Eur. Phys. J. B (2013) 86: 305
DOI: 10.1140/epjb/e2013-30653-y

THE EUROPEAN
PHYSICAL JOURNAL B

Regular Article

Phase transition in Pr0.5Ca0.5CoO3 and related cobaltites
J. Hejtmánek1,a , Z. Jirák1 , O. Kaman1 , K. Knı́žek1, E. Šantavá1, K. Nitta2 , T. Naito3 , and H. Fujishiro3
1

Institute of Physics, ASCR, Cukrovarnicka 10, 162 00 Prague 6, Czech Republic
Japan Synchrotron Radiation Research Institute, Sayo, 679-5198 Hyogo, Japan
3
Faculty of Engineering, Iwate University, 4-3-5 Ueda, 020-8551 Morioka, Japan
2

Received 20 July 2012 / Received in ﬁnal form 7 December 2012c EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2013
Published online 3rd July 2013 – 
Abstract. We present an extensive investigation (magnetic, electric and thermal measurements and X-ray
absorption spectroscopy) of the Pr0.5 Ca0.5 CoO3 and (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.0625−0.15) perovskites, in which a peculiar metal-insulator (M-I) transition, accompanied with pronounced structuraland magnetic anomalies, occurs at 76 K and 40−132 K, respectively. The inspection of the M-I transition
using the XANES data of Pr-L3 -edge and Co-K-edge proofs the presence of Pr4+ ions at low temperaturesand indicates simultaneously the intermediate spin to low spin crossover of Co species on lowering thetemperature. The study thus deﬁnitively conﬁrms the synchronicity of the electron transfer between Pr3+
ions and Co3+/4+ O3 subsystem and the transition to the low-spin, less electrically conducting phase. The
large extent of the transfer is evidenced by the good quantitative agreement of the determined amount of
the Pr4+ species, obtained either from the temperature dependence of the XANES spectra or via integration of the magnetic entropy change over the Pr4+ related Schottky peak in the low-temperature speciﬁcheat. These results show that the average valence of Pr3+ /Pr4+ ions increases (in concomitance with thedecrease of the formal Co valence) below TMI for (Pr0.925 Y0.075 )0.7 Ca0.3 CoO3 up to 3.16+ (the doping
level of the CoO3 subsystem decreases from 3.30+ to 3.20+), for (Pr0.85 Y0.15 )0.7 Ca0.3 CoO3 up to 3.28+(the decrease of doping level from 3.30+ to 3.13+) and for Pr0.5 Ca0.5 CoO3 up to 3.46+ (the decrease of
doping level from 3.50+ to 3.27+).

1 Introduction
The hole doped cobaltites of the general formulaLn1−x Aex CoO3 (x > 0.2; Ln, Ae – lanthanide and alkali
earth ion, respectively) exhibit generally a robust metallic state with ferromagnetic ordering at low temperatures.
As an exception, a sharp metal-insulator (M-I) transition
at TMI ∼ 90 K has been observed on the “half-doped”
Pr0.5 Ca0.5 CoO3 by Tsubouchi et al. [1] and, later on,
also on related compounds of lower dopings. It turns out
now that the transition is conditioned not only by presence of praseodymium ions, but also by a suitable structural distortion which depends on the average ionic radius and size mismatch of the perovskite A-site ions [2].
Furthermore, the stability of the low-temperature insulating phase can be controlled by external means. In particular, the high pressure increases TMI and may even induce the transition in some samples with otherwise stable
metallic state [3]. On the other hand, TMI is lowered bymagnetic ﬁeld when the “metallic” state can be ﬁnallystabilized under a high magnetic ﬁeld, in particular at
8 T in (Pr1−y Yy )0.7 Ca0.3 CoO3 for y = 0.0625 [4]. Similarly, the signiﬁcant oxygen deﬁciency in polycrystallineContribution to the Topical Issue “New Trends in Magnetism and Magnetic Materials”, edited by Francesca Casoli,
Massimo Solzi and Paola Tiberto.
ae-mail: hejtman@fzu.cz

