectroscopic changes were

FIG. 4. (Color online) Examples of the fitting of the XANES
spectrum at the (a) Pr L3 and (b) Tb L3 edge for the(Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 sample at 8 K. For the fitting of the Pr L3edge, one arctangent function and three Lorentzian functions (A, B1,
and B2) are used. For the fitting of the Tb L3 edge, one arctangent
function and two Lorentzian functions (C and D) are used.

detected, the spectrum, in particular the intensity ratio ID /IC ,was further considered as a standard for Tb3.5+ . The valenceof Tb ions in the (Pr1−y Tby )0.7 Ca0.3 CoO3 samples was simply
deduced from observed intensity ratio ID /IC of the D to C
spectral peaks,23 taking the data at 300 K as a standard of
Tb3.0+ . This has been done with use of the calibration line in
the inset of Fig. 3(b); ID /IC = 0.856 for Tb3.5+ in Tb4 O7 andID /IC = 0.136 for Tb3.0+ in (Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 .
The fit of XANES spectra is exemplified in Fig. 4(a) using
the data for the (Pr0.8 Tb0.2 )0.7 Ca0.3 CoO3 sample at 8 K, wherethe valence change achieves a maximum. It turns out that the
XANES spectrum at Pr L3 edge is well reproduced within theenergy range from 5944 to 5985 eV including the peaks related
to Pr3+ and Pr4+ . The B2 component at the slope of main
peak A is also resolved and the presence of Pr3+ /Pr4+ mixture
is thus obvious. Figure 4(b) shows the fit of the XANES
spectrum at Tb L3 edge for the same sample, also at 8 K. The
XANES spectrum cannot be reproduced without consideringboth characteristic peaks of Tb3+ and Tb4+ (features C andD, respectively).
The results based on the curve fitting of XANES spectra aresummarized in Figs. 5(a) and 5(b), which show, respectively,

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FIG. 5. (Color online) Temperature dependence of the average valence of (a) Pr ion and (b) Tb ion for the (Pr1−y Tby )0.7 Ca0.3 CoO3 samples estimated using the XANES spectra and curve fitting. In (a), theestimated valence of Pr ion for the reported (Pr1−y Yy )0.7 Ca0.3 CoO3
samples (y = 0.075 and 0.15; Ref. 16) is also presented. The
uncertainty of the estimated valence values is within 1% of theabsolute value.

the temperature dependence of the valence of Pr and Tb ions inthe (Pr1−y Tby )0.7 Ca0.3 CoO3 samples. Contrary to the sharp SS
transitions detected by the magnetic susceptibility and electric
resistivity, the average Pr valence changes on cooling from300 K gradually, though with the steepest increase at TSS .
Finally at the lowest temperatures, the valence reaches final
values of 3.19 + and 3.25 + for the y = 0.1 and 0.2 samples,
respectively. The uncertainty of the estimated valence values
is within 1% of the absolute value, which arises from the arbitrariness of parameters used in the arctangent and Lorentzianfunctions. Comparing the results for (Pr1−y Tby )0.7 Ca0.3 CoO3
and those for the (Pr1−y Yy )0.7 Ca0.3 CoO3 samples (y = 0.075
and 0.15)16 also presented in Fig. 5(a), one finds that theenhancement of the Pr valence systematically increases withincreasing contents of the substitution with smaller Tb or Y
ions, i.e., with decreasing of the mean size of the A-site cationin the perovskite structure.
As the Tb valence in (Pr1−y Tby )0.7 Ca0.3 CoO3 is concerned,
it changes gradually around TSS and reaches final values of
3.01 + and 3.03 + at 8 K for the y = 0.1 and 0.2 samples,
respectively [see Fig. 5(b)]. We note, however, that the increase

FIG. 6. (Color online) Temperature dependence of the XANES
spectra at (a) Sm L3 edge for (Pr0.7 Sm0.3 )0.7 Ca0.3 CoO3 and (b) Eu L3edge for (Pr0.85 Eu0.15 )0.7 Ca0.3 CoO3 samples. Inset shows the magnetic
susceptibility χ (T ) of each sample.

in the Tb valence is about one order of magnitude smaller andits “gradual” character is even more pronounced than in thecase of Pr valence.
D. XANES measurements at RE edges for(Pr1− y R E y )0.7 Ca0.3 CoO3 (RE = Sm, Eu)

