PHYSICAL REVIEW B 82, 165107 共2010兲

Metal-insulator transition and the Pr3+ Õ Pr4+ valence shift in (Pr1−yYy)0.7Ca0.3CoO3J. Hejtmánek, E. Šantavá, K. Knížek, M. Maryško, and Z. Jirák
Institute of Physics, ASCR, Cukrovarnická 10, 162 00 Prague 6, Czech Republic

T. Naito, H. Sasaki, and H. Fujishiro
Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan
共Received 23 April 2010; revised manuscript received 22 July 2010; published 5 October 2010兲
The magnetic, electric, and thermal properties of the 共Ln1−yYy兲0.7Ca0.3CoO3 perovskites 共Ln = Pr, Nd兲 were
investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator 共M-I兲
transition, which is observed in the praseodymium based samples with y = 0.075 and 0.15 at TM-I = 64 K and
132 K, respectively. The study suggests that the transition, reported originally in Pr0.5Ca0.5CoO3, is not due toa mere change in cobalt ions from the intermediate to the low-spin states but is associated also with asignificant electron transfer between Pr3+ and Co3+ / Co4+ sites, so that the praseodymium ions occur below TM-I
in a mixed Pr3+ / Pr4+ valence. The presence of Pr4+ ions in the insulating phase of the yttrium doped samples
共Pr1−yYy兲0.7Ca0.3CoO3 is evidenced by Schottky peak originating in Zeeman splitting of the ground-state
Kramers doublet. The peak is absent in pure Pr0.7Ca0.3CoO3 in which metallic phase, based solely on nonKramers Pr3+ ions, is retained down to the lowest temperature.
DOI: 10.1103/PhysRevB.82.165107

PACS number共s兲: 71.30.⫹h, 65.40.Ba

I. INTRODUCTION

La0.5Sr0.5CoO3 to diamagnetic insulator LaCoO3.9,10
The
present
work
concerns
thesystem
共Pr1−yYy兲0.7Ca0.3CoO3 共y = 0 – 0.15兲, which is advantageouswith respect to an easier preparation of oxygenstoichiometricsamplesascomparedwiththePr0.5Ca0.5CoO3 system. The M-I transition observed fory ⱖ 0.075共TM-I = 60– 130 K兲 is manifested by pronounced
anomalies in the temperature course of transport, magnetic,
and thermal properties. Fundamental information on the nature of the insulating phase is obtained at very low temperatures. This refers especially to the occurrence of a Schottky
peak in the specific heat, centered at 1–2 K. A comparison
with the specific-heat data in Nd0.7Ca0.3CoO3 where similar
Schottky peak is currently also observed, allows us to conclude that the peak arises due to an exchange field splittingof the doublet ground state of Kramers rare-earth ions, inparticular Nd3+. Its occurrence in the yttrium-doped
共Pr1−yYy兲0.7Ca0.3CoO3 provides a direct and quantitative evidence for the fact that in the insulating phase, in addition tocommon Pr3+ 共non-Kramers ion兲, there is a significant significant population of Pr4+ 共Kramers ion兲.

Thermally induced transitions in LnCoO3 共Ln = La, Y, rareearths兲 have been studied for decades. Recent interpretation
associates them with a local excitation of the octahedrally
6兲 to HS 共highcoordinated Co3+ ions from LS 共low spin, t2g
4 2spin, t2geg兲 state, followed at higher temperature by a formation of a metallic phase of IS character 共intermediate spin,5 ⴱ␴ 兲—see, e.g., Ref. 1 and citations therein. An attention is
t2g
attracted also to the mixed-valence cobaltites such as
La1−xSrxCoO3 or La1−xCaxCoO3 where a similar transition
from insulating LS Co3+ ground state toward the metallic
5 ⴱ␴ one is obtained in the course of doping 共0 ⬍ x ⬍ 0.2兲.
t2g
As concerns the metallic region beyond x = 0.2, some heavily
doped Pr-based cobaltites, in particular Pr0.5Ca0.5CoO3 behave anomalously. At ambient temperature, they appear in
5 ⴱ␴ phase as expected but on cooling they
the metallic t2g
undergo a sharp metal-insulator 共M-I兲 transition at
TM-I = 90 K, documented for the first time by Tsubouchi
et al.2,3 The same transition was observed also on the lessdoped samples Pr1−xCaxCoO3 共x = 0.3兲 under high pressuresor upon a partial substitution of praseodymium by smaller
rare earth cations or yttrium.4–6 The effect was tentatively
ascribed to a change in the cobalt states from itinerant ones
5 ⴱ
6␴ to a mixture of localized LS Co3+共t2g, S = 0兲 andt2g
4+ 5
LS Co 共t2g , S = 1 / 2兲 states. Very recently, an alternative explanation has been proposed based on electronic-structure
calculations and some indirect experimental data, namely,
the significant lattice contraction and shortening of Pr-O
bond lengths that accompany the M-I transition.7,8 It is suggested that the formal cobalt valency in Pr0.5Ca0.5CoO3 ischanged at TM-I from the mixed-valence Co3.5+ toward pureCo3+ with strong preference for LS state, and the praseodymium valence is simultaneously increased from Pr3+ toward
Pr4+. The spin-state transition and formation of an insulatingstate in Pr0.5Ca0.5CoO3 is thus an analogy of the compositional transition from ferromagnetic 共FM兲 metal
1098-0121/2010/82共16兲/165107共7兲