Pr0.5 Ca0.5 CoO3−δ samples can lead to complete suppression of the TMI as recently shown by Yoshioka et al. [5].
For such highly oxygen deﬁcient sample the analysis of
the Pr-L3 and Co-K X-ray absorption near-edge structure(XANES) complemented by the ﬁrst-principle calculations
revealed, that the valence state of Co ions is instead of
3.5+ (stoichiometric Pr0.5 Ca0.5 CoO3 ) only 3.106+ leading to considerable oxygen deﬁciency of δ ∼ 0.197. Thisobservation corroborates both the absence of the metalinsulator transition and the trivalent character of Pr ions,derived from Pr-L3 spectra, and underlines the necessity of the control of the oxygen stoichiometry during the
preparation procedure.
The M-I transition observed in the mentioned
praseodymium-based cobaltites is further manifested bya pronounced peak in the speciﬁc heat, by striking susceptibility drop and large volume contraction. Althoughthese signatures bear resemblance to common spin-statecrossover of Co ions, there is a growing evidence that inherent part of the transition is a signiﬁcant drop of thehole doping, formally from the mixed valence Co3+ /Co4+
closer to pure Co3+ , enabled by closeness in energy of
the Pr4+ and Pr3+ states. This scenario was originallysuggested in order to account for anomalous shorteningof some Pr-O bondlengths, whereas the expected shrinkof the CoO6 octahedra due to the stabilization of lower
spin states of Co has not been observed [6]. The existencePage 2 of 8

of Pr → CoO3 electron transfer was supported later
theoretically by GGA + U electronic structure calculations exploiting the temperature dependence of the structural experimental data for Pr0.5 Ca0.5 CoO3 [7], and experimentally, in particular by observation of Schottky
peak in the low temperature speciﬁc heat, related to
Zeeman splitting of the ground doublet of Kramers ionsPr4+ [8]. The direct proof of the mixed Pr3+/Pr4+ valence, was provided for Pr0.5 Ca0.5 CoO3 using the Pr-L3
and M4,5 edge X-ray absorption spectroscopy (XAS) by
Garcia-Muñoz et al. [9] and Herrero-Martin et al. [10],demonstrating also the spin change on cobalt sites bymeans of the Co-K and L-edge spectroscopies [11]. Using the measurement of the temperature dependence of
the X-ray absorption near-edge structure (XANES) spectra at the Pr-L3 edge on (Pr1−y Yy )0.7 Ca0.3 CoO3 system,
Fujishiro et al. [12] observed similar eﬀect, i.e. the presence of mixed Pr3+ / Pr4+ valence at temperatures below
the TMI .
In this paper, we present an extensive investigation of the Pr0.5 Ca0.5 CoO3 (50% Co4+ ) and(Pr1−y Yy )0.7 Ca0.3 CoO3 (30% Co4+ ) systems by meansof magnetic, electric, thermal measurements and X-ray
absorption spectroscopy. Both the magnetic entropy associated with the low temperature Schottky-like anomaly
in speciﬁc heat, quantitatively linked with the presenceof Pr4+ ions, and the analysis of XANES spectra of
Pr-L3 edge, point to a signiﬁcant valence shift of Pr below the TMI . In Pr0.5 Ca0.5 CoO3 , the essentially trivalentpraseodymium ions at 300 K change to a Pr3+ /Pr4+ mixture. The average valence values determined at the lowest
temperature, Pr3.52+ from Schottky anomaly and Pr3.40+
from XANES spectra, are signiﬁcantly higher than thevalue previously deduced using the Pr-L3 and M4,5 edge
spectra by Herrero-Martin et al. [10,11] (from Pr3.0+ at
300 K ⇒ Pr3.15+ at 10 K) and are also higher thanpraseodymium valence shifts found by us in the less doped(Pr1−y Yy )0.7 Ca0.3 CoO3 systems [8,12]. The simultaneous
inspection of the M-I transition using the XANES spectra of Co-K-edge conﬁrms the change of Co spin state on
crossing the M-I transition.

2 Experimental
Polycrystalline (Pr1−y Yy )0.7 Ca0.3 CoO3 samples were prepared by a solid-state reaction, the detailed sample preparation procedures and chemical characterization proving both the phase purity and oxygen stoichiometry
are described elsewhere [8]. The Pr0.5 Ca0.5 CoO3 sample
was, however, prepared using “soft chemistry route” bymeans of sol-gel procedure. The stoichiometric amountsof Pr6 O11 and CaCO3 with chemically determined metal
content were dissolved in nitric acid and mixed with volumetric solution of Co(NO3 )2 . After removal of the excessive nitric acid by heating and adjustment of pH to 3
by ammonia, the citric acid (1.5nmetals) and ethylene glycol (3nmetals ) were added. The precursor was formed by
gradual evaporation and subjected to slow heating above
250 ◦ C. The pulverized material was calcinated in air atEur. Phys. J. B (2013) 86: 305

