Physica C: Superconductivity and its applications xxx (xxxx) xxx

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Physica C: Superconductivity and its applications
journal homepage: www.elsevier.com/locate/physc

Enhanced superconducting properties of double-chain based
superconductor Pr2 Ba4 Cu7 O15− δ synthesized by citrate pyrolysis technique
Kazuma Honnami a, Michiaki Matsukawa *, a, Tatsuya Senzaki a, Tomoki Toyama a,
Haruka Taniguchi a, Koichi Ui a, Takahiko Sasaki b, Kohki Takahashi b, Makoto Hagiwara cabcFaculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Kyoto Institute of Technology, Kyoto 606-8585, JapanA R T I C L E I N F O

A B S T R A C T

PACS:
74.25.Ha
74.25.F74.90.+n

We report the enhanced superconducting properties of double-chain based superconductor Pr2 Ba4 Cu7 O15− δsynthesized by the citrate pyrolysis technique. The reduction heat treatment in vacuum results in the appearanceof superconducting state with Tc =22-24 K, accompanied by the higher residual resistivity ratios. The super­
conducting volume fractions are estimated from the ZFC data to be 50 ∼ 55%, indicating the bulk supercon­
ductivity. We evaluate from the magneto-transport data the temperature dependence of the superconducting
critical field, to establish the superconducting phase diagram. The upper critical magnetic field is estimated to beabout 35 T at low temperatures from the resistive transition data using the Werthamer-Helfand-Hohenberg
formula. The Hall coefficient RH of the 48-h-reduced superconducting sample is determined to be − 0.5 × 10− 3
cm3 /C at 30 K, suggesting higher electron concentration. These findings have a close relationship with homo­
geneous distributions of the superconducting grains and improved weak links between their superconducting
grains in the present synthesis process.

1. Introduction
Since the discovery of high-Tc copper-oxide superconductors, re­searches have focused on the unconventional superconductivity on thetwo-dimensional CuO2 planes in some cuprates. In quasi onedimensional (1D) ladder system without CuO2 planes, it is well known
that the superconductivity at Tc = 12 K appears only under the appli­
cation of high pressure [1]. In a previous study of Pr-based cuprates withmetallic CuO double chains and insulating CuO2 planes,
Pr2 Ba4 Cu7 O15− δ is found to be a new superconductor with a higher Tc(15 K) after a reduction treatment [2]. A nuclear quadrupole resonance(NQR) study has revealed that the newly discovered superconductivity
is realized at the CuO double-chain block [3].
Structurally, the Pr-based cuprates, PrBa2 Cu3 O7− δ (Pr123) and
PrBa2 Cu4 O8 (Pr124), are identical to their corresponding Y-based highTc superconductors, YBa2 Cu3 O7− δ (Y123) and YBa2 Cu4 O8 (Y124).
Pr123 and Pr124 compounds have insulating CuO2 planes and are nonsuperconductive [4,5]. The suppression of superconductivity in the Pr
substitutes has been explained by the hybridization of Pr-4f and O-2p

orbitals [6]. The crystal structure of Pr124 with CuO double chainsdiffers from that of Pr123 with CuO single chains. It is well known thatCuO single chains in Pr123 and CuO double chains in Pr124 show
semiconducting and metallic behaviors, respectively [7]. The carrier
concentration of doped double chains of Pr124 is difficult to vary,because it is thermally stable up to high temperatures.
The compound Pr2 Ba4 Cu7 O15− δ (Pr247) is an intermediate between
Pr123 and Pr124. In this compound, CuO single-chain and double-chain
blocks are alternately stacked along the c-axis such as {-D-S-D-S-}
sequence [8,9] (see Fig. 1). Here, S and D denote CuO single-chain anddouble-chain blocks along the b-axis, respectively. The physical prop­
erties of the metallic CuO double chains can be examined by controllingthe oxygen content along the semiconducting CuO single chains.
Anisotropic resistivity measurements of single-crystal Pr124 haverevealed that metallic transport arises by the conduction along the CuO
double chains [10]. In oxygen removed polycrystalline Pr2 Ba4 Cu7 O15− δ ,the superconductivity appears at an onset temperature Tc,on of ∼15 K[2]. Hall coefficient measurements of superconducting Pr247 with
Tc,on = 15 K have revealed that at intermediate temperatures below 120* Corresponding author.
E-mail address: matsukawa@iwate-u.ac.jp (M. Matsukawa).
https://doi.org/10.1016/j.physc.2021.1353869
Received 22 January 2021; Received in revised form 5 March 2021; Accepted 8 April 2021
Available online 17 April 2021
0921-4534/© 2021 Elsevier B.V. All rights reserved.

