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Physica C 480 (2012) 1–5

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Physica C
journal homepage: www.elsevier.com/locate/physc

Magnetization and transport properties in the superconducting Pr2Ba4Cu7O15 dwith metallic double-chain
Shibuki Toshima a, Michiaki Matsukawa a,⇑, Taiji Chiba a, Satoru Kobayashi a, Sigeki Nimori b,
Makoto Hagiwara cabc

Department of Materials Science and Engineering, Iwate University, Morioka 020-8551, Japan
National Institute for Materials Science, Tsukuba 305-0047, Japan
Kyoto Institute of Technology, Kyoto 606-8585, Japana r t i c l e

i n f oArticle history:
Received 28 February 2012
Accepted 17 May 2012
Available online 26 May 2012
Keywords:
Double-chain
Pr2Ba4Cu7O15 d
PressureeffectThermopowera b s t r a c t
We have reported the effect of pressure on the magnetization, and transport properties in the nominal
composition Pr2Ba4Cu7O15 d synthesized by a sol–gel technique. A reduction treatment of the as-sinteredsample in vacuum causes higher superconductivity achieving Tc,on = 30 K for d = 0.94. Application ofhydrostatic pressure on the oxygen depleted sample enhances its onset temperature up to 36 K at
1.2 GPa, indicating the nearly optimum doping level of the charge carrier in comparison to the pressure
dependence of lower Tc samples with d = 0.45. Seebeck coefﬁcient of the superconducting sample shows ametallic conduction, followed by a clear drop below Tc,on and is in its temperature dependence below
100 K quite different from that of the non-superconducting one. This ﬁnding strongly suggests a dramaticchange of the electronic state along the CuO double chain due to the reduction treatment for the appearance of superconductivity.
Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction
Since the discovery of high-Tc copper-oxide superconductors,
extensive studies on strongly electron correlated system have beenin progress on the basis of physical properties of two-dimensional(2D) CuO2 planes. Moreover, from the viewpoint of low-dimensional physics, particular attention is paid to the physical role ofone-dimensional (1D) CuO chains included in some families ofhigh-Tc copper oxides such as Y-based superconductors with thetransition temperature Tc = 92 K. It is well known that the Pr-substitution for Y-sites in YBa2Cu3O7 d (Y123) and YBa2Cu4O8 (Y124)
compounds dramatically suppresses Tc and superconductivity in
CuO2 planes disappears beyond the critical value of Pr, xc = 0.5
and 0.8, respectively [1,2]. Such a suppression effect due to Pr-substitution on superconductivity has been explained in terms of thehybridization model with respect to Pr-4f and O-2p orbitals [3].
Y124 compound with double chains is thermally stable up to
800 °C, while in Y123/7-d oxygen deﬁciencies are easily introduced
at lower annealing temperatures [4]. Intermediate between
PrBa2Cu3O7 d (Pr123) with single chains and PrBa2Cu4O8 (Pr124)
with double chains is the Pr2Ba4Cu7O15 d (Pr247) compound withan alternative repetition of the CuO single chain and double chain
blocks along the c-axis, isostructural with superconducting⇑ Corresponding author.
E-mail address: matsukawa@iwate-u.ac.jp (M. Matsukawa).
0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.physc.2012.05.008

Y2Ba4Cu7O15 d (Y247) [5,6]. In contrast to Y123, Y247 retains its
orthorhombic structure and its superconducting properties, evenafter oxygen along the single chains is completely depleted [7].
This ﬁnding is considered to be due to the presence of the double
CuO chain block in Y247. For Pr247, it is possible to examine physical properties of metallic double chains in terms of varying oxygen
contents along single chains.
Recently, Matsukawa et al. discovered that oxygen removed
polycrystalline Pr2Ba4Cu7O15 d with CuO metallic double chainshows superconductivity around Tc,on = 15 K. Matsushita et al.
[8] revealed that Hall coefﬁcient of oxygen defect Pr247 is varied
from positive sign to negative one upon decreasing temperatures,indicating an electron doped superconductor [9]. The nuclear quadrapole resonance experiment indicated that the superconductivity of reduced Pr247 is realized at the CuO double chains [10].
For the appearance of Pr247 superconductor, it is essential to remove oxygen from the as sintered sample of Pr247 [8,11]. Throughseveral works on synthesis of higher Tc samples in nominal Pr247
composition, it has been made clear that the heterogeneous structure containing both Pr123 and Pr124 phases in Pr247 system
plays a crucial role on attaining higher Tc [12,13]. High-resolution
transmission electron microscopy (TEM) analyses of the heterogeneous Pr247 sample exhibits that there exists an irregular longperiod stacking structure along the c axis such as {–S–S–S–S–D–}
sequence, where S and D denote single CuO chain and double
CuO chain blocks, respectively [12].