400 ◦ C. After being pressed into pellets, it was sintered at
1200 ◦ C, followed with a long-term annealing at 800 ◦ C inoxygen atmosphere. The ceramic material was found single phase but oxygen deﬁcient, so that an additional annealing was undertaken in high pressure cell (200 atm O2 )at 600 ◦ C to achieve an oxygen stoichiometric compound.
The X-ray diﬀraction proved single perovskite phaseof orthorhombic Pbnm symmetry in all the systems.
Among the (Pr1−y Yy )0.7 Ca0.3 CoO3 samples the parameters a = 5.3472(9) Å, b = 5.3513(8) Å, c = 7.5584(9) Å,V = 216.28(6) Å3 have been obtained in particular forthe y = 0.10 composition. The lattice parameters of
Pr0.5 Ca0.5 CoO3 have been reﬁned to a = 5.3384(6) Å,b = 5.3395(7) Å, c = 7.5415(6) Å, V = 214.96(4) Å3 . Considering the excellent agreement of our crystallographic
data with lattice parameters and unit cell volumes reported by Tsubouchi et al. [1] or Barón-González et al. [6]for Pr0.5 Ca0.5 CoO3 samples, which oxygen stoichiometry
has been conﬁrmed by iodometric titration and by neutron diﬀraction reﬁnement, respectively, we consider ourPr0.5 Ca0.5 CoO3 sample as oxygen stoichiometric.
The magnetic and speciﬁc heat measurements were
performed using Quantum Design MPMS and PPMS devices; the experiments at very low temperatures down
to 0.4 K were done using the He3 option. The magnetic susceptibility was measured under an applied ﬁeldof 0.1 T, employing the zero-ﬁeld and ﬁeld-cooled regimes
during warming and cooling the sample, respectively.
Thermal conductivity, thermoelectric power and electricalresistivity were measured using a four-probe method witha parallelepiped sample cut from the sintered pellet. The
measurements were done on sample cooling and warming
using a close-cycle cryostat down to 3.5 K, the detailed
description of the cell including calibration is described
elsewhere [13].
For the XANES measurements, a part of the samples
was pulverized, mixed with 3N boron nitride (BN) powder with proper molar ratios in order to optimize the absorption and pelletized 6 mm in diameter and 0.5 mm inthickness. The Pr-L3 -edge and Co-K-edge XANES spectra were measured at BL01B1 of SPring-8 in Japan undertransmission mode with the detectors of ionization chambers and were obtained at various temperatures from 8 to
300 K using a cryocooler [12]. The valence of Pr ions inperovskite samples was supposed to be 3.0+ at 300 K, forthe determination of the mixed Pr4+ content below TMI , a
comparative measurement on the oxygen balanced Pr6 O11
was made. The XANES spectra were analyzed using thesum of Lorentz functions and one arctangent function representing the “baseline edge” of continuum excitations.
For more detailed description see details in reference [12].

3 Results and discussion
The M-I transitions in our samples are illustrated by peaksin the speciﬁc heat in Figure 1, located at TMI = 76 K
for the prototypical compound Pr0.5 Ca0.5 CoO3 and at
TMI = 40−132 K for the (Pr1−y Yy )0.7 Ca0.3 CoO3 systems

Eur. Phys. J. B (2013) 86: 305

Fig. 1. Temperature dependence of the speciﬁc heat=
0.0625−0.15) andof (Pr1−y Yy )0.7 Ca0.3 CoO3 (yPr0.5 Ca0.5 CoO3 . The critical temperatures of M-I transitions are marked.

Fig. 2. The temperature dependence of electrical resistivity of (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.0625−0.15) andPr0.5 Ca0.5 CoO3 , plotted in log-log scale. The data for
Pr0.7 Ca0.3 CoO3 with ferromagnetic metallic ground state areadded for comparison.

with y = 0.0625−0.15. The electrical resistivity of thestudied samples is presented in Figure 2 together with thedata for ferromagnetic “metallic” Pr0.7 Ca0.3 CoO3 sample. Although the character of conduction can be somewhat “masked” by the polycrystalline form of samples,
we clearly identify two distinct behaviors: (i) the real
macroscopically “insulating” state with steeply increasing resistivity at low temperatures, represented by the(Pr1−y Yy )0.7 Ca0.3 CoO3 sample y = 0.15 and (ii) the
behavior typical for “bad” or “highly disordered” metals characterized by slowly increasing but ﬁnite resistivity at low temperatures, which is represented by thePr0.7 Ca0.3 CoO3 compound. Most interestingly, similar ﬁnite resistivity is also observed in the low-temperature“insulating” state of prototypical Pr0.5 Ca0.5 CoO3 . The
corresponding ground state of the Pr0.5 Ca0.5 CoO3 sample is thus quite complex, likely linked to the ﬁrst ordertransition associated with magnetic disorder and strong
electronic correlations [14]. Let us note that our sample