Please cite this article as: Kazuma Honnami, Physica C: Superconductivity and its applications, https://doi.org/10.1016/j.physc.2021.1353869

K. Honnami et al.

Physica C: Superconductivity and its applications xxx (xxxx) xxx

enhanced zero-resistance states under magnetic field, and higher carrier
concentrations. The final section is devoted to the summary.
2. Experiment
Polycrystalline samples of Pr2 Ba4 Cu7 O15− δ were synthesized byusing the citrate pyrolysis technique [14,15]. In the first step, stoichio­
metric mixtures of high purity Pr6 O11 , Ba(NO3 )2 , and CuO were dis­solved in a nitric acid solution at 50-60 ◦ C. After adding citric acid andneutralizing it by aqueous ammonia, we then obtained the porous
products through the self-ignition process using halogen lamp stirrer. In
the next step, the precursors were ground and resultant fine powders
were annealed under ambient oxygen pressure at 890-891 ◦ C for an
extended period over 110-120h. In the present citrate pyrolysis synthesis
procedure, we adopted the electric tube furnace with three zone tem­
perature controlled system, to achieve the temperature uniformity
within 1 ◦ C.
For scanning electron microscope (SEM) measurements, we prepared
the polycrystalline film sample, to reveal microstructures of single-phase
Pr2 Ba4 Cu7 O15 powders synthesized by the citrate pyrolysis technique.
The polycrystalline film on Ag substrate was fabricated from the assintered powders by an electrophoretic deposition technique. The elec­
trophoretic deposition was conducted in the acetone and iodine bathunder the application of electric voltage up to 300 V for 120 s [16]. We
set Pt and Ag plates as anode and cathode electrodes, respectively.
Furthermore, we performed the magnetic field assisted electrophoretic
deposition process for the fabrication of c-axis aligned Pr2 Ba4 Cu7 O15
polycrystalline film [17]. Here, the magnetic anisotropy of rare earth ion
Pr including Pr2 Ba4 Cu7 O15 shows that the magnetic susceptibility ofχ H‖c is larger than that of χ H⊥c [18]. A magnetic field up to 10 T wasapplied to the colloid bath including the suspended Pr247 powders using
a superconducting magnet with a 100 mm diameter bore at room tem­
perature at the High Field Laboratory for Superconducting Materials,
Institute for Materials Research (IMR), Tohoku University. The magnetic
field was applied parallel to the direction of applied electric field for theelectrophoretic deposition.
We performed X-ray diffraction measurements on the produced
samples at room temperature with an Ultima IV diffractometer (Rigaku)
using Cu-Kα radiation. The lattice parameters were estimated from the xray diffraction data using the least-squares fits.
The oxygen in the as-sintered sample was removed by reduction
treatment in a vacuum at 500 ◦ C, yielding a superconducting material.
Typical dimensions of the pelletized rectangular sample were 4.2 ×
3.4 × 1.3 mm3 . X-ray diffraction data revealed that the as-sinteredpolycrystalline samples are an almost single phase with an ortho­rhombic structure (Ammm), as shown in Fig. 1(a).
The lattice parameters of the as-sintered sample are a = 3.89293 Å,
b = 3.91394 Å, and c = 50.79270 Å, which are in fair agreement withthose obtained by a previous study [19]. The oxygen deficiencies in the
48 h and 72 h reduced samples in a vacuum were estimated fromgravimetric analysis to be δ = 0.43 and 0.54, respectively. As a functionof the oxygen deficiency, the Tc,on rises rapidly at δ ≥∼ 0.2, thenmonotonically increases with increasing δ, and finally saturates around26-27 K at δ ≥∼ 0.6 [20]. Accordingly, we expect that the carriers in thepresent sample are concentrated around the optimally doped region.
The electric resistivity in zero magnetic field was measured by the dc
four-terminal method. The magneto-transport up to 9 T was measured
by the ac four-probe method using a physical property measuring system(PPMS, Quantum Design), increasing the zero-field-cooling (ZFC) tem­
peratures from 4 K to 40 K. The high field resistivity (up to 14 T) was
measured in a superconducting magnet at IMR, Tohoku University. The
electric current I was applied longitudinally to the sample ; conse­
quently, the applied magnetic field H was transverse to the sample(because H⊥I). We performed Hall coefficient measurements on the assintered and 48 h reduced samples with the five-probe technique using