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S. Toshima et al. / Physica C 480 (2012) 1–5

In this paper, we have reported the magnetic, electronic andthermal transport properties of the oxygen removed
Pr2Ba4Cu7O15 d. In particular, the d = 0.94 sample exhibits higher
superconductivity attaining Tc,on = 30 K.

Polycrystalline samples of the nominal composition
Pr2Ba4Cu7O15 d (Pr247) were synthesized using sol–gel technique.
High purity powders of praseodymium oxide (Pr6O11), barium carbonate (BaCO3), and copper carbonate (CuCO3) were mixed to thestoichiometric composition, they were then added to a suitableamount of citric acid solution and ﬁnally their mixture was magnetically stirred in a container. The solution was condensed on a
hot plate at 80 °C for 30 min and the resultant citrate gel was pyrolyzed at 450 °C for 2 h, to remove the organic compounds. The obtained precursor was ground into a powder by a glass rod. The
precursor was calcined in air at 800 °C for 24 h. The resultant powder was pressed into a pellet and then sintered at 875–876 °C for along time over 180 h under ambient oxygen pressure (as-sinteredsample). For the sintered process, the electric furnace equippedwith three temperature controllers was used to reduce thermal
gradient along the axis of furnace tube. The resultant sample waspost annealed in vacuum at 500 °C for 48–72 h (reduced sample).
X-ray powder diffraction measurement was carried out with Cu
Ka radiation at diffraction angle range of 2h = 5° 80°. The electron probe micro analyzer (EPMA) analysis on the superconductive
sample annealed in vacuum revealed that the atomic ratio of
Pr:Ba:Cu = 1.99:4.18:6.72, through an average of composition analyses at several points within its sample. The oxygen deﬁciency wasestimated to be d = 0.70 and 0.94 from gravimetric analysis for 48 h
and 72 h reduction treatments, respectively. The electric resistivity
was measured with dc four terminal method. Seebeck coefﬁcient
S(T) was determined from both measurements of a thermoelectric
voltage and temperature difference along the longitudinal direction of the measured sample. The Seebeck coefﬁcient of copper
lead was subtracted from the measured values using the publisheddata. The thermal conductivity data were collected with a conventional heat ﬂow method. The dc magnetization was performed
using the commercial superconducting quantum interference device (SQUID) magnetometers both at Iwate University and National
Institute for Materials Science. In particular, to remove the remanent magnetic ﬁeld below 1 mOe, the ﬂuxgate device was utilized
in the zero-ﬁeld cooled (ZFC) and ﬁeld cooled (FC) magnetization
measurements under H = 1 Oe. Hydrostatic pressure in magnetization was applied by using a clamp-type CuBe cell up to 1.2 GPa.
Fluorinert was used as a pressure transmitting medium. The magnitude of pressure was calibrated by the pressure dependence ofthe critical temperature of lead.
3. Results and discussion
First of all, let us show in Fig. 1, X-ray powder diffraction pattern of the superconductive sample with the nominal
Pr2Ba4Cu7O15 d. The low angle diffraction data enlarged near
2h = 5° 8° are presented in the inset of Fig. 1a, where three arrows point to the peaks corresponding to the Miller indexes ofPr124, Pr247, and Pr123 upon increasing 2h. These ﬁndings indicate that the nominal phase of Pr247 obtained coexists with both
Pr124 and Pr123 phases. Under ambient pressure condition, it
seems to be difﬁcult to synthesize high quality polycrystalline
sample with nominal composition. However, to our knowledge,
the heterogeneous samples have shown good superconductive
properties in comparison to pure Pr247 with lower Tc,on = 15 K[12]. The scanning electron microscope (SEM) micrographs of the