II. EXPERIMENTAL

Polycrystalline samples of 共Ln1−yYy兲0.7Ca0.3CoO3
共Ln = Pr, Nd兲 were prepared by a solid-state reaction. Raw
powders of Pr6O11, Nd2O3, Y2O3, Co3O4, and CaCO3 were
weighted with proper molar ratios and ground using an agate
mortar and pestle for 1 h. Mixed powders were calcined at1000 ° C for 24 h in air. Then they were pulverized, ground,and pressed into pellets of 20 mm diameter and 4 mm thickness. Pellets were sintered at 1200 ° C for 24 h in 0.1 MPa
flowing oxygen gas. The measured densities of each sample
were greater than 90% of the ideal density. Powder x-ray
diffraction patterns were taken for each sample using Cu K␣
radiation; the samples were confirmed to have a single phaseorthoperovskite 共Pbnm兲 structure.

165107-1©2010 The American Physical Society

PHYSICAL REVIEW B 82, 165107 共2010兲

HEJTMÁNEK et al.

10

9

10

7

160
Thermoelectric power (µVK-1)

Electrical resistivity (mΩcm)

1011
Pr0.7Ca0.3CoO3(Pr0.925Y0.075)0.7Ca0.3CoO3(Pr0.85Y0.15)0.7Ca0.3CoO3
Nd0.7Ca0.3CoO3

105
TM-I

103
101
10

TM-I

-1

0

50

100

(a)

150
200
T (K)

250

Pr0.90Ca0.10CoO3

140(Pr0.85Y0.15)0.7Ca0.30CoO3(Pr0.925Y0.075)0.7Ca0.30CoO3

120

TM-I

100
TM-I

80
60
40

Nd0.70Ca0.30CoO3
Pr0.70Ca0.30CoO3

20

300

0
0

50

100

150

200

250

300

T (K)

109
107
T=40 K

105

TM-I

103
101
10-1
0.2

(b)

FIG. 2. 共Color online兲 The enhanced Seebeck coefficient in
共Pr1−yYy兲0.7Ca0.3CoO3 共y = 0.075 and 0.15兲 below the M-I transition. The data for metallic Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3 arealso shown. The solid line gives data for Pr0.9Ca0.1CoO3 taken of
Ref. 13 共see the text兲.

T=6 K

0.3

0.4
0.5
T -1/4 (K -1/4)

0.6

0.7

0.8

FIG. 1. 共Color online兲 The temperature dependence of electricalresistivity in 共Pr1−yYy兲0.7Ca0.3CoO3 共y = 0, 0.075, and 0.15兲 andNd0.7Ca0.3CoO3. The data measured on cooling and warming areoverlapping. The lower panel shows the dependence on T−1/4.

The magnetic measurements were performed in the temperature range from 2 to 400 K using a superconducting
quantum interference device magnetometer Magnetic Property Measuring System device 共Quantum Design兲 . The hysteresis loops at T = 2 and 4.5 K were recorded between thefield −7 and 7 T. The susceptibility was measured under anapplied field of 0.1 T, employing the zero-field and fieldcooled regimes during warming and cooling the sample, respectively.
Thermal conductivity, thermoelectric power and electricalresistivity were measured using a four-probe method with aparallelepiped sample cut from the sintered pellet. The electrical current density varied depending on the sample resistivity between 10−1 A / cm2 共metallic state兲 and 10−7 A / cm2
共insulating state兲. The measurements were done on sample
cooling and warming using a close-cycle cryostat working
down to 2–3 K. The detailed description of the cell including
calibration is described elsewhere.11
The specific heat was measured by Physical Property
Measuring System device 共Quantum Design兲 using the two-␶model. The data were collected generally on sample cooling.
The experiments at very low temperatures 共down to 0.4 K兲
were done using the He3 option.

tivity of about 1 m⍀ cm at 0 K. The yttrium doped
samples, apparently in the same metallic state at room temperature, exhibit a sudden increase in the electrical resistivity
and thermopower 共Seebeck coefficient兲 at TM-I = 64 K and
132 K for y = 0.075 and y = 0.15, respectively. Concurrently,
the thermal conductivity exhibits a kink, followed with arecovery at lower temperatures 共Fig. 3兲. The magnetic susceptibility drops markedly 共Fig. 4兲, which is a strong signature that cobalt ions transform to LS states at TM-I. The M-Itransition in 共Pr1−yYy兲0.7Ca0.3CoO3 is further manifested by apronounced peak in the specific-heat data 共Fig. 5兲. The peakis very narrow ⬃1 − 2 K for y = 0.075 with lower TM-I, suggesting a first-order character of the transition while it ismuch broader ⬃15 K for y = 0.15 with higher TM-I. It is seen
also that the values of specific heat below the M-I transition
are small compared to Pr0.7Ca0.3CoO3 retaining the metallic
phase, which is indicative of a significant change in latticedynamics in the low-temperature phase of yttrium-doped
samples.
The anomalies in the transport, magnetic and thermal
data, similar to those in Figs. 1–5, were observed earlier forthe prototypical compound Pr0.5Ca0.5CoO3 共Refs. 2 and 3兲
and also for the Pr0.7Ca0.3CoO3-derived systems with analogous M-I transition. In important distinction to these previous reports, the transition in present samples
共Pr1−yYy兲0.7Ca0.3CoO3 exhibits very small thermal hysteresis,
which is about 0.2 K for both y = 0.075 and 0.15. Another
3.5
Thermal conductivity (Wm-1K-1)