Fig. 1. (color online)(a) X-ray diffraction patterns of as-sintered polycrystalline
Pr2 Ba4 Cu7 O15− δ . The (004) peak corresponds to one of typical Miller indexes of
Pr247. The calculated curve is obtained using the lattice parameters in the text.
Inset shows the crystal structure of Pr247 with CuO single-chain and doublechain blocks along the b-axis. Here, S and D denote CuO single-chain anddouble-chain blocks along the b-axis. (b)X-ray diffraction pattern for poly­
crystalline film prepared by an electrophoretic deposition technique under 10
T. The magnetic field was applied parallel to the direction of applied electricfield between anode and cathode electrodes.

K, the main carriers change from holes to electrons, as the temperature
decreases. Accordingly, this compound is an electron-doped supercon­
ductor [11]. In our previous study, we examined the effect of magnetic
fields on the superconducting phase of Pr247 [12]. Despite of theresistive drop associated with the superconducting transition, we foundthat the diamagnetic signal was strongly suppressed as expected in the
1D superconductivity of CuO double chains. We also reported the effectof pressure on magneto-transport properties in the superconducting andnormal phases of the metallic double chain compound Pr2 Ba4 Cu7 O15− δ[13]. The model of slightly warped Fermi surfaces explains not only themagneto-resistance (MR) effect of the non-superconducting sample, butalso the pressure-induced MR phenomena of the superconducting
sample.
In this paper, we demonstrate the magneto-transport properties of
Pr2 Ba4 Cu7 O15− δ with Tc,on (22-24 K) accompanied by the strongly
metallic conduction, for our understanding of the effect of magnetic fieldon the superconducting properties of electron-doped metallic doublechain compound. In the next section, the experimental outline is
described. In Sect. 3, we show the outstanding data including higher
residual resistivity ratios, larger superconducting volume fractions,2K. Honnami et al.