To illustrate the valence states of the substituted Sm and Eu
ions in (Pr1−y REy )0.7 Ca0.3 CoO3 , the temperature-dependent
XANES spectra at the Sm L3 edge and Eu L3 edge arecompiled in Figs. 6(a) and 6(b). The insets present the dataon magnetic susceptibility χ (T ) on the heating run, which
demonstrate the transition at 85 K and 55 K for the RE = Sm
(y = 0.3) and Eu (y = 0.15) samples, respectively.
For the samarium-substituted sample, although the intensityof the main peak of Sm3+ at 6720 eV slightly decreased withdecreasing T , no isomeric shift or new spectral feature is
detected. This result suggests that the Sm valence remains
essentially as 3.0 + over the entire temperature range. For the
europium-substituted sample, the spectrum does not change at
all down to 8 K and, consequently, the valence of the Eu ion
also remains 3.0 + . Let us note that properties of the Sm-doped
system became a subject of very recent paper by Guillou et al.19

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function of the average ionic radius of the perovskite A
site, rA . The experimental results for the RE = Y system(y = 0.075, 0.1, and 0.2) are also shown.16 It is seen that theinterrelation between TSS and rA  is not universal; the curves
gradually shift to the large-rA  side with decreasing ionicradius of RE. In this comparison, the curve for RE = Tb is not
peculiar, which can be understood, since the actual shift of Tb
valence is fairly small (to 3.03 + in maximum). Nonetheless,the possibility of Tb3+ /Tb4+ crossover in cobaltites is confirmed and additional studies aiming to exemplify this valencechange are under way. As a final note to Fig. 7(a) we may
mention that the TSS vs rA  relation is applicable only forthe (Pr1−y REy )0.7 Ca0.3 CoO3 systems and does not hold forthe intensively studied system Pr0.5 Ca0.5 CoO3−δ , in which theCo, Pr valences and TSS strongly depend on the actual oxygen
content.4,7,17,30,31
Finally, in Fig. 7(b) we plot the formal valence of Co ions inthe (Pr1−y REy )0.7 Ca0.3 CoO3 at 8 K as a function of rA , calculated based on the Pr or Tb valences estimated from XANES
data. This is enabled by the fact that, unlike Pr0.5 Ca0.5 CoO3−δ ,the Pr0.7 Ca0.3 CoO3 derived systems are prepared generally
as stoichiometric ones, as proved for selected samples bythermogravimetry or neutron diffraction method. The oxygenstoichiometry can be inferred also indirectly, comparing theelectric transport properties, as shown in our previous paper on
the Y-substituted samples.15 The results in Fig. 7(b) show thatthe doping level of the cobalt subsystem decreases from 0.30
hole per f.u. to about 0.15 hole per f.u., similar for Tb content of
y = 0.1 and 0.2. Analogous data for other RE system studied
by us are also presented.
FIG. 7. (Color online) (a) The transition temperature TSS and(b) the calculated valence of Co ion at 8 K, as a function of theaverage ionic radius of the perovskite A site, rA , for various(Pr1−y REy )0.7 Ca0.3 CoO3 systems. The results for the RE = Y werecited from Ref. 16. In (b), the uncertainty of the estimated valencevalues is within 1% of the absolute value.

E. Discussion

The unusual SS transition occurring in some Pr-based
cobaltites is intimately connected with a charge transfer
between praseodymium and cobalt sites. Such process isenabled by energy closeness of the Pr3+ /Pr4+ valence states.
In the previous subsections, a possibility of more complex valence equilibria has been probed on (Pr1−y REy )0.7 Ca0.3 CoO3
systems with the RE = Tb, Sm, Eu substitutions at Pr sites,in which mixed valence states, RE 3+ /RE 4+ or RE 3+ /RE 2+ ,can eventually also occur. Indeed, a small but unquestionable
increase in the valence of RE ion is observed for the
Tb-substituted samples below TSS , besides the substantial
enhancement of the Pr ion valence to 3.25 + . On the other
hand, no temperature variation of the Sm and Eu valence stateis detected for such RE-substituted samples. This means that
hypothetical redox reaction between rare-earth elements, suchas Pr3+ +Sm3+ → Pr4+ +Sm2+ , does not play any role bothabove and below the MI transition.
To demonstrate the effect of particular rare-earth elements,
the transition temperature TSS is plotted in Fig. 7(a) as a

