J Supercond Nov Magn (2012) 25:2019–2023DOI 10.1007/s10948-012-1553-x

O R I G I NA L PA P E R

Effect of Cu Doping on the Crystal Structureof (Ru1−xCux)(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ

H.K. Lee · Y.I. Kim

Received: 17 February 2012 / Accepted: 26 March 2012 / Published online: 14 April 2012© Springer Science+Business Media, LLC 2012

Abstract The structural effects of Cu doping in (Ru1−xCux)(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ (0 ≤ x ≤ 0.5) havebeen investigated by using X-ray and neutron diffraction.X-ray diffraction showed that the phase purity of the sam-ples increased as the Cu-doping concentration increased andnearly-single-phase materials could be obtained for sam-ples with x ≥ 0.25. The Rietveld refinements of the neutrondiffraction data around 5 K for x = 0.25 and x = 0.5 sam-ples show that the oxygen vacancies are located on boththe O(1) site of the RuO2 basal plane and the O(4) site ofthe fluorite-type (Nd,Ce)O2−δ block, and the overall oxygencontent is found to decrease with increasing Cu content x,mainly due to an increase in oxygen vacancies located onthe O(1) site. It is also observed that the rotation angle ofthe RuO6 octahedra away from the crystallographic c-axisincreases as the Cu content x increases.

Keywords (Ru, Cu)-1222 · Structure · Neutrondiffraction · Oxygen vacancy

1 Introduction

Layered ruthenocuprates, both RuSr2R2−xCexCu2O10−δ

(Ru-1222) [1] and RuSr2RCu2O8 (Ru-1212) [2] (R standsfor a rare earth, typically Gd, Eu or Sm) compounds, have

H.K. Lee (�)Department of Physics, Kangwon National University,Chuncheon 200-701, Republic of Koreae-mail: hklee221@kangwon.ac.kr

Y.I. KimKorea Research Institute of Standards and Science,Daejeon 305-340, Republic of Korea

been extensively investigated due to the observation of co-existing superconductivity and weak ferromagnetism. Weakferromagnetism was observed in the ruthenate layer be-low Tm = 125–180 K and superconductivity in the cop-per oxide planes below Tc ∼ 50 K, depending on the rare-earth element and oxygen concentration. Neutron diffrac-tion measurements on Ru-1212 compounds have shownthat the magnetic order is antiferromagnetic with a smallferromagnetic component [3]. This ferromagnetic compo-nent of the Ru-1212 compound is considered to origi-nate from the canting of the Ru moment [4]. This cant-ing is caused by rotation of the RuO6 octahedra [5]. How-ever, detailed neutron diffraction experiments for the Ru-1222 system have been hampered not only because mostruthenocuprates [6] prepared under ambient pressure arestabilized by the mid-series rare earths Gd, Eu, and Sm,which have a very large neutron absorption cross-section,but also because preparation of the phase-pure Ru-1222compound is relatively difficult compared to that of theRu-1212 compound [7, 8]. In these respects, synthesis ofNd–based Ru-1222 samples is very promising because ofthe low neutron absorption cross section of Nd. However,few structural studies [9, 10] have been reported for theNd-based Ru-1222 compounds, mainly because prepara-tion of phase-pure Nd-based Ru-1222 compounds is muchmore difficult than for those of R = Gd, Eu, or Sm. Re-cently, it has been found that nearly-single-phase sam-ples can be obtained in the nominal compositions of the(Ru0.5Cu0.5)(Sr1.67−xBaxNd0.33)(Nd1.34Ce0.66)Cu2O10−δ

system [11]. To address the crystal structure of Nd-basedRu-1222 compounds and the role of Cu doping, polycrys-talline samples with compositions of (Ru1−xCux )(Sr1.47

Ba0.2Nd0.33)(NdCe) Cu2O10−δ were prepared and exam-ined by using X-ray and neutron diffraction.

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2 Experiments

The investigated samples with nominal compositions of(Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ were pre-pared by a solid state reaction from starting powdersRuO2, CuO, SrCO3, BaCO3, Nd2O3 and CeO2 with pu-rity above 99.9 %. The procedure of heat treatments wasthe same as that applied earlier for preparing phase-pure(Ru0.5Cu0.5)(Sr1.67−xBaxNd0.33)(Nd1.34Ce0.66)Cu2O10−δ

samples [11]. X-ray diffraction (XRD) was used to examinethe phase purity of the samples by using a Bruker D5005powder diffractometer and Cu Kα radiation at room tem-perature. Measurement of dc magnetic susceptibility wascarried out using a Quantum Design SQUID magnetometer.