Physica C: Superconductivity and its applications xxx (xxxx) xxx

PPMS and the 15T-superconducting magnet [21]. The dc magnetization
was performed under ZFC in a commercial superconducting quantum
interference device magnetometer (Quantum Design, MPMS).
3. Results and Discussion
Figure 1 (a) shows X-ray diffraction (XRD) patterns of as-sinteredpolycrystalline Pr2 Ba4 Cu7 O15− δ (Pr247). The (004) peak corresponds toone of typical Miller indexes of Pr247. The calculated curve is obtainedusing the lattice parameters. The inset of Fig. 1(a) displays the crystal
structure of Pr247 with CuO single-chain and double-chain blocks along
the b-axis. The unknown impurity peaks near 2θ ∼30 ∘ are identified as
the BaCuO2 phase [15].
For comparison, X-ray diffraction pattern of polycrystalline film
prepared by an electrophoretic deposition technique under 10 T isshown in in Fig. 1(b). We note that the XRD pattern of the Pr247 film
fabricated by the electrophoretic deposition process without the appliedmagnetic field (not shown here) was the same as that of the Pr247
powders. (00l) peaks did not appear except for the (004) and (0026)
peaks. When the magnetic field was applied parallel to the direction of
applied electric field, the peak intensities of (00l) were strongly
enhanced as shown in Fig. 1(b), indicating the c-axis alignment ofpolycrystalline grains. The SEM image of the Pr247 film in Fig. 2 reveals
that plate-like grains with sub micron size are homogeneously dispersed.
Now, the temperature dependences of electric resistivities of the assintered, 48-h-reduced and 72-h-reduced Pr2 Ba4 Cu7 O15− δ compounds
are shown in Fig. 3(a). The reduction heat treatment on the as-sinteredsample in vacuum results in the appearance of superconducting statewith Tc =22-24 K, accompanied by the strongly metallic properties overa wide range of temperature. Here, we define the residual resistivity
ratio (RRR) as ρ(300K)/ ρ(30K), For the present samples reduced in
vacuum, we obtained the higher RRR values (10 ∼ 12.5), in comparison
with the previous data [12].
Furthermore, to check the bulk superconductivity, we performed tomeasure low-temperature dependences of magnetic susceptibilities χ ofthe 48-h and 72-h-reduced superconducting samples measured at 5 mT
under ZFC scan. Figure 3(b) exhibits diamagnetic signals below
Tc,on =22.5 K and 24.0 K for the 48-h-reduced and 72-h-reduced samples,
respectively. In addition, the superconducting volume fractions due tothe shielding effect are estimated to be 50 ∼ 55% from the ZFC values at
5 K, which are much higher than the previous data (∼ 30%) [12]. In theinset of Fig. 3(b), the magnified data are plotted to clarify the definitionof Tc,on . The characteristic values for the present and previous samples
are listed in Table 1.
Next, we try to measure the magneto-transport properties of the
reduced samples, to examine the magnetic effect on the superconducting
phase of the Pr247 compound. Figure 4(a) shows the low-temperature

Fig. 3. (color online)(a) Temperature dependences of electric resistivities of theas-sintered, 48-h-reduced and 72-h-reduced Pr2 Ba4 Cu7 O15− δ compounds. (b)low-temperature dependences of magnetic susceptibilities χ of the 48-h-reduced
and 72-h-reduced superconducting samples measured at 5 mT under ZFC scan.
In the inset, the magnified data are plotted to clarify the definition of Tc,on .

dependences of electric resistivities of the 48-h-reduced super­
conducting Pr2 Ba4 Cu7 O15− δ compound measured under zero field andapplied fields of 10 and 14T.
In zero field, the resistivity starts to decrease around Tc,on = 22 K,then follows a rapid drop, and finally achieves a zero-resistance state atTc,zero = ∼18 K. The observed transition width Tc,on = 22 K is relatively
sharp in comparison with the published data. Inset plots the magnetoresistance data of the 48-h-reduced sample at 4.2 and 30 K. At thehigh field of 10 T, we observed Tc,on = 18 K and Tc,zero = ∼5 K. Upon
increasing the applied field up to 14 T, the superconductive critical field
at 4.2K is determined to be 10.6 T. Even at the maximum field of 14 T,
we judge that the zero-resistance state is almost realized over the wholesample since the magnitudes of ρ are negligibly small with those of thenormal state at 30 K. These findings strongly suggest that for the present
sample the superconducting properties under relatively high fields are
considerably improved. For the previous Pr247 samples, it has beenreported that the critical field is as low as a few tesla at low tempera­
tures.
In Fig. 4(b), we show the low-temperature dependences of electric
resistivities of the 72-h-reduced superconducting Pr2 Ba4 Cu7 O15− δcompound measured at the several applied fields (H = 0, 1, 3, 6 and 9 T).
The inset displays the enlarged data of the 72-h-reduced sample aroundTc,on = 24.3 K. In a similar to the data of the 48-h-reduced sample, we
observed a zero-resistance state below Tc,zero = ∼18 K and 5 K, at 0 T andat 9 T, respectively. Here, we evaluate from the magneto-transport data