Page 3 of 8

Fig. 3. The thermoelectric power of (Pr1−y Yy )0.7 Ca0.3 CoO3
(y = 0.0−0.15) and Pr0.5 Ca0.5 CoO3 . The transition is evident from the increase below the TMI , the data for cobaltites
Pr1−x Cax CoO3 of lower hole doping are given for comparison.

and the original one of Tsubouchi et al. [1], exhibit
both a thermal hysteresis at M-I transition. For the(Pr1−y Yy )0.7 Ca0.3 CoO3 samples, however, the transitionshows itself as non-hysteretic within the experimental uncertainty, as documented simultaneously by the thermoelectric power data in Figure 3. Here, the sharp jumpof thermopower coeﬃcient below the TMI indicates thatthe charge carrier concentration is decreased and theiritinerancy is strongly inhibited in the low-temperaturephase. In contrast to the 30% doping of holes in the Co
subsystem at 300 K, the ﬁnal level can be estimated tohole concentration between 10−20% based on a comparison of the low-temperature thermopower with data on
the Pr0.9 Ca0.1 CoO3 and Pr0.8 Ca0.2 CoO3 compounds, included also in Figure 3.
It is worth mentioning to the Pr0.5 Ca0.5 CoO3 samples
and their highly hysteretic transition that the electricalresistivity after a cooling run never recovers the anteriorvalue, and further cycling over the transition gradually
increases its absolute value. This fact can be explained
supposing that this 1st order transition is accompanied
by high elastic constraints of the crystal lattice, which areat the origin of the gradual deterioration of the ceramic
sample when cycling over the transition.
The magnetic data of Pr0.5 Ca0.5 CoO3 and(Pr1−y Yy )0.7 Ca0.3 CoO3 samples are confronted in
Figures 4 and 5. The magnetic susceptibility, plotted
as 1/χ vs. temperature in Figure 4 jumps markedly at
TMI . At high temperatures, after the subtraction of theparamagnetic contribution of the Pr3+ ions, the simple
analysis enables us to estimate the eﬀective moment of Co
species to μ2eﬀ ∼ 10μ2B . This value matches the theoreticalvalue for intermediate-spin (IS) Co3+ /Co4+ mixture –see also similar results obtained for the La1−x Srx CoO3
samples in the range 300−600 K by Wu and Leighton [15].
With decreasing temperature, the inverse susceptibility
associated with the cobalt subsystem does not decreaselinearly, and thus does not follow the Curie-Weiss

Page 4 of 8

Fig. 4. The inverse of the magnetic susceptibility as a functionof the temperature for (Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.0−0.15)
and Pr0.5 Ca0.5 CoO3 .

Fig. 5. Magnetization loops measured at 2 K for(Pr1−y Yy )0.7 Ca0.3 CoO3 (y = 0.0−0.15) and Pr0.5 Ca0.5 CoO3 .

behaviour with temperature independent parameters.
This fact corroborates the hypothesis that Co ions
gradually, i.e. starting at temperatures well above the
TMI , change their spin state from the high-temperaturealloy with main weight of IS states towards the “novel”
ground-state based on mixture of low-spin (LS) species(LS Co3+ is diamagnetic while LS Co4+ has spin 1/2
yielding μ2eﬀ ∼ 3 μ2B ), which is ﬁnally stabilized below
TMI [8].
To shed more light on the magnetic background of
the in principle “utmost” non-magnetic ground-state, we
present in Figure 5 the magnetization loops measured at
2 K. The metallic counterpart Pr0.7 Ca0.3 CoO3 with FMlike transition at TC ∼ 55 K exhibits nearly rectangular
hysteresis with large coercivity. For Pr0.5 Ca0.5 CoO3 , thehysteresis is preserved though in smaller extent, whereas
the magnetization curves for (Pr1−y Yy )0.7 Ca0.3 CoO3
are of Brillouin type without perceptible coercivity
and remanence. In addition to observed ferro- and/or
Brillouin-like magnetism the superposed paraprocess ofsigniﬁcant magnitude is observed in all samples. This lowtemperature feature is associated with Van Vleck susceptibility of Pr3+ ions in the nonmagnetic ground sin-