Pr124

Intensity (arb. units)

2. Experiment

(a)

Pr247
Pr123

Intensity (arb. units)

2

5

5.5

6

6.5

7

7.5

8

2θ (deg)

10

20

30

40

50

60

70

80

2θ (degree)

Fig. 1. (a) X-ray powder diffraction pattern and (b) SEM image of the superconductive sample with the nominal Pr2Ba4Cu7O15 d. The inset of (a) shows the low angle
diffraction data enlarged near 2h = 5° 8°. Here, three arrows point to the peaks
corresponding to the Miller indexes (0 0 2), (0 0 4), and (0 0 1) of Pr124, Pr247, andPr123, respectively upon increasing 2h.

superconducting sample with d = 0.94 displayed in Fig. 1b revealthat its typical grain size is estimated to be several lm.
Fig. 2a shows the temperature variation of zero ﬁeld cooled(ZFC) and ﬁeld cooled (FC) magnetization of the oxygen removed
Pr247 sample under an applied ﬁeld of 1 Oe. In the inset of
Fig. 2, the magniﬁed data are plotted, to clarify the deﬁnition of onset Tc. The as-sintered sample exhibits no superconducting anomaly as previously reported. However, the reduction treatment on its
as-sintered one in vacuum results in diamagnetic character from
Tc,on = 21 K for 48 h up to Tc,on = 30 K for 72 h, which reaches
highest value over the previous works [8,11,12]. The magnetic susceptibility data taken under 1 Oe using the ﬂuxgate device are presented for the 72 h reduction sample. From the ZFC magnetization
at 2 K, the superconducting volume fraction is estimated to be10%, indicating bulk superconductivity. Next, let us examine
the effect of external pressure on the low-T magnetization in theoxygen removed Pr247. First of all, application of hydrostatic pressure on the superconducting sample enhances Tc,on = 30 K at
0 GPa up to 36 K at 1.2 GPa as shown in Fig. 3, where the data
including the magnetization of the pressure cell are normalized
by the value of magnetization at 40 K. In the inset of Fig. 3a, the en-Author's personal copy

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S. Toshima et al. / Physica C 480 (2012) 1–5

2(a) H= 1 Oe

0(a) ZFC 50 Oe
T

0GPa

0

= 30.5 K

C ON

0.8GPa-4

1.2GPaFC-5

3.2x10

-1x10

-5

3x10

T

C ON

= 30.5 K

2.8x10-5 ZFC

ZFC 50 Oe
T

-4

-6

-3

-1.5x100GPa
0.8GPa1.2GPa1.06

M/M(40K)

FC-3

M /M (40K)-2

M (emu/g)

M (emu/g)

-5x10

= 30K

C ON

1.04
1.02

T

1

-5

=33.5K

C ON

2.6x10

ZFC

T
28

30

32

-3

-2x10

0

5

10

15

20

25

34
T (K)

36

38

0.96
25

30

=36K

C ON

0.98

-8

-5

2.4x10

35

40

-10

Temperature (K)

0

5

10

15

30

20

25

35

T (K)

30

40

35

40

Temperature (K)0.005(b) ZFC 20 Oe

TC ON = 21 K

40

TC ON = 30K(b)0

as-sintered
48 h
72 h

δ=0.9430-0.0155.2x10

-4

5x10

-4

4.8x10

-4

ZFC

T

C ON

2.5GPa

(K)

-4

c on

5.4x10= 30K

δ=0.62

25

T

FC

-0.01

M (emu/g)

