Physica C 411 (2004) 101–106
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Superconductivity in Pr2Ba4Cu7O15
double chainsd with metallic

M. Matsukawa a,*, Yuh Yamada b, M. Chiba a, H. Ogasawara a, T. Shibata c,
A. Matsushita d, Y. Takano da

Department of Materials Science and Technology, Iwate University, Ueda 4-3-5, Morioka 020-8551, Japanb
Department of Physics, Niigata University, Niigata 950-2181, Japanc
Interdisciplinary Faculty of Engineering, Shimane University, Matsue 690-8504, JapandNational Institute for Materials Science, Tsukuba 305-0047, Japan
Received 23 April 2004; accepted 2 July 2004
Available online 5 August 2004

Abstract
We report superconductivity with Tc, onset = 10 K in polycrystalline samples of Pr2Ba4Cu7O15 d possessing metallic double chains. A reduction treatment on as-sintered samples causes not only the enhanced metallic conduction but
also the appearance of superconductivity, in strongly contrast with the oxygen removal eﬀect of superconducting
Y2Ba4Cu7O15 d.
 2004 Elsevier B.V. All rights reserved.
PACS: 74.25.Fy; 74.72.Jt
Keywords: Pr2Ba4Cu7O15 d; Double-chains

1. Introduction
Since the discovery of high-Tc copper-oxide
superconductors, extensive studies on strongly
electron correlated system have been in progresson the basis of physical properties of two-dimensional (2D) CuO2 planes. Moreover, from the*Corresponding author. Tel./fax: +81 19 621 6358.
E-mail address: matsukawa@iwate-u.ac.jp (M. Matsukawa).

viewpoint of low-dimensional physics, particularattention is paid to the physical role of one-dimensional (1D) CuO chains included in some familiesof high-Tc copper oxides such as Y-based superconductors with the transition temperature
Tc = 92 K. It is well known that the Pr-substitution for Y-sites in YBa2Cu3O7 d (Y123/7 d) andYBa2Cu4O8 (Y124/8) compounds dramaticallysuppresses Tc and superconductivity in CuO2
planes disappears beyond the critical value ofPr, xc = 0.5 and 0.8, respectively [1,2]. Such a0921-4534/$ - see front matter  2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.physc.2004.07.002

102M. Matsukawa et al. / Physica C 411 (2004) 101–106

suppression eﬀect due to Pr-substitution on superconductivity has been explained in terms of thehybridization model with respect to Pr-4f and O2p orbitals [3]. Y124/8 compound with doublechains is thermally stable up to 800 C, while inY123/7 d oxygen deﬁciencies are easily introduced at lower annealing temperatures [4]. Intermediate between PrBa2Cu3O7 d (Pr123/7 d)with single chains and PrBa2Cu4O8 (Pr124/8) withdouble chains is the Pr2Ba4Cu7O15 d (Pr247/15 d) compound with an alternative repetition of
the single and double chains along the c-axis, isostructural with superconducting Y2Ba4Cu7O15 d(Y247/15 d) [5,6]. The crystal structure of orthorhombic Pr247 is schematically shown in Fig. 1(space group Ammm). In Pr247, it is possible toFig. 1. Schematic view of the Pr2Ba4Cu7O15 d structure with a repetition of the corner-sharing CuO single chain and the egde-sharing
CuO double chains. Also shown is the direction of the orthorhombic principal axes. The inset indicates models of parallel resistors
along the b direction, where Rp and Rc denote resistances of CuO2 planes and CuO chains, respectively.

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103

examine physical properties of metallic double
chains varying oxygen contents along single chains[6,7].
In this paper, we report superconductivity in
Pr2Ba4Cu7O15 d compound possessing metallic
double chains.
2. Experimental
Polycrystalline samples of Pr2Ba4Cu7O15 dwere synthesized using a powder sintering method
under high pressure oxygen [2,6]. The Pr6O11,
BaO2 and CuO powders with high purity weremixed to the stoichiometric composition and werethen pressed into a cylindrical pellet. The pelletwas calcined at 850–900 C in air and was then sintered at 975 C for 18 h with P(O2) = 5 atm. Assintered samples were annealed in argon gas at650 C. The resistivity measurement was performed by a conventional four-probe technique.
The magnetization measurement was carried out
down to 2 K under zero-ﬁeld cooling (ZFC) andﬁeld cooling (FC) conditions using a SQUID
magnetometer.
3. Results and discussion
Fig. 2 shows the X-ray diﬀraction patterns for
as-sintered and Ar reduced Pr247/15 d at roomtemperature. All diﬀraction lines were indexed interms of the Pr247 majority phase, except for asmall amount of the BaCuO2 phase. Here, we givesome comments on impurity phases in Pr247.
From our previous studies on synthesis of Pr247
compounds, we have pointed out possible impurity
phases such as PrBaO3, BaCuO2 and Pr124/8, except for the starting materials [6,2]. X-ray diﬀraction patterns on both as-sintered and reduced
Pr247 reveal at a small amount of the BaCuO2
impurity phase although the precipitation ofPrBaO3 and Pr124/8 is strongly suppressed in thissintering process. It is truly expected that in thepresent samples, the PrBaO3 and Pr124/8 phases
may be included as a very small volume fraction
hardly detectable by X-ray diﬀraction. However,the lower-T heat treatment never causes superconductivity for the PrBaO3, BaCuO2 and Pr124/8