Electrical resistivity (mΩcm)

1011

III. RESULTS
A. Physical characterization

The electric transport measurements on the
共Pr1−yYy兲0.7Ca0.3CoO3 samples for y = 0, 0.075, and 0.15 andon Nd0.7Ca0.3CoO3 are presented in Figs. 1 and 2. It is seenthat the y = 0 and analogous neodymium sample are metallic
over the whole temperature range, tending to a finite resis-

Pr0.70Ca0.30CoO3(Pr0.925Y0.075)0.7Ca0.30CoO3(Pr0.85Y0.15)0.7Ca0.30CoO3

3.0
2.5
2.0
1.5
1.0

TM-I

0.5
TM-I

0.0
0

50

100

150

200
T (K)

250

300

FIG. 3.
共Color online兲 Thermal conductivity
共Pr1−yYy兲0.7Ca0.3CoO3 for y = 0, 0.075, and 0.15.

165107-2of(Pr1-yYy)0.7Ca0.3CoO3, H=1000 Oe

1
0.5

y = 0.0 0.075 0.15
ZFC
FCC
FCW

y=0

0.1
0.05

y=0.075

TM-I
0.01
0.005
0

y=0.15

20

TM-I

40

60

(a)
Inverse susceptibility (mol Oe emu-1)

PHYSICAL REVIEW B 82, 165107 共2010兲

Moment (µB /formula unit)

Susceptibility (emu mol-1 Oe-1)

METAL-INSULATOR TRANSITION AND THE Pr3+ / Pr…

140

Pr4+

60

Pr0.7Ca0.3CoO3(Pr0.925Y0.075)0.7Ca0.3CoO3(Pr0.85Y0.15)0.7Ca0.3CoO3
Nd0.7Ca0.3CoO3(Nd0.925Y0.075)0.7Ca0.3CoO3

40
20
0
0

50

100

150

(b)

200

250

300

T (K)

FIG. 4. 共Color online兲 Magnetic susceptibility of
共Pr1−yYy兲0.7Ca0.3CoO3 for y = 0, 0.075 and 0.15 共in log scale兲. The
lower panel shows the temperature dependence of inverse susceptibility. The solid lines mark the theoretical contribution for Pr3+ andPr4+ ions, calculated for the praseodymium amount in y = 0.15 using
the data in Ref. 12 共see the text兲. The data for
共Nd1−yYy兲0.7Ca0.3CoO3 for y = 0 and 0.075 without M-I transition
are added for comparison.

property deserving attention is practical absence of residual
metallic phase in the insulating ground state, especially for
y = 0.15. As seen in the plot of inverse susceptibility in lower
panel of Fig. 4, the metallic sample Pr0.7Ca0.3CoO3 undergoes FM-like transition at TC ⬃ 55 K and, consistently, themagnetization data taken at low temperatures show nearlyrectangular hysteresis loops with large coercivity 共Fig. 6兲. On
the other hand, the sample y = 0.15 exhibits essentially paramagnetic behavior of inverse susceptibility and magnetization curves are of Brillouin type, though some very weakcoercivity and remanence appear at 2 K.
The very low susceptibility below TM-I makes sample y= 0.15 suitable for a more quantitative analysis. The observed
values are obviously smaller than the calculated contribution
120

80

-1

Cp (J mol )

100

60
40

Pr0.70Ca0.3CoO3(Pr0.925Y0.075)0.7Ca0.3CoO3(Pr0.85Y0.15)0.7Ca0.3CoO3
Nd0.70Ca0.3CoO3

20
0
0

50

100

150

-0.2
2K

4.5K

(Pr0.85Y0.15)0.7Ca0.30CoO3-1

0
Magnetic field (T)1

2

FIG. 6. 共Color online兲 Magnetization loops measured at 2 K
共open symbols兲 and 4.5 K 共solid symbols兲 on 共Pr1−yYy兲0.7Ca0.3CoO3
共y = 0 and 0.15兲.