M (emu/g)-0.005

35

20
-0.02
4.6x10-4

28-0.025

0

5

10

15

20

30

32

25

34
T (K)

30

36

15

38

35

δ=0.4540

Temperature (K)Fig. 2. (a) Temperature variation of zero ﬁeld cooled (ZFC) and ﬁeld cooled (FC)magnetization of oxygen removed Pr247 under an applied ﬁeld of 1 Oe using theﬂuxgate device. (b) The magnetization curves of the oxygen removed Pr247 under
20 Oe for several reduced conditions. In the inset, the magnetization data aremagniﬁed to clarify the deﬁnition of Tc,on.

larged plots are displayed to emphasize the variation of Tc,on withpressure. In Fig. 3b, the effect of pressure on Tc,on of pure Pr247
with d = 0.45 prepared by a high pressure oxygen technique citedfrom Ref. [14] shows a rapid rise of Tc,on from 15 K at ambient pressure up to 24 K at 1 GPa, indicating the stronger dependence of Tc,onon pressure in comparison to the present result. Recently, Ishikawa
has reported that the application of pressure on the onset of Tc in
Pr2Ba4Cu7O15 d (d = 0.62) increases from 20.6 K at 0 GPa up to
33.1 K at 2.5 GPa. Furthermore, increasing pressure above 3 GPa,
the superconductivity disappeared and under high pressure above
5 GPa the temperature variation of the resistivity showed a semiconducting like behavior [15]. These ﬁndings suggest that if thepure Pr247 sample is located in the underdoped region, the present
Pr247 sample is near the optimum one. Second, the magnitude ofnegative magnetization (the volume of the superconducting state)is suppressed with the applied pressure of 1.2 GPa, which is contrast to the pressure effect of Tc,on. Sano et al. discussed the suppression of the superconductivity in the CuO double chain underpressure on the basis of Tomonaga–Luttinger liquid theory [16].
If the volume shrinkage due to the application of pressure gives
rise to a decrease of the distance between the two chains of a
CuO double chain, we then expect that an increase of the hopping
term between the nearest neighbor oxygen sites occupying 2p

10

0

0.2

0.4

0.6

0.8

1

1.2

P (GPa)Fig. 3. (a) Temperature variation of normalized magnetization of oxygen removed
Pr247 under hydrostatic pressures of 0, 0.8 and 1.2 GPa. The enlarged plots areshown in the inset, to clarify the effect of external pressure on Tc,on. (b) The pressure
dependence of Tc,on of the Pr247 sample with the oxygen defect d = 0.94 as afunction of the applied pressure up to 1.2 GPa. For comparison, the data of pure
Pr247 with d = 0.45 and 0.62 prepared by a high oxygen pressure technique arecited [14,15].

orbitals, resulting in a suppression of the superconducting correlation. Their calculation does not take into account for the pressure
dependence of carrier concentration, giving no explanation forthe overall change of the superconducting state with pressure.
The d dependence of Tc,on shows a monotonous increase from15 K at d = 0.45 up to 30 K at d = 0.94, which is qualitatively in good
agreement with a theoretical prediction by Nakano [17]. Comparing the effect of pressure on Tc,on with the d dependence of it, we
expect that the application of pressure gives rise to an increasein the superconducting carrier concentration.
For several reduction conditions, the temperature variation ofelectrical resistivity of the present sample Pr247 is given in
Fig. 4a. For 72 h reduced case, q(T) curve exhibits its dramatic drop
near 30 K followed by zero resistance at low temperatures. We notice that the resistivity in the normal state for except lower temperature region rises gradually with increasing the reduction
time. This tendency seems to be not anomalous but common
behavior in the present system because an increase of the resistivity in CuO2 plane, qpl, due to the reduction treatment is predominant over a decrease of that along CuO double chain, qch at
higher temperatures.

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S. Toshima et al. / Physica C 480 (2012) 1–5

0.03

(a)

as sintered
48h
72h

0.025

ρ (Ω cm)

0.02

0.015

0.01

0.005

0

0

50

100

150

200

250

300

T (K)
35

(b)

30

as-sintered
72h reduction

20

5

15
4

Tc on

10

S (μV/K)

S (μV/K)

25

3
2

5
1

0

0
0

-5

0

50

100

150

10

200

20
30
T (K)

250

40

50

300

T (K)
Fig. 4. (a) Temperature dependence of the resistivity of oxygen removed Pr247 forseveral annealing conditions. (b) Temperature dependence of Seebeck coefﬁcientfor the as-sintered and oxygen removed Pr247. In the inset, the magniﬁed data of
the reduced sample are presented.