Fig. 2. X-ray diﬀraction patterns of Pr2Ba4Cu7O15 d compounds at room temperature: (a) as-sintered and (b) annealed in
Ar for 4 days. The indices represent reﬂections of the Pr247
phase.

phases. The oxide materials of PrBaO3 and BaCuO2 are highly insulator. Moreover, the Pr124/8
phase is thermally stable under lower temperatureannealing and the resistivity data of metallic Pr124
single crystals exhibit no superconductivity down
to 2 K [8]. Therefore, we believe that even if there
exist minor impurity phases hardly observed by Xray probe, low-T reduction process induces nosuperconductivity for the impurity phases inPr247.
The lattice parameters of the as-sintered sample
are estimated from the least-squared ﬁtting tothe X-ray diﬀraction data to be a = 3.8880(3),b = 3.9037(3) and c = 50.660(4) Å, respectively.
The oxygen deﬁciency d of as-sintered Pr247 is104

M. Matsukawa et al. / Physica C 411 (2004) 101–106

taken as almost zero because pure Y247 prepared
under the same sintering condition as Pr247,exhibits a sharp superconducting transition withTc = 89 K [2]. In fact, Tallon et al., reported thatthe resistivity of Y247/15 shows a superconducting
transition near Tc = 92 K [9]. For comparison,
the unit length of c-axis in Pr247/15 [=2 · (Pr123/7 unit) + (Pr124/8 unit)] is estimated to be 50.77
Å from c = 11.73 Å for Pr123/7 and c = 27.308 Å
for Pr124/8 [6,10]. This estimation is almost comparable with the c-axis lattice constant of Pr247/
15. For the sample reduced in Ar for 4 days, thelattice parameters are determined to bea = 3.8923(5), b = 3.9020(3) and c = 50.814(6) Å,respectively. The oxygen deﬁciency d of Pr247
due to Ar annealing is estimated to be d = 0.5
from the thermogravimetry and results in a substantial elongation of the c-axis length up to0.3%, keeping its orthorhombic structure. These
results are similar with a variation in the lattice
parameters of Y247 as a function of oxygen deﬁciency. Irizawa et al., reported that the c-axis
length of Y247/15 d reaches an increase by
0.36% over the range of oxygen deﬁciency fromd = 0.05 to 0.6 [11].
Fig. 3(a) shows the temperature variation ofelectrical resistivity in sintered Pr247/15 d. The
value of q of the as-sintered sample exhibits asemiconducting behavior at high T, it reaches abroad maximum at Tm = 150 K and graduallydecreases down to 4 K. This ﬁnding has been well
explained on the basis of the parallel circuit model(the inset of Fig. 1) composed of both the semiconducting CuO2 planes and metallic CuO doublechains, as ﬁrst proposed by Bucher et al., in YBCO
system [7,12]. A previous study on both sintered Pr
124 and Pr247 implicated that the negative temperature dependence of the resistivity above Tm is ascribed to the CuO2 plane conduction, while thepositive T-dependence of the resistivity below Tmto the CuO double-chain conduction [7]. On theother hand, the T dependence of q of the reduced
samples follows a strongly metallic conduction up
to 300 K. For the 4-days reduced sample, the superconducting transition evidently appears around
10 K and the zero-resistivity temperature Tc, zerois determined to be 3.8 K from low-T resistivity
data. The value of q of the superconducting

Fig. 3. The temperature variation of electrical resistivity in
Pr247, (a) over the whole range of temperature up to 300 K,and (b) below 50 K. The arrows denote the value of Tc, onset.

sample is proportional to T2 below 150 K, whichis the same temperature dependence of q along
the b-axis of Pr124 single crystal [8]. Accordingly,
the oxygen defects in Pr247 system cause not only
the enhanced metallic state but also the appearance of superconductivity, in strongly contrast
with the oxygen removal eﬀect of Y247 on its resistivity [11].
Next, the temperature dependence of magnetic
susceptibility (v) in Pr247 is shown in Fig. 4. For
the 4-days reduced sample, the value of v gradually
increases with decreasing T, then exhibits a humpnear 10 K and ﬁnally a clear diamagnetic signal
is detected below 9 K on both ZFC and FC data.
The superconducting volume fraction is estimatedto be about 4% at 2 K from the ZFC data. Moreover, for 2-days reduced one, a sharp drop in the v

M. Matsukawa et al. / Physica C 411 (2004) 101–106

(a)

(b)