TM-I

80

0.0

-2

120
100

2K
4.5K

0.2

-0.4

80 100 120 140 160 180
T (K)

TM-I

Pr3+

Pr0.7Ca0.30CoO3

0.4

200

250

300

350

T (K)

FIG. 5. 共Color online兲 Specific heat of 共Pr1−yYy兲0.7Ca0.3CoO3for y = 0, 0.075, and 0.15. The data for Nd0.7Ca0.3CoO3 are addedfor comparison.

of praseodymium ions in trivalent state 共see data of Ref. 12
in lower panel of present Fig. 4兲, disregarding that there
should also be contribution of the LS Co4+ spins. A plausible
interpretation provides the new model of M-I transition stating that some praseodymium ions are changed to tetravalentstates with lower magnetic moments, and corresponding
number of LS Co4+ ions are transformed to diamagnetic LS
Co3+. The actual valence shift can be inferred from the thermopower data in Fig. 2. Namely, the thermopower in mixedvalence cobaltites is primarily determined by number of carriers, i.e., by the formal cobalt valence, and, in a less extent,
by the average ionic size of large cations in perovskite A
sites. Having this in mind, we refer to the fact that Seebeckcoefficient in the metallic phase of yttrium doped sample y= 0.15
共T ⬃ 150– 300 K兲 matches well to Nd0.7Ca0.3CoO3 withcomparable A-site size. This means that cobalt valence inthese two samples is practically the same and, consequently,
praseodymium ions are essentially in trivalent state, similarly
to neodymium ones. Below TM-I = 132 K, Seebeck coefficient is enhanced and, on further cooling, it becomes comparable to samples with much lower number of carriers, inparticular, to Pr0.9Ca0.1CoO3.13 In a rough estimate, this may
mean that the formal cobalt valence in y = 0.15 is changed
upon the M-I transition from original Co3.3+ to about Co3.1+.
B. Low-temperature specific heat

At the lowest temperatures, the insulating y = 0.075 and0.15 samples exhibit a steep increase in specific heat 共see thec p / T vs T2 plot in Fig. 7兲. Though a similar effect was reported originally for Pr0.5Ca0.5CoO3 the present experiments,
performed down to 0.4 K, document for the first time that theincrease is due to Schottky peak, centered at T = 1 – 2 K, that
adds to common lattice, electronic and nuclear contributionsof low-temperature specific heat 共Fig. 8兲. With increasing
external field the peak position shifts rapidly to higher temperatures. The Schottky peak is absent in metallic
Pr0.7Ca0.3CoO3 but is found with even larger intensity in
Nd0.7Ca0.3CoO3 with analogous metallic ground state 共seeFigs. 7 and 9兲.
The occurrence of Schottky peak in Nd0.7Ca0.3CoO3 canbe understood considering the Kramers character of the rareearth ions. In the orthoperovskite structure, the 4I9/2 multiplet

165107-3

PHYSICAL REVIEW B 82, 165107 共2010兲

HEJTMÁNEK et al.

0.4

3

expected lattice heat βTfor ΘD=460 K (β~0.0002)

H=9T

2.0
1.5
1.0
0.5
Cp=γT+βT 3+αT −2
γ=0.03; β=0.00025; α=0.005

0.0

-1

0.2

0

2

4

6

8 10 12 14 16 18 20 22

0.0
0

50

100

150
2

200

T (K)

250

2

T (K )

FIG. 7. 共Color online兲 The low-temperature specific heat of
共Pr1−yYy兲0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3 shown in the c p / T vs T2
plot. Beside common cubic lattice term ␤T3 and linear ␥T term, one
may notice an additional contribution for y = 0, associated with excitations within the crystal-field split multiplet of Pr3+, and an important Schottky anomaly for y = 0.075, 0.15 and Nd0.7Ca0.3CoO3that shifts with external field.

field 0 T
1T
3T
5T
9T

2.5
-1

Pr0.7Ca0.3CoO3 in Figs. 7 and 8. It originates in an excitation
to the next singlet state at an energy difference of 5 meV.
Analogously to Nd0.7Ca0.3CoO3 the Schottky peak in theyttrium-doped samples 共Pr1−yYy兲0.7Ca0.3CoO3 can be related
to the presence of Kramers ions, which are Pr4+ states
formed presumably below TM-I. A splitting of the 2F5/2 multiplet to three doublets with large spacing is anticipated. The
total entropy change is determined from data in Fig. 8 to
0.98 J K−1 mol−1 for y = 0.15. This quantitative analysis enables to estimate the number of Pr4+ states to about 0.18 performula. For y = 0.075, the entropy change makes
0.61 J K−1 mol−1 and corresponding number of Pr4+ states is0.12 per formula.
The change in Zeeman splitting of the ground-state doublets with external field is presented in Fig. 10. There is alittle shift of Tmax共Schottky兲 for Nd0.7Ca0.3CoO3 between
zero field and 3 T, before it starts to rise nearly linearly. Thisshows that the external field does not simply add to the molecular field, in other words, there must be antiparallel orperpendicular orientation of the Nd3+ moments with respect
to the FM polarized Co spins in the ground state. On theother hand, the shift with external field in the y = 0.15 sample
共Pr1−yYy兲0.7Ca0.3CoO3 is monotonous from the very beginning, which is indicative of a parallel orientation of the Pr4+
and Co moments.