Fig. 2. (color online) SEM image for polycrystalline film prepared by an elec­
trophoretic deposition technique under 10 T.
3

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Table 1
Physical and superconducting properties for Pr2 Ba4 Cu7 O15− δ . In details, see the corresponding text and references. δ and RRR denote oxygen deficiency and residual
resistivity ratio ρ(300K)/ ρ(30K), respectively. fSC : the superconducting volume fraction estimated from the ZFC magnetization data at low temperatures. Hc∗ is definedfrom the field sweep data at fixed temperatures as the critical field where the zero-resistance state is violated with increasing the field.
Synthesis

Reduced time

δ

RRR

fSC

Tc,on

Tc,zeroH∗c

RH (30K)

(%)

(K)

(K)

(T)

(10− 3 cm3 /C)

citrate pyrolysis
citrate pyrolysis
citrate pyrolysis

48h
72h
48h

0.43
0.54
0.52a

10
12
4a

55
50
30a

22
24.3
26.5a

18
18
15a

10.6(4.2K)∼2(5K)-0.5-1.1b

sol-gel [22]

72h

0.94

2

∼10

30.5

∼10

-

-

high pressure [11]

48h

0.5

-

30

16

∼10

-

-1.5

a
b

see ref. [12]ref. [21]resistance state. The upper critical field at 0 K is estimated to be about
35 T from the data of the onset Tc using the Werthamer-HelfandHohenberg(WHH)
formula
in
the
dirty
limit,
μ0 Hc2 (0) = − 0.69Tc (dHc2 (T) /dT)|T=Tc [23].
There are quite differences in the critical fields determined fromTc,zero between the present and previous samples, strongly suggesting theenhanced superconducting properties in the presence of magnetic field
as represented by a right arrow in Fig. 5.
Furthermore, in Fig. 6, we show the temperature dependences of theHall coefficients RH for the as-sintered non-superconducting and 48-hreduced superconducting samples of Pr2 Ba4 Cu7 O15− δ . The RH data of
the as-sintered sample are very similar, in the both magnitude andtemperature dependence, to those of the previous sample. For the 48-hreduced sample, the RH data exhibit negative values in the limited
temperature range between 30 and 80 K, accompanied by electron
doping due to the reduced heat treatment in vacuum. Moreover, weestimate RH = − 0.5 × 10− 3 cm3 /C at 30 K, which is as about half as the
published data [21], indicating higher carrier concentration. These re­sults have a close relationship with the enhanced superconducting
properties, leading to the superconducting phase diagram of the present
samples.
Finally, the physical and superconducting properties for
Pr2 Ba4 Cu7 O15− δ compounds are summarized in Table 1. In the present

Fig. 4. (color online) (a) Low-temperature dependences of electric resistivitiesof the 48-h-reduced superconducting Pr2 Ba4 Cu7 O15− δ compound measured
under 0T, 10T, and 14T. Inset plots the magneto-resistance data of the 48-hreduced sample at 4.2 K and 30 K. (b) Low-temperature dependences of elec­
tric resistivities of the 72-h-reduced superconducting Pr2 Ba4 Cu7 O15− δ com­pound measured at the several applied fields (H = 0, 1, 3, 6 and 9 T). The inset
displays the enlarged data of the 72-h-reduced sample around Tc .

the temperature dependence of the superconductive critical field, toestablish the superconducting phase diagram. The onset Tc (Tc,on ) is
determined from the intersection between the two lines extrapolated
from the normal and superconducting transition resistivity data, justabove and bellow Tc , respectively. (see Fig. 4). The zero-point Tc (Tc,zero )denotes the value of the critical temperature reaching the zero-Fig. 5. (color online) Temperature dependence of the superconducting (SC)critical field of the reduced samples of Pr2 Ba4 Cu7 O15− δ . The onset Tc (Tc,on ) is
determined from the resistivity data as described in the text. For comparison,
the previous data of the reduced Pr247 are referred (open symbol). The solid
curve separating the superconducting and normal phases was the best fit to theexperimental data and the upper critical field of ∼35 T. Here, the corresponding
Hc2 (0) is estimated using the WHH formula in the text.
4