Eur. Phys. J. B (2013) 86: 305

glet due to crystal ﬁeld eﬀects. Our experimentally estimated value χVV ∼ 0.03−0.06 emu mol−1 Oe−1 per Pr3+
matches the absolute value determined by Knı́žek et al. forPr0.3 CoO2 [16]. Turning back to the open hysteresis loopof the Pr0.5 Ca0.5 CoO3 , we note that its occurrence canbe related to the fact that due to some composition inhomogeneity and/or oxygen nonstoichiometry in the sample,
not all Co species are transformed into the low spin state.
A minor phase of supposedly FM character is thus developed below 70 K and interacts with the major glassy
FM phase (Tf ∼ 6.5 K) [17]. This complication is absent
in (Pr1−y Yy )0.7 Ca0.3 CoO3 samples and, in our opinion,
the observed Brillouin-like magnetism cannot be explained
without considering a contribution of magnetic Pr4+ ions.
More detailed analysis exceeds, however, the scope of thepresented work and will be published later.
The thermodynamics of the Pr0.5 Ca0.5 CoO3 and(Pr1−y Yy )0.7 Ca0.3 CoO3 systems has been investigatedusing the speciﬁc heat measurements from room
temperature down to 0.4 K (see Fig. 1). Important information on the nature of the low-temperature insulating phase is, nonetheless, contained in the temperature range 0.4−10 K. As we demonstrated previously
for the (Pr1−y Yy )0.7 Ca0.3 CoO3 sample with y = 0.15,
the low-temperature speciﬁc heat of insulating Pr-based
cobaltites, contrary to their metallic counterparts, is accompanied by Schottky peaks which we associate with theground doublet of Kramers ion Pr4+ , split by the internal
magnetic ﬁeld existing in the samples [8]. The existenceof internal magnetic ﬁeld acting on Pr sites is, however,somewhat contradicted by apparent absence of long-range
magnetic order of the Co and/or Pr sublattice, according
to our neutron diﬀraction patterns taken on the y = 0.15
compound down to 0.2 K. Also for Pr0.5 Ca0.5 CoO3 no observable long-range order has been detected in the studyof Barón-González et al. [18], though the magnetization
curve in present Figure 4 suggests an existence of saturated moment of about 0.3μB per f.u., which is above theresolution limit of powder neutron diﬀraction. We consider
this point as highly interesting and controversial and, consequently, we intend to analyze it in a separate paper.
The low temperature speciﬁc heat for Pr0.5 Ca0.5 CoO3
(TMI = 76 K) and Pr0.63 Y0.07 Ca0.3 CoO3 (the y =0.10 sample, TMI = 93 K), measured at various magnetic
ﬁelds, is shown in Figure 6. To separate the Pr4+ related
Schottky peaks, a background line is modeled using common contributions of speciﬁc heat, the hyperﬁne nuclear
Cphyp ∼ αT −2 , lattice Cplatt ∼ βT 3 and linear “glassy”
Cpglass ∼ γT terms, and includes, as well a Schottky-like
contribution due to thermal excitation of Pr3+ ions fromthe ground singlet to excited one at ΔE = 6 meV [19].
This latter contribution becomes important for T > 15 K
and peaks at about 30 K. The values actually used are
α ∼ 0.060 J mol−1 K, β ∼ 0.000130 J mol−1 K−4 ,γ ∼ 0.050 J mol−1 K−2 for Pr0.5 Ca0.5 CoO3 and α ∼0.028 J mol−1 K, β ∼ 0.000175 J mol−1 K−4 , γ ∼0.028 J mol−1 K−2 for Pr0.63 Y0.07 Ca0.3 CoO3 . The hyperﬁne nuclear contribution, emphasized in Figure 6 by useof log-log scale, is especially large for Pr0.5 Ca0.5 CoO3 . It

Eur. Phys. J. B (2013) 86: 305

Fig. 6. The low-temperature speciﬁc heat of Pr0.5 Ca0.5 CoO3
and Pr0.63 Y0.07 Ca0.3 CoO3 measured in ﬁelds 0–9 T and plotted in log-log scale. In order to separate the Pr4+ related
Schottky peaks, the background contribution comprising common terms is marked by full line (see the text).

is characterized by coeﬃcient α ∼ 0.060 J mol−1 K, which
is apparently ﬁeld independent and surpassing by morethen one order the nuclear speciﬁc heat originating fromthe spin I = 7/2 multiplet of 59 Co nuclei in the ferromagnetic cobaltites [20]. The origin of such anomalously large
value may be associated to the contribution of 141 Pr nuclei
with spin I = 5/2 in the hyperﬁne ﬁeld of Pr4+ electronic
pseudospins [8].
After subtraction of the background, the Pr4+ contribution for Pr0.5 Ca0.5 CoO3 and Pr0.63 Y0.07 Ca0.3 CoO3
samples is plotted as Cp /T vs. T in Figure 7. The analysis is made supposing anisotropic g-factor of axial symmetry, so that it is described by two components g andg⊥ only. This model leads to a modiﬁed Schottky form,
where the energy splitting ΔE for a particular direction
is given by the angle θ corresponding to the deviation fromthe direction of the magnetic ﬁeld. The partial contribution to the overall Schottky-like anomaly is calculated as[(ΔE  cosθ)2 + (ΔE⊥ sin θ)2 ]1/2 and the contribution to
speciﬁc heat is weighted by sin θ, which corresponds to
the random orientation of crystallites in the sample. Theﬁt for Pr0.5 Ca0.5 CoO3 , represented by solid lines in upper panel of Figure 7, gives the g factors for Pr4+ species
as g⊥ ∼
= 1.4 and g ∼= 4.0, respectively. Furthermore themagnetic ﬁeld dependence of the characteristic energies
ΔE⊥ (ΔE ) = f (B) in Figure 8 enables us to determine
the molecular ﬁeld experienced by rare earth moments
as the intercept with B axis to Bmol (Pr4+ ) ∼ 7.5 T.