2.0(Pr0.85Y0.15)0.7Ca0.3CoO3
Nd0.7Ca0.3CoO3

10(Pr0.85Y0.15)0.7Ca0.3CoO3

3.0
Cp (J mol )FIG. 9. 共Color online兲 Specific heat of Nd0.7Ca0.3CoO3 down to
0.4 K, measured in fields 0–9 T.

Tmax(Schottky) (K)of Nd3+ is split by crystal-field effects into five relatively
distant doublets.14 The double degeneracy is lifted by an exchange field arising due to FM ordering of cobalt spins.
Hence, the Schottky peak under discussion is related to thermal excitations within the ground-state doublet, the locationof the maximum defines the energy splitting due to exchange
field 共⌬ = 0.42kBTmax兲 and its shift with applied field bears
information on the effective gyromagnetic gJ⬘ value that governs the Zeeman splitting 共⌬ = gJ⬘J⬘␮B共B + Bm兲, whereJ⬘ = ⫾ 1 / 2 denotes two pseudospin levels of the Kramers
doublet and Bm is molecular field acting on the rare-earth
moment兲. The shape of observed Schottky peak is nearly
ideal with only a little extra broadening, and the integrationof cSchottky / T over T gives the total change in entropy of
3.95 J K−1 mol−1, in good agreement with the theoreticalvalue 0.7 N kB ln 2 = 4.04 J K−1 mol−1 for Nd0.7Ca0.3CoO3composition.
In the case of Pr0.7Ca0.3CoO3 the 3H4 multiplet of Pr3+ is
split into nine singlets.15 In accordance with the nonmagnetic
共singlet兲 ground state, there is no Schottky peak at very low
temperatures but another Schottky-type contribution emerges
at T ⬎ 10 K, as can be seen by increased specific heat of
3.5

field 0 T
1T
3T
5T
9T

Nd0.7Ca0.3CoO3

2.5

-1

0.6

3.0

Cp (J mol-1)

H=0T
Cp / T (J mol K )3.5

Nd0.7Ca0.3CoO3(Pr0.85Y0.15)0.7Ca0.3CoO3(Pr0.925Y0.075)0.7Ca0.3CoO3
Pr0.7Ca0.3CoO3

0.8

Pr0.7Ca0.3CoO3
0T
9T

1.5

8

gJ'= 3.30

6
4
2

gJ'= 1.85

1.0
3

0.5

-2

Cp=γT+βT +αT
γ=0.07; β=0.0002; α=0.032

0.0

0
0

2

4

6

8

10

12

14

B (Tesla)
0

2

4

6

8 10 12 14 16 18 20 22
T (K)FIG. 8. 共Color online兲 Specific heat of 共Pr1−yYy兲0.7Ca0.3CoO3
共y = 0.15兲 down to 0.4 K, measured in fields 0–9 T. The heat capacity for metallic Pr0.7Ca0.3CoO3 共y = 0兲 added for comparison is displaced to −0.3.

FIG. 10. 共Color online兲 The shift of the Schottky peak maximum 共Tmax in the cSchottky vs T dependence兲 with applied field. Thelines define the effective g factors of the Kramers ground-state doublets of Pr4+ and Nd3+. The molecular field experienced by rareearth moments at B = 0 is determined to ⬃1.6 T in
共Pr1−yYy兲0.7Ca0.3CoO3 共y = 0.15兲 and ⬃2.5 T in Nd0.7Ca0.3CoO3.