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Physica C: Superconductivity and its applications xxx (xxxx) xxx

establish the superconducting phase diagram. The upper critical mag­netic field was estimated to be about 35 T from the resistive transition
data using the WHH formula. For the 48-h-reduced sample, the RH data
exhibit negative values in the limited temperature range between 30 and
80 K, accompanied by electron doping due to the reduced heat treatment
in vacuum. Moreover, we estimated RH = − 0.5 × 10− 3 cm3 /C at 30 K,
which is as about half as the published data, indicative of higher carrier
concentration. In spite of limited heat-treatment conditions for thePr247 phase, we attained homogeneous distributions of the super­
conducting grains and improved weak links between their super­
conducting grains, leading to the enhanced superconducting properties
in the present samples.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Fig. 6. (color online) Temperature dependences of the Hall coefficients RH for
the as-sintered non-superconducting and 48-h-reduced superconducting sam­ples of Pr2 Ba4 Cu7 O15− δ .

Acknowledgement

citrate pyrolysis synthesis procedure, we adopted the electric tube
furnace with three zone temperature controlled system, to achieve thetemperature uniformity within 1 K. In spite of limited heat-treatment
conditions, we obtained homogeneous distributions of the super­
conducting grains and improved weak links between their super­
conducting grains, leading to the enhanced superconducting properties
in the present samples. The former is closely related to the narrowertransition width of superconductivity in zero magnetic field. The latter
improvement is mainly responsible for the considerable enhancement ofsuperconducting properties in high field. In addition, we give somecomments on the high-Tc samples synthesized by a sol-gel technique[22]. The transmission electron microscopy image of the high-Tc sample
with Tc,on =30.5 K revealed that there exists an irregular long-period
stacking structure along the c-axis such as {-D-D-D-S-S-S-D-D-S-S} se­quences [12]. Here, S and D denote CuO single-chain and double-chain
blocks along the b-axis. This finding is close to higher concentration,
resulting in higher Tc , in comparison to the regular stacking case of{-S-D-S-D-S-D}. The sol-gel sample showed highly inhomogeneous fea­tures such as the irregular stacking structures and the coexistence of thenominal Pr247 phase with the Pr123 and Pr124 phases. For the corre­
sponding inhomogeneous sample, we also observed a broad width be­
tween the onset and zero transition temperatures as listed in Table 1.

We would like to thank Mr. K. Sasaki for the SEM measurement, M.
Nakamura for his assistance in the PPMS experiments. M. M. thanks Dr.
F. Ishikawa for variable discussions. This work was supported in part by
MEXT Grands-in-Aid for Scientific Research (JPSJ KAKENHI Grants No.
JP19K04995).
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4. Summary
We have demonstrated the enhanced superconducting properties ofdouble-chain based superconductor Pr2 Ba4 Cu7 O15− δ synthesized by thecitrate pyrolysis technique using the 3-zone controlled electric furnace.
The outstanding experimental data include the higher residual re­sistivity ratios, the larger superconducting volume fractions, theenhanced zero-resistance state under magnetic field, and the higher
carrier concentrations.
The SEM image of the Pr247 film revealed that plate-like grains withsub micron size are homogeneously dispersed. For the polycrystalline
bulk samples, we obtained the higher residual resistivity ratio ranging
from 10 to 12 for the 48-72 h reduction. The reduction heat treatment on
the as-sintered sample in vacuum results in the appearance of super­
conducting state with Tc =22-24 K, accompanied by the metallic prop­
erties over a wide range of temperature. The superconducting volume
fractions are estimated from the ZFC data to be 50 ∼ 55%, indicating thebulk superconductivity. We evaluated from the magneto-transport data
the temperature dependence of the superconductive critical field, to5