Page 5 of 8

Fig. 7. The Schottky Pr4+ contribution for Pr0.5 Ca0.5 CoO3
and Pr0.63 Y0.07 Ca0.3 CoO3 plotted as Cp /T vs. T together withthe ﬁt based on anisotropic g-factor as described in the text
(solid lines).

Fig. 8. Zeeman splitting (ΔE and ΔE⊥ ) of the Pr4+
ground doublet for Pr0.5 Ca0.5 CoO3 (open symbols) andPr0.63 Y0.07 Ca0.3 CoO3 (full symbols), determined from the ﬁtof Schottky peaks in Figure 7.

For Pr0.63 Y0.07 Ca0.3 CoO3 , somewhat larger g⊥ ∼= 1.65and g ∼= 4.7 are obtained, but the molecular ﬁeld issmaller, Bmol (Pr4+ ) ∼ 2.5 T. The most important resultof this analysis is, however, oﬀered by the integration of
the Schottky speciﬁc heat as:ΔΣ Schottky =CpSchottky /T dT,Page 6 of 8

Fig. 9. The temperature dependence of the normalized
XANES spectra at the Pr-L3 -edge for Pr0.5 Ca0.5 CoO3 sample. The lower panel shows the magniﬁcation of the spectrawith the most obvious contribution of Pr4+ ions.

which provides the quantitative determination of entropy change associated with the temperature induced
redistribution of Pr4+ pseudospins between the split
levels of the ground doublet. Comparing the evalu−1
ated entropy change ΔΣ Schottky ∼K−1= 1.50 J mol(identical value calculated for all magnetic ﬁelds) for
Pr0.5 Ca0.5 CoO3 and ΔΣ Schottky ∼= 0.74 J mol−1 K−1
for Pr0.63 Y0.07 Ca0.3 CoO3 with the theoretical limit
R ln 2 = 5.76 J mol(Pr4+ )−1 K−1 , we can estimate
the absolute amount of Pr4+ . This is evaluated as
0.26 per f.u. in the case of Pr0.5 Ca0.5 CoO3 , which gives
the “real” chemical formula below the M-I transition as
4+
2+
3+
4+
Pr3+
0.24 Pr0.26 Ca0.5 Co0.76 Co0.24 O3 (average valence of Pr
ions of 3.52+). Similarly, the number of Pr4+ ions in they = 0.10 sample is determined to 0.13 per f.u., corre4+
3+
2+
3+
4+
sponding to formula Pr3+
0.5 Pr0.13 Y0.07 Ca0.3 Co0.83 Co0.17 O3 .
This latter value can be compared with previously reported Pr4+ contents in the low-temperature phaseof (Pr1−y Yy )0.7 Ca0.3 CoO3 samples y = 0.075 and
0.15, which amounted to 0.12 per f.u. and 0.18 per f.u.,
respectively [8].
Apart of this “indirect” proof of the electronic transfer between the Pr3+ and Co4+ species we have probed
directly the evolution of the Pr and Co valency using the
XANES spectra of Pr-L3 -edge and Co-Kedge. In Figure 9,
we present the temperature dependence of XANES spectra at the Pr-L3-edge for the Pr0.5 Ca0.5 CoO3 sample. It

Eur. Phys. J. B (2013) 86: 305

Fig. 10. The ﬁt of XANES spectra for Pr0.5 Ca0.5 CoO3 at (a)300 K and (b) 8 K. The observed data are marked by symbols
and ﬁtting curve is represented by the thick dashed line. One
arctangent and three Lorentzian functions (A, B1 and B2) areused as components.