165107-4

METAL-INSULATOR TRANSITION AND THE Pr3+ / Pr…

PHYSICAL REVIEW B 82, 165107 共2010兲

IV. DISCUSSION

As established in earlier works, the M-I transition inheavily doped cobaltites, such as Pr0.5Ca0.5CoO3 or
共Pr1−yYy兲0.7Ca0.3CoO3 is conditioned by presence ofpraseodymium ions, combined with a suitable structural distortion which depends on an average ionic radius and size
mismatch of the perovskite A-site ions 共see Ref. 6 and references therein兲. Important finding of the recent generalized
gradient approximation plus U electronic-structure calculations for Pr0.5Ca0.5CoO3 is the location of the occupied
Pr3+ 4f states closely below EF at ambient temperature, andtheir splitting and partial transfer above EF due to latticecontraction experimentally observed at TM-I = 90 K. In ionicpicture, this means that praseodymium valence in the metallic phase is Pr3+, and Pr4+ valence states are formed in the
low-temperature insulating phase, compensated by a valence
shift of cobalt ions toward pure Co3+. Such scenario is currently supported using different experiments on
共Pr1−yYy兲0.7Ca0.3CoO3, namely, the low-temperature specificheat, thermopower, and magnetic susceptibility. In addition,some new information on physical properties over a broad
temperature range is obtained and deserves more discussion.
The first issue is the character of electronic transport. The
data in Fig. 1 suggest that the resistivity in purePr0.7Ca0.3CoO3 tends to a finite value. The metallicity is thus
evident despite of the granular character of the ceramic
sample. On the other hand, the y = 0.15 sample of
共Pr1−yYy兲0.7Ca0.3CoO3 shows a clear localization that is best
fitted by Mott’s formula for variable range hopping 共VRH兲,␳ = ␳o exp共To / T兲1/4, valid for T ⬍ 40 K 关see Fig. 1共b兲兴. The
characteristic parameters are ␳o ⬃ 4 ⫻ 10−5 m⍀ cm andTo ⬃ 8 ⫻ 106 K. The VRH mechanism is associated with aphonon-assisted tunneling of electrons from initial sites located near EF to target sites close in energy, for which theMiller-Abrahams transfer rate applies 共see, e.g., recent review of Tessler et al.16兲. This type of conduction is generally
manifested with a specific T−1/2 dependence of thermopower,
which is, however, not obeyed for present samples. Instead,the Seebeck coefficient increases steeply from zero value atthe lowest temperatures in a linear metalliclike manner andthen tends to a saturation, which is not reached completely
because of ingoing transition. Such behavior is suggestingthat first, the present system possesses a quasicontinuous,very narrow band of electronic levels at EF, and second, theSeebeck coefficient is related to the presence of carriers
rather than to their motion, i.e., the dominant contribution isthe change in the net entropy of a solid upon the addition ofa charge carrier while the energy transported by carriers, divided by the absolute temperature, seems to be marginal—see, e.g., Ref. 17.
Major experimental data refer, however, to specific heat in
the y = 0.075 and 0.15 samples with occurrence of the M-Itransition, compared to samples Pr0.7Ca0.3CoO3 andNd0.7Ca0.3CoO3 with metallic ground state 共Figs. 5 and 7兲.
The dominant contribution to the specific heat above the liquid helium temperature is the phononic term that follows in
the metallic samples a standard Debye-type dependence,
though there is also extra contribution due to thermally
activated population of the crystal-field split levels of therare earths 共see, e.g., Ref. 18兲. At room temperature the c pvalue reaches 118 J K−1 mol−1 for all the studied samples.
This makes about 93% of the saturated lattice heat taking
into account the Dulong-Petit high-temperature limit
cV = 15 NkB ⬃ 125 J K−1 mol−1 in ABO3 perovskites
and a minor correction for thermal expansionc p − cV ⬃ 1.2 J K−1 mol−1 at 300 K.18
The phononic term in the low-temperature phase of theyttrium-doped 共Pr1−yYy兲0.7Ca0.3CoO3 samples is clearlylower than that of the undoped samples. This is a strong
indication that M-I transition is accompanied by a significant
change in orthoperovskite structure, presumably by the lattice contraction and Pr-O bond length shortening, similar to
those in Pr0.5Ca0.5CoO3.
As concerns the specific-heat anomaly at TM-I, the associated entropy change is determined, by integration of an excess of c p / T, to 2.17 and 4.78 J K−1 mol−1 for
共Pr1−yYy兲0.7Ca0.3CoO3 with y = 0.075 and 0.15, respectively.
It is of interest that the total entropy change at the gradualspin-state transitions in LnCoO3 is about 20 J K−1 mol−1 andis distributed evenly between the first step from the LS Co3+
ground state to a LS/HS mixture and the subsequent formation of the metallic phase of IS Co3+ character.18,19
At low-enough temperatures, there are two standard contributions of the specific heat, that can be easily distinguished in the c p / T vs T2 plot in Fig. 7. One is the vanishingphononic term, approximated by the cubic term ␤T3 where ␤is directly related to Debye temperature ⌰D. The parameters␤ ⬃ 0.00020 and 0.00024 J K−4 mol−1 for the y = 0.15
sample and Nd0.7Ca0.3CoO3 shown in the fit in Figs. 8 and 9,
give an estimate ⌰D = 460 K and 430 K for the insulating
and metallic phase, respectively. The latter value should becompared with ⌰D ⬃ 400 K determined for metallic
La0.7Sr0.3CoO3.20 The second contribution is the linear term
␥T. The ␥ value ⬃30 mJ K−2 mol−1 for metallic
Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3 is comparable to⬃40 mJ K−2 mol−1 for La0.7Sr0.3CoO3.20 The linear
term for the insulating phase of the yttrium-doped
samples 共Pr1−yYy兲0.7Ca0.3CoO3 is significantly larger,␥ ⬃ 70 mJ K−2 mol−1 for y = 0.15. Since the enhancement
correlates with steeper rise of the low-temperature Seebeckcoefficient in Fig. 2, we conclude that the observed linear
term is of electronic origin. It arises in consequence of continuous spectrum of density of states, similarly to the electronic heat in metallic conductors but its actual form for essentially localized states in the VRH regime becomes
nontrivial because of role of on-site correlations.21
The remaining term, as concerns the baseline in
Figs. 8 and 9, is the nuclear contribution, manifested as the␣T−2 upturn at the lowest temperature. The value␣ ⬃ 0.032 mJ K−3 mol−1, observed on the y = 0.15 sample
共Pr1−yYy兲0.7Ca0.3CoO3 is exceptionally large and apparently
field independent. It is worth mentioning that recent specificheat study on La1−xSrxCoO3 report much smaller nuclear
specific heat despite the bulk ferromagnetic state.20 In thesecompounds, the nuclear heat originates in the Zeeman splitting of the spin I = 7 / 2 multiplet of 59Co nuclei in the hyperfine field induced by FM ordering of electronic spins. Its
intensity thus probes the amount of FM phase in the sample.
The very large ␣T−2 term in the y = 0.15 sample should be165107-5