IV. CONCLUSION

Temperature dependence of the x-ray absorption nearedge structure (XANES) spectra at the Pr L3 and RE L3
edges was measured for the (Pr1−y REy )0.7 Ca0.3 CoO3 samples(RE = Tb, Sm, and Eu), in which a spin-state (SS) transition
took place at a critical temperature TSS . The important
experimental results and conclusions are summarized asfollows.
(1) In all the studied Pr-based cobaltites, the trivalentpraseodymium ions change to a Pr3+ /Pr4+ mixture below TSS , which anticipates that, at the same time, theformal cobalt valence should decrease from 3.3 + closer
to 3.0 + .
(2) Besides the enhancement of the Pr valence, a small
increase in the valence of the Tb ion is found in the(Pr1−y Tby )0.7 Ca0.3 CoO3 samples below TSS . In the y = 0.2
sample, the average valence determined at 8 K makes 3.25 +and 3.03 + for the Pr and Tb ion, respectively. The calculated
Co valence is 3.156 + in this case. It should be noted thatthese results do not necessarily mean very different tendenciesof the two rare-earth ions for tetravalent states, since the final
valence balance in such chemically inhomogeneous systems
depends also on steric factors, including the size mismatch atperovskite A sites.
(3) The observed valence shift of Tb ion indicates the energy
closeness of trivalent and tetravalent states in this rare-earth
element. This fact suggests that the unusual SS transition need

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not be unique for the Pr-based cobaltites but can be expectedalso for the Tb-based ones.
(4) In the (Pr1−y REy )0.7 Ca0.3 CoO3 samples (RE = Sm andEu) with similar transition, no valence shift of the RE ion isdetected at the RE ion absorption edge in the XANES spectra.
This suggests that Eu3+ and Sm3+ are stable in the system overthe entire temperature range, and may signify that the energy
closeness of trivalent and divalent states in these rare-earth
elements has no effect on the praseodymium/cobalt valence
equilibria.

1

Z. Jirák, J. Hejtmánek, K. Knı́žek, and M. Veverka, Phys. Rev. B
78, 014432 (2008).
2
M. A. Korotin, S. Yu. Ezhov, I. V. Solovyev, V. I. Anisimov, D. I.
Khomskii, and G. A. Sawatzky, Phys. Rev. B 54, 5309 (1996).
3
J.-Q. Yan, J.-S. Zhou, and J. B. Goodenough, Phys. Rev. B 69,
134409 (2004).
4
S. Tsubouchi, T. Kyômen, M. Itoh, P. Ganguly, M. Oguni,
Y. Shimojo, Y. Morii, and Y. Ishii, Phys. Rev. B 66, 052418(2002).
5
S. Tsubouchi, T. Kyômen, M. Itoh, and M. Oguni, Phys. Rev. B 69,
144406 (2004).
6
T. Saitoh, Y. Yamashita, N. Todoroki, T. Kyomen, M. Itoh,M. Higashiguchi, M. Nakatake, and K. Shimada, J. Electron
Spectrosc. Relat. Phenom. 144–147, 893 (2005).
7
T. Fujita, T. Miyashita, Y. Yasui, Y. Kobayashi,
M. Sato, E. Nishibori, M. Sakata, Y. Shimojo, N. Igawa, Y.
Ishii, K. Kakurai, T. Adachi, Y. Ohishi, and M. Takata, J. Phys.
Soc. Jpn. 73, 1987 (2004).
8
T. Fujita, S. Kawabata, M. Sato, N. Kurita, M. Hedo, andY. Uwatoko, J. Phys. Soc. Jpn. 74, 2294 (2005).
9
T. Naito, H. Sasaki, and H. Fujishiro, J. Phys. Soc. Jpn. 79, 034710(2010).
10
M. Maryško, Z. Jirák, K. Knı́žek, P. Novák, J. Hejtmánek, T. Naito,
H. Sasaki, and H. Fujishiro, J. Appl. Phys. 109, 07E127 (2011).
11
T. Naito (unpublished).
12
K. Knı́žek, J. Hejtmánek, P. Novák, and Z. Jirák, Phys. Rev. B 81,
155113 (2010).
13
J. Wu and C. Leighton, Phys. Rev. B 67, 174408 (2003).
14
K. Knı́žek, Z. Jirák, J. Hejtmánek, and P. Novák, J. Magn. Magn.
Mater. 322, 1221 (2010).
15
J. Hejtmánek, E. Šantavá, K. Knı́žek, M. Maryško, Z. Jirák,
T. Naito, H. Sasaki, and H. Fujishiro, Phys. Rev. B 82, 165107(2010).
16
H. Fujishiro, T. Naito, S. Ogawa, N. Yoshida, K. Nitta, J. Hejtmánek,
K. Knı́žek, and Z. Jirák, J. Phys. Soc. Jpn. 81, 064079 (2012).