Finally, we check thermal transport properties of the superconducting and non-superconducting Pr247 (Fig. 4b). (Thermal conductivity data are not shown here) Seebeck coefﬁcient S is asensitive probe to examine the electronic state of highly inhomogeneous materials since the thermal current is hard to be disturbed
by the grain boundaries in contrast to the electric current. Upondecreasing T, Seebeck coefﬁcient S(T) of the superconducting sample reaches a maximum peak, then shows a rapid decrease below
200 K, and ﬁnally followed by a substantial drop below Tc,on. It istrue that the behavior of S(T) of the as-sintered non-superconducting sample is, both in its temperature dependence and magnitude,
similar to that of Pr124 with metallic double chain [18]. The magnitude of S of Pr124 reaches a small value less than 1 lV/K below
100 K, which is explained on the basis of the one- to two-dimensional (1D–2D) crossover. For conventional metal, Seebeck coefﬁcient is expressed in terms of S = (pk)2T/3e{@ln (N(E))/@E}, where
N(E) is the density of electronic state [19]. In our rough estimation,
if we assume that the magnitude of S is proportional to the energy
derivative of N(E), the density of state for 2D metal is independentof the energy, giving S(T)  0. However, the S value of the superconducting Pr247 shows a ﬁnite value of at most a few lV/K down
to Tc,on. This ﬁnding is probably ascribed to a variation of the electronic state of the CuO double chain associated with carrier doping

into the double-chain block due to the reduction treatment. Upondecreasing T, the Hall coefﬁcient RH of the 24 h reduced sample
changes its sign across 100 K and then exhibits a rapid increases
with negative sign. We expect that the disagreement in sign between RH and S in the oxygen removed sample is ascribed to thedramatic variation of the density of states with reduction treatment since @(N(E))/@E near the Fermi energy is related to the signof Seebeck coefﬁcient in addition to the sign of charge carrier.
A previous work [20] on role of oxygen in Pr123 compound
revealed that the oxygen deﬁcient Pr123 phase remains highly
insulating state, causing neither metallic or superconducting
sign. Double chain conduction in single crystalline Pr124 exhibitsa strongly metallic character but no superconducting transition
down to 2 K [21]. We concentrate on single-chain rich microdomains present in the nominal Pr247 as observed by TEM measurements [12], which is probably close to the appearance of
higher Tc,on = 30 K accompanied by the larger oxygen defectd = 0.94. It is, thus, expected that a reduction treatment removes
oxygen from CuO single chain rich sequence such as {S–D–S–S–
S–D} and results in higher carrier doping into its surrounding
CuO double chain in comparison to that in the regular sequence{S–D–S–D–S–D}.
In summary, we have demonstrated the pressure effect of Tc,on,
and the transport properties of the nominal composition
Pr2Ba4Cu7O15 d. Magnetization and resistivity measurements ofthe d = 0.94 sample reduced in vacuum exhibit Tc,on = 30 K. The
pressure dependence of Tc,on of the d = 0.94 sample strongly suggests the nearly optimum doping of the superconducting carries.
Comparing the effect of pressure on Tc,on with the d dependenceof it, we notice that the application of pressure gives rise to an increase in the carrier concentration. We believe that the heterogeneous structure of the nominal Pr247 is closely related to theappearance of higher Tc. Seebeck coefﬁcient of the d = 0.94 sample
shows a metallic conduction below 100 K and is in its temperaturedependence quite different from the data of the as-sintered one,leading to a dramatic change of the density of state along the
CuO metallic double chain.
Acknowledgments
The authors are grateful for Dr. K. Nonaka for his assistance in
EPMA experiments. This work was partially supported by aGrant-in-Aid for Scientiﬁc Research from Japan Society of the Promotion of Science.
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