PHYSICAL REVIEW B 82, 165107 共2010兲

HEJTMÁNEK et al.

thus ascribed primarily to contribution of 141Pr nuclei withspin I = 5 / 2 in the hyperfine field induced by dressed Pr4+
electronic pseudospins. On the other hand, the specific-heatdata for pure Pr0.7Ca0.3CoO3 also included in Fig. 8, do notshow observable nuclear contribution at zero field but a comparable ␣T−2 term is induced in field of 9 T.
The Schottky peaks observed in Nd0.7Ca0.3CoO3 共Fig. 9兲
and yttrium-doped 共Pr1−yYy兲0.7Ca0.3CoO3 共Fig. 8兲 serve as
local probe of the Kramers ions Nd3+ and Pr4+. Their position in the zero-field specific heat and shift with applied field
allow to estimate that Nd3+ pseudospins in Nd0.7Ca0.3CoO3
are characterized by effective gJ⬘ = 1.85 and experience a molecular field of Bm = 2.5 T while for Pr4+ pseudospins in they = 0.15 sample 共Pr1−yYy兲0.7Ca0.3CoO3 the respective values
are gJ⬘ = 3.30 and Bm = 1.6 T 共see Fig. 10兲. The origin ofmolecular field in metallic Nd0.7Ca0.3CoO3 is in ferromagnetic ordering of cobalt ions in IS states at TC ⬃ 25 K
共M s ⬃ 0.16␮B per f.u.兲 and their interaction via 3d-4f
exchange mechanisms with spin component of the Nd3+ moments. The situation in the low-temperature phase of
共Pr1−yYy兲0.7Ca0.3CoO3 which exhibits essentially paramagnetic characteristics 共see Figs. 4 and 6兲 is not clear and will
require further experimental and theoretical investigation. If
Pr4+ pseudospins were ordered spontaneously, significantly
sharper and higher peak 共so called lambda peak兲 would beobserved as is, e.g., in the case of antiferromagnetic orderingof Nd3+ moments in NdCoO3 at TN = 1.2 K.22 The observation of standard Schottky peak thus suggests that also in theyttrium-doped samples the rare earths experience a stablemolecular field induced by the cobalt subsystem. We notethat the cobalt subsystem in the y = 0.15 sample contains 12%of LS Co4+ ions that represent spins 1/2 in a diamagnetic
matrix of LS Co3+. Anticipating their itinerancy they may
lead to certain ferromagnetic polarization of the narrow t2g
cobalt bands, low compared to Nd0.7Ca0.3CoO3 but sufficient
to mediate relatively strong magnetic interactions among thePr4+ pseudospins 共Ruderman-Kittel-Kasuya-Yoshida model
known for 3d-4f intermetallics can be envisaged23兲. Suchinterpretation seems to be supported by two findings—theparallel orientation of praseodymium moments with cobalt
spins, manifested in the field-induced shift of Schottky peakin Fig. 10, and the unusually large term ␣T−2 contributed by
the 141Pr nuclear spins, seen for the y = 0.15 sample
共Pr1−yYy兲0.7Ca0.3CoO3 in Fig. 8.

taken with an aim to elucidate the character of a peculiar
first-order M-I transition in some Pr-based cobaltites.
Though the transition is typical for Pr0.5Ca0.5CoO3, thepresent yttrium-doped systems 共Pr1−yYy兲0.7Ca0.3CoO3 with
TM-I = 64 K and 132 K for y = 0.075 and 0.15, respectively,
appear preferable because of easier stabilization of the stoichiometric phase and complete transformation from the metallic to insulating state. The study shows that the M-I transition is manifested by a huge peak in the specific-heat data
and marked changes in the electrical resistivity, thermopower, and thermal conductivity. A sudden drop of magnetic susceptibility indicates a change in cobalt states from5 ⴱ
6␴ to the mixture of LS Co3+ 共t2g
兲 and LSthe metallic t2g4+ 5
Co 共t2g兲.
An important novelty is an observation of Schottky peakin specific-heat data at very low temperatures, 0.4–10 K.
This peak is absent in pure Pr0.7Ca0.3CoO3 in which metallic
phase is retained down to the lowest temperature but appears
with large intensity in Nd0.7Ca0.3CoO3 with similar metallic
phase. Its occurrence follows from the Kramers character of
Nd3+ and Pr4+ whose 4I9/2 and 2F5/2 multiplets are split by
crystal field associated with the distorted dodecahedral coordination of the rare earths in the Pbnm perovskites. The
Schottky peak thus probes the Zeeman splitting of theground-state doublet. The total entropy change associatedwith the Schottky peak is kB ln 2 per ion, which allows todetermine the concentration of Kramers ions in the samples
experimentally, by integration of cSchottky / T over T.
The analysis for the y = 0.075 and 0.15 samples
共Pr1−yYy兲0.7Ca0.3CoO3 provides values 0.12 and 0.18 Pr4+ per
formula unit, respectively. Considering that praseodymiumions are essentially in trivalent state in the high-temperaturephase, the observation of Pr4+ in the low-temperature phaseis in accordance with idea that the simultaneous M-I andspin-state transition in Pr-based cobaltites is accompanied byan electronic transfer between the praseodymium and cobalt
ions. In particular, for y = 0.15, the present results are indicative of a significant change from common valence distribu3+
3+
2+
3+
4+ 2−
Y0.15
兲0.7Ca0.3
Co0.7
Co0.3
O3 ,
tion in the metallic state, 共Pr0.85
3+
4+
3+
2+
3+
4+
2−
to 共Pr0.59Pr0.26Y0.15兲0.7Ca0.3Co0.88Co0.12O3 in the insulatingstate.