ACKNOWLEDGMENTS

The synchrotron radiation experiments were performedat the BL01B1 of SPring-8 with approval of Japan Synchrotron Radiation Research Institute (JASRI) (Proposals No.
2011A1060, No. 2011B1075, and No. 2012A1118). Part of thework was performed under the financial support of a Grant-inAid for Scientific Research (No. 24540355) from the Ministryof Education, Culture, Sports, Science, and Technology, Japan,
and the Grant Agency of the Czech Republic within the ProjectNo. 204/11/0713.

17

J. L. Garcı́a-Muñoz, C. Frontera, A. J. Barón-González, S. Valencia,
J. Blasco, R. Feyerherm, E. Dudzik, R. Abrudan, and F. Radu, Phys.
Rev. B 84, 045104 (2011); J. Herrero-Martı́n, J. L. Garcı́a-Muñoz,
S. Valencia, C. Frontera, J. Blasco, A. J. Barón-González, G. Subı́as,
R. Abrudan, F. Radu, E. Dudzik, and R. Feyerherm, ibid. 84, 115131(2011).
18
J. Herrero-Martı́n, J. L. Garcı́a-Muñoz, K. Kvashnina, E. Gallo,
G. Subı́as, J. A. Alonso, and A. J. Barón-González, Phys. Rev. B
86, 125106 (2012).
19
F. Guillou, Q. Zhang, Z. Hu, C. Y. Kuo, Y. Y. Chin, H. J. Lin,C. T. Chen, A. Tanaka, L. H. Tjeng, and V. Hardy, Phys. Rev. B 87,
115114 (2013).
20
E. Largeau, M. El-Ghozzi, and D. Avignant, J. Solid State Chem.
139, 248 (1998).
21
M. Mizumaki, S. Tsutsui, H. Tanida, T. Uruga, D. Kikuchi,
H. Sugawara, and H. Sato, J. Phys. Soc. Jpn. 76, 053706 (2007).
22
C. Felser, J. Kohler, A. Simon, O. Jepson, G. Svensson, S. Cramm,and W. Eberhardt, Phys. Rev. B 57, 1510 (1998).
23
H. Yamaoka, H. Oohashi, I. Jarrige, T. Terashima, Y. Zou,
H. Mizota, S. Sakakura, T. Tochio, Y. Ito, E. Ya. Sherman, andA. Kotani, Phys. Rev. B 77, 045135 (2008).
24
A. Bianconi, A. Marcelli, H. Dexpert, R. Karnatak, A. Kotani,
T. Jo, and J. Petiau, Phys. Rev. B 35, 806 (1987).
25
Z. Hu, S. Bertram, and G. Kaindl, Phys. Rev. B 49, 39 (1994).
26
B. Ravel and M. Newville, J. Synchrotron Rad. 12, 537 (2005).
27
H. Dexpert, R. C. Karnatak, J. M. Esteva, J. P. Connerade, M.
Gasgnier, P. E. Caro, and L. Albert, Phys. Rev. B 36, 1750 (1987).
28
J. B. Gruber, K. L. Nash, R. M. Yow, D. K. Sardar, U. V. Valiev,
A. A. Uzokov, and G. W. Burdick, J. Lumin. 128, 1271 (2008).
29
G. Kalkowski, G. Kaindl, G. Wortmann, D. Lentz, and S. Krause,
Phys. Rev. B 37, 1376 (1988).
30
P. Tong, Y. Wu, B. Kim, D. Kwon, J. M. S. Park, and B. G. Kim,
J. Phys. Soc. Jpn. 78, 034702 (2009).
31
A. Chichev, M. Dlouhá, S. Vratislav, J. Hejtmánek, Z. Jirák,
K. Knı́žek, and M. Maryško, Z. Kristallogr. Suppl. 26, 435 (2007).

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