is obvious that the shape of the spectra changes markedly
when the temperature crosses the transition temperature
TMI = 76 K (see the lower panel of Fig. 9). The dropof the white line together with increase of the peak at
5979 eV indicate unequivocally the appearance of Pr4+
species. The quantitative analysis was actually based on
the arctangent background subtraction and Lorentzian
three-peak ﬁtting (5966, 5969 and 5979 eV) as illustrated
for 300 K and 8 K in Figure 10. The Pr4+ population was deduced from the integral intensity of the peakat 5979 eV−I(5979 eV), which is a ﬁngerprint of Pr4+
(though some spectral weight due to Pr3+ is also present),
relative to the peak at 5966 eV−I(5966 eV), representing
the white line of Pr3+ . The calibration was done using thespectra of Pr6 O11 as standard for Pr4+/Pr3+ mixture of average valence 3.67 + (I(5979 eV)/I(5966 eV) = 0.635) andthe spectra of Pr0.5 Ca0.5 CoO3 taken at 300 K as standard
for pure Pr3+ (I(5979 eV)/I(5966 eV) = 0.175). The temperature evolution of Pr valence in the cobaltite was thendeduced from the observed peak ratios by simple interpolation, in particular I(5979 eV)/I(5966 eV) = 0.452 foundat 8 K yielded the average valence of Pr ions of 3.40+.
The complete data on variable praseodymium valence inPr0.5 Ca0.5 CoO3 are shown in Figure 11, including also theresults on two previously studied (Pr1−y Yy )0.7 Ca0.3 CoO3
compounds.

Eur. Phys. J. B (2013) 86: 305

Page 7 of 8

Fig. 11. The temperature dependence of the average valenceof Pr ions in the samples with occurrence of M-I transition.
The uncertainty of the estimated valence values is ±0.03, resulting from the arbitrariness of parameters in the arctangent background and three Lorentzian peaks, which ﬁt thePr-L3 -edge XANES spectra. For more details see the text andreference [12].

Fig. 12. The temperature dependence of the normalized XANES spectra at the Co-K-edge for thePr0.63 Y0.07 Ca0.3 CoO3 sample. The inset shows the magniﬁcation of the pre-edge peak including the data after
subtraction of arctangent background (full lines). The arrows
indicate the most marked changes of spectral weight between
300 and 8 K.

The low temperature chemical formula derived from the XANES result on average Pr3.40+
4+
2+
3+
4+
valence, Pr3+
0.30 Pr0.20 Ca0.5 Co0.70 Co0.30 O3 , is in reasonable agreement with the above-mentioned formula
4+
2+
3+
4+
3.52+
Pr3+
va0.24 Pr0.26 Ca0.5 Co0.76 Co0.24 O3 , based on Pr
lence as derived from Schottky analysis. Taking a mean
between the two valence values, Pr3.46+ , and supposing
the charge neutrality and full oxygen stoichiometry, we
may estimate that the formal cobalt valence changes from
Co3.50+ to Co3.27+(±0.03) below the TMI transition. This
result presumes the electron transfer of ∼0.23e between
Pr and Co in case of Pr0.5 Ca0.5 CoO3 , which is still larger
than the electron transfer of ∼0.10 and 0.17e observedfor (Pr1−y Yy )0.7 Ca0.3 CoO3 systems with y = 0.075 and
0.15, respectively.
The spin-state change that occurs concomitantly with
the M-I transition has been investigated using XANESat Co-K-edge. The spectra seen in Figure 12 refer to thePr0.63 Y0.07 Ca0.3 CoO3 (or y = 0.10) sample, where theelectron transfer of ∼0.13e between Pr and Co is determined based on the Schottky peak analysis. Two dominant features can be noticed in the XANES region –the main absorption peak at 7726 eV due to the dipole
1s → 4p transitions, and the pre-edge peak centeredaround 7710 eV that reﬂects the quadrupole 1s → 3dor nonlocal dipole transitions. As evidenced in Figure 12,
the main absorption peak does not show any detectable
shift with decreasing temperature. This is quite surprisingconsidering the signiﬁcant population of Pr4+ ions below
the TMI that should be compensated by an appropriate
decrease of Co valence. A plausible explanation why thechange of Co valence is not directly reﬂected by the chemical shift of the main peak could be found in an interplay of competing eﬀects between: (i) the chemical shift,which may make about 0.4 eV to low energy side in thecase of Co valence change 3.3+ to 3.13+ encountered in
Pr0.63 Y0.07 Ca0.3 CoO3 , and (ii) the lowering of spin state,