V. CONCLUSIONACKNOWLEDGMENT

A comparative study of perovskite cobaltites
共Ln1−yYy兲0.7Ca0.3CoO3 perovskites 共Ln = Pr, Nd兲 was under-This work was supported by Project No. 202/09/0421 of
the Grant Agency of the Czech Republic.

1

Z. Jirák, J. Hejtmánek, K. Knížek, and M. Veverka, Phys. Rev. B
78, 014432 共2008兲.
2 S. Tsubouchi, T. Kyômen, M. Itoh, P. Ganguly, M. Oguni, Y.
Shimojo, Y. Morii, and Y. Ishii, Phys. Rev. B 66, 052418
共2002兲.
3 S. Tsubouchi, T. Kyômen, M. Itoh, and M. Oguni, Phys. Rev. B
69, 144406 共2004兲.

T. Fujita et al., J. Phys. Soc. Jpn. 73, 1987 共2004兲.
T. Fujita, S. Kawabata, M. Sato, N. Kurita, M. Hedo, and Y.
Uwatoko, J. Phys. Soc. Jpn. 74, 2294 共2005兲.
6
T. Naito, H. Sasaki, and H. Fujishiro, J. Phys. Soc. Jpn. 79,
034710 共2010兲.
7 K. Knížek, J. Hejtmánek, P. Novák, and Z. Jirák, Phys. Rev. B
81, 155113 共2010兲.
45165107-6

METAL-INSULATOR TRANSITION AND THE Pr3+ / Pr…

PHYSICAL REVIEW B 82, 165107 共2010兲
by D. M. Rowe 共CRC Press, London, 2006兲, Chap. 5.

8

A. J. Barón-González, C. Frontera, J. L. García-Muñoz, J.
Blasco, and C. Ritter, Phys. Rev. B 81, 054427 共2010兲.
9
J. Wu and C. Leighton, Phys. Rev. B 67, 174408 共2003兲.
10 K. Knížek, Z. Jirák, J. Hejtmánek, and P. Novák, J. Magn. Magn.
Mater. 322, 1221 共2010兲.
11
J. Hejtmánek, Z. Jirák, M. Maryško, C. Martin, A. Maignan, M.
Hervieu, and B. Raveau, Phys. Rev. B 60, 14057 共1999兲.
12 K. Sekizawa, M. Kitagawa, and Y. Takano, J. Magn. Magn.
Mater. 177-181, 541 共1998兲.
13 H. Masuda, T. Fujita, T. Miyashita, M. Soda, Y. Yasui, Y. Kobayashi, and M. Sato, J. Phys. Soc. Jpn. 72, 873 共2003兲.
14
A. Podlesnyak, S. Rosenkranz, F. Fauth, W. Marti, A. Furrer, A.
Mirmelstein, and H. J. Scheel, J. Phys.: Condens. Matter 5,8973 共1993兲.
15 A. Podlesnyak, S. Rosenkranz, F. Fauth, W. Marti, H. J. Scheel,
and A. Furrer, J. Phys.: Condens. Matter 6, 4099 共1994兲.
16 N. Tessler, Y. Preezant, N. Rappaport, and Y. Roichman, Adv.
Mater. 21, 2741 共2009兲.
17
D. Emin, in Thermoelectrics Handbook: Macro to Nano, edited

18 K. Knížek, J. Hejtmánek, Z. Jirák, P. Tomeš, P. Henry, and G.

André, Phys. Rev. B 79, 134103 共2009兲.
Tachibana, T. Yoshida, H. Kawaji, T. Atake, and E.
Takayama-Muromachi, Phys. Rev. B 77, 094402 共2008兲.
20
C. He, S. Eisenberg, C. Jan, H. Zheng, J. F. Mitchell, and C.
Leighton, Phys. Rev. B 80, 214411 共2009兲.
21 The simple relation between electronic heat and density of states
in metals derives from Fermi-Dirac statistics that is based onprinciple of double 共spin-up, spin-down兲 occupancy of electronic levels. At the presence of correlations and preference of
single occupancy as in Hubbard model, the electronic heat issignificantly modified, see J. Spałek and W. Wójcik, Phys. Rev.
B 37, 1532 共1988兲.
22 F. Bartolomé, M. D. Kuz’min, J. Bartolomé, J. Blasco, J. García,
and F. Sapiña, Solid State Commun. 91, 177 共1994兲.
23 J. J. M. Franse and R. J. Radwanski, in Handbook of Magnetic
Materials, edited by K. H. J. Buschow 共Elsevier, New York,
1993兲, Vol. 7, Chap. 5.
19 M.

165107-7