Neutron powder-diffraction data were taken with a high-resolution powder diffractometer at the research reactorHigh-flux Advanced Neutron Application Reactor (HA-NARO) (30 MWth) at the Korea Atomic Energy ResearchInstitute. The white beam of neutrons from the reactor wasmonochromatized by using a Ge(331) monochromator to awavelength of 1.8367 Å. The sample weighed ∼4 g and wassealed in a thin-walled vanadium can with helium exchangegas to ensure homogeneous cooling. Diffraction data werecollected at a low temperature of ∼5 K by using 32 3Hecounters and refined using a Rietveld program RIETAN [12]and a pseudo-Voigt profile function. The intensity data in the2θ range from 15° to 150° with steps of 0.05° were used inthe refinements. The refinements were performed accordingto the following steps. The first step involved the refine-ment of the scale factor, background parameters, zero pointshift, cell parameters and profile parameters including peakshape, half width, and asymmetry parameter. In the secondstep, the atomic position parameters, the site occupanciesand the isotropic parameters were refined step-by-step foreach atom. In the last step, all the parameters were refinedsimultaneously. The coherent scattering lengths, b, used forthe Rietveld refinements were 7.030 fm (Ru), 7.020 fm (Sr),5.070 fm (Ba), 7.690 fm (Nd), 4.840 fm (Ce), 5.803 fm (O)and 7.718 fm (Cu).

3 Results and Discussion

Figure 1 shows the XRD patterns for (Ru1−xCux )(Sr1.47

Ba0.2Nd0.33)(NdCe)Cu2O10−δ with x = 0,0.1,0.2,0.25,

0.33 and 0.5, respectively. It can be seen that the x = 0 sam-ple contains a slight trace of impurity phases, as markedwith asterisks. The phase purity of the samples improvedwith increasing x and the samples with 0.25 ≤ x ≤ 0.5 ex-hibit a single-phase nature. Based on this result, we sep-arately prepared two samples with nominal compositionsof (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ with

Fig. 1 Powder XRD patterns for (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ (x = 0 ∼ 0.5) samples. Peaks due to impurity phasesare marked with asterisks

x = 0.25 and x = 0.5 for neutron diffraction measure-ments. Figure 2 shows the magnetization versus tempera-ture behavior in the temperature range of 5 ∼ 150 K for thex = 0.25 and x = 0.5 samples in an applied field of 20 Oe,measured in both zero-field-cooled (ZFC) and field-cooled(FC) modes. Figure 2 also shows the susceptibility data forthe x = 0.25 sample, which was annealed at 400 °C for24 hours in N2 and subsequently cooled to room temper-ature. The ZFC and FC parts of susceptibility for the as-prepared x = 0.25 sample start to branch up around 25 K.Interestingly, the branching of the ZFC and FC magnetiza-tion becomes negligible for the N2-treated x = 0.25 samplein the entire measuring temperature range. A similar absenceof branching between ZFC and FC magnetization is also ob-served for the as-prepared x = 0.5 sample, as shown in theinset of Fig. 2, and the magnetization curve shows a param-agnetic behavior in the entire range from 150 K down to 5 K.The collapse of the ZFC and FC magnetization curves to asingle paramagnetic-like behavior suggests the absence offerromagnetic-like ordering in the present samples. The N2-treated x = 0.25 sample and the as-prepared x = 0.5 samplewere used in the neutron diffraction analysis.

The structure refinements of the two samples were car-ried out using an initial model based on the tetragonalI4/mmm (No. 139) space group and the crystal structureof RuSr2(Gd1.3Ce0.7)Cu2Oz determined by Knee et al. [13]from neutron powder diffraction data. Atoms occupying the

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Fig. 2 Temperature dependences of the zero-field-cooled (ZFC)and field-cooled (FC) magnetization (M) for (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ (x = 0.25 and 0.5) sam-ples measured at 20 Oe. The magnetization data for the x = 0.25sample, which was annealed at 400 °C for 24 hours in N2 andsubsequently cooled to room temperature, are also shown in the figure

same position were constrained to have the same thermalparameter and the sum of their occupancy factors was con-strained to be equal to 1. Since Ru and Cu, Sr and Nd haveclose scattering cross sections for neutrons, their site oc-cupancies were fixed at those of the nominal composition.The coordination of Ru is sixfold, with four in-plane oxygenatoms O(1) and two apical oxygen atoms O(2). In the courseof refinements, a large thermal parameter of ∼9 Å2 for O(1)was observed for the site with coordinates 4c (0,1/2,0) inthe I4/mmm symmetry. A displacement of the O(1) sitefrom 4c (0,1/2,0) to 8j (x,1/2,0), which suggests the pres-ence of disordered rotation of the RuO6 octahedron, wasthus modeled. We could also obtain an improved fit to theneutron diffraction data when a displacement of the apicaloxygen O(2) site from (0,0, z) to (x,0, z) was allowed. Thisindicates that there is an appreciable tilt of the RuO6 octa-hedra, as reported in Refs. [9, 10, 13]. The refined atomicpositions are 4c (0,0, z) for Cu, 4e (1/2,1/2, z) for Nd/Ceand Sr/Ba/Nd(1), 2a (0,0,0) for Ru/Cu(1), 8j (x,1/2,0)for O(1), 16n (x,0, z) for O(2), 8g (0,1/2, z) for O(3), and4d (0,1/2,1/4) for O(4). The neutron diffraction data andRietveld refinement profile