Fig. 4. (a) The magnetic susceptibility(M/H) of the 4-days
reduced sample in Pr247 as a function of temperature underzero ﬁeld cooling (ZFC) and ﬁeld cooling (FC) conditions at 1Oe. In the inset, the M/H data of the 2-days one are also shown.
(b) The susceptibility data of the as-sintered sample in Pr247. A
solid curve represents a least-squared ﬁtting according to theCurie–Weiss law taken into account the T independent component v0. The arrow denotes the antiferromagnetic ordering
temperature of Pr ions. The inset of Fig. 4(b) indicates(v v0) 1 versus T plots up to 300 K.

data is also observed below 10 K. On the other
hand, the value of v for the as-sintered one showsa Curie–Weiss like behavior and a slight anomaly
is observed at 3 K indicating the formation ofsuperconducting state. The Curie–Weiss plot suggests that the Néel temperature of Pr spins TN is
16 K almost the same as TN(=17 K) in Pr124[2]. Furthermore, the (v v0) 1 data in the insetof Fig. 4(b) show a nonlinear deviation near160 K associated with the Néel ordering of Cu
spins, which is not far from TN(=200 K)in Pr124

105[13]. This ﬁnding is consistent with the evidence
for an antiferromagnetic order of the planar Cu
spins in the NQR experiment [14].
Here, we would like to comment on superconductivity with Tc = 85 K in a Pr123 single crystal
reported by Zou et al. [15]. Surely, the origin ofbulk superconductivity in Pr123 has not been
made clear, but several authors claimed that thesuperconductivity originates from the inhomogeneities in the chemical composition such as theBa-rich Pr123 [16]. The partial substitution of Bafor Pr-site gives rise to a strong suppression of
the hybridization between Pr-4f and O-2p orbits,
resulting in doping mobile carriers into CuO2planes and the appearance of superconductivity.
This issue seems to depend on the criterion
whether the orbital hybridization leading to carrier
localization is retained or not. The negative Tdependence of resistivity for as-sintered Pr247(Fig. 3(a)) demonstrates a clear contribution fromthe insulating CuO2 planes as well as the insulating
Pr123. Moreover, Matsushita et al., showed thatthe value of q in as-sintered Pr247 gradually rises
at high temperatures with applied pressure, indicating the enhancement of the hybridization state
between Pr and O orbits [7]. It is easily understoodthat a reduction process does not alter the electronic state of the Pr–O hybridization because alower temperature annealing in Ar hardly cause
the Ba substitution for Pr sites in the Pr247 composition. In fact, a previous work on stoichiometric Pr123/7 d reported that its resistivity increases
by more than three orders of magnitude as theoxygen content 7 d is reduced from 6.93 to
6.46 [10]. In this study, the c-axis elongation due
to oxygen deﬁciency gives no changes on the Pr–
O hybridization state in Pr123 system. In a similarway, it is expected that there also exists the hybridization in the reduced Pr247 accompanied by the caxis elongation due to oxygen removal. In the view
point of sample preparation, we have to point out
outstanding diﬀerences between Pr123 and Pr247.
As-grown crystal of Pr123 was annealed in ﬂowing
pure oxygen gas for over 4 days under two steps ofannealing temperatures in order to realize superconductivity. Zou et al., reported that the c-axis
lattice constant of the annealed crystal is shorter
than that of the as-grown one. Therefore, it is106

M. Matsukawa et al. / Physica C 411 (2004) 101–106

K, which is probably related to the enhanced
metallic state due to oxygen removal. A systematic
study on the pressure eﬀect on both transport andmagnetic properties in the superconducting Pr247
is in progress [18]. A microscopic research suchas the NMR will be also carried out, in order tospecify the superconducting regions.
In summary, we have reported superconductivity in polycrystalline samples of Pr2Ba4Cu7O15 dpossessing metallic double chains. Both resistivity
and magnetization measurements exhibit thesuperconducting transition temperature Tc, onset =10 K and the superconducting volume fraction
is estimated to be about 4% at 2 K. The reduction
treatment causes not only the enhanced metallic
conduction but also the realization for superconductivity, accompanied by the c-axis elongation
due to oxygen deﬁciency.

Acknowledgment
This work was partially supported by a Grantin-Aid for Scientiﬁc Research from the Ministry ofEducation, Science and Culture, Japan.

Fig. 5. (a) The resistivity and (b) magnetization data for Pr247
samples annealed in a vacuum (at 400 C for 24 h) as a functionof temperature. In the inset of Fig.5(a), its magniﬁed data below
50 K are shown. A low-temperature vacuum reduction
enhances the value of Tc, onset = 18 K.

diﬃcult to identify the observed superconductivity
in reduced Pr247 with the bulk superconductivity
in oxidized Pr123 crystal.
Recently, we have found out that a low-T
reduction treatment in a vacuum causes a higher
superconductive sample of Pr247, as shown inFig. 5, through our seeking for more suitableannealing conditions. It should be noted thatsuperconductive samples are obtained reproducibly under the similar annealing procedure fromstarting materials. Moreover, Hall coeﬃcientmeasurement [17] on reduced Pr247 reveals thatRH(T) clearly shows its negative sign below 100

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