which increases the main edge energy for the same valence of Co. The impact of the spin state is diﬃcult toestimate quantitatively, but very roughly, based on thepaper of Vankó et al. [21] on LaCoO3 the shift from LS to
HS leads to increase of the main edge energy of ∼0.5 eV.
The pre-edge peak, shown in more detail in the inset of
Figure 12, exhibits also little change with decreasing temperature. Compared to the spectra of LnCoO3 , that allow
a decomposition into two components and the temperature induced spin states can be readily probed [21–23], thepre-edge peak in mixed-valent cobaltites is much broader
and rather featureless. The same vague shape applies also
for the pre-edge peak of present Pr0.63 Y0.07 Ca0.3 CoO3
sample. The close inspection of the pre-edge feature after
the subtraction of the background (shown by the dashedline) reveals, nevertheless, a small transfer of the spectral
weight from the low to high-energy side at low temperatures, as expected for the transition to lower spin states.
The surplus of spectral weight seems to be less diﬀusive,
forming a rather separated bump at 7711.5 eV. We relate this feature to a stabilization of well-localized LS
Co3+ states in the Pr0.63 Y0.07 Ca0.3 CoO3 sample below
TMI . Let us note that practically identical evolution of the
Co-K-edge pre-edge peak was presented most recently forPr0.5 Ca0.5 CoO3 by Herrero-Martin et al. [11].

4 Conclusions
Magnetic, electric, thermal and X-ray absorption
spectroscopy data have been accumulated for thePr0.5 Ca0.5 CoO3 and (Pr1−y Yy )0.7 Ca0.3 CoO3 (y =0.0625 − 0.15) perovskites, which exhibit the unusual M-Itransition at 76 K and between 40−132 K, respectively.
The study provides conclusive arguments that thetransition is accompanied with large electron transferfrom the Pr3+ ions to the CoO3 subsystem on coolingPage 8 of 8

through the transition. The latter process is quantiﬁed by
two independent experimental means: (i) by the analysis of Schottky peak in the low temperature speciﬁc heat,
which are related to Zeeman splitting of the ground-state
doublet of Kramers ions Pr4+ , and (ii) by the comparative XANES investigation at Pr-L3-edge with use of thePr3+ /Pr4+ standard. The mutually consistent data on
the charge transfer between Pr species and CoO3 subsystem are obtained. In particular, for the prototypicalcompound Pr0.5 Ca0.5 CoO3 , the amount of Pr4+ ions at
8 K is determined to 0.23 ± 0.03 per f.u., corresponding
to the change of average praseodymium valence from common value 3+ at room temperature to about 3.46+ in the
low-temperature phase. This is a surprisingly large valence
shift in view of recent report, where a combined analysis of
X-ray spectroscopic data, including the behavior at boththe Pr and Co absorption or emission edges, gave threetimes lower change [11]. For (Pr1−y Yy )0.7 Ca0.3 CoO3 , theamount of Pr4+ ions is smaller, but still important:∼0.13 per f.u. for the Pr0.63 Y0.07 Ca0.3 CoO3 (y = 0.10)
compound.
Somewhat controversial result is obtained in the temperature dependence of XANES spectra at Co-K-edge on
Pr0.63 Y0.07 Ca0.3 CoO3 . Two observations are noticeable.
The ﬁrst one is the apparent lack of chemical shift oncooling the sample below the TMI , which is naturally anticipated as a consequence of the electron transfer fromPr species to CoO3 subsystem. This eﬀect might originate
eventually from competing tendencies acting oppositely on
the position of main edge, i.e. the spin state transition andthe charge transfer. More important ﬁnding is, however,
the empirically deduced very small transfer of spectral
weight in the pre-edge peak. This observation questionsin principal not only the existence of signiﬁcant shift of
Co valence, but also the complete IS → LS crossover, considered as the basic constituent of the M-I transition in Prbased, mixed valence cobaltites. On the other hand, we are
ﬁrmly convinced that the electron transfer from Pr sites to
CoO3 subsystem does exist, and is manifested in the electric transport properties as signiﬁcant drop of the hole carrier concentration. In particular for Pr0.63 Y0.07 Ca0.3 CoO3
the observed amount 0.13 Pr4+ per. f.u. means that thedoping level is decreased from the 30% doping level atroom temperature to about 17% doping level below theM-I transition. We thus conclude that the nature of the
low-temperature phase is not a simple issue, leaving thequestion on the carrier character and spin states, as wellon the character of Pr4+ pseudospins and origin of strong
molecular ﬁeld acting on them, open and worth to be further studied.

We thank Prof. Nan Lin Wang of Institute of Physics,
Beijing for fruitful discussion and possibility of the highpressure oxygenation. The work was performed underthe ﬁnancial support of the Grant Agency of the Czech
Republic within the Project No. 204/11/0713. The synchrotron radiation experiments were performed at theEur. Phys. J. B (2013) 86: 305
BL01B1 of SPring-8 with approval of Japan Synchrotron Radiation Research Institute (Proposal No. 2011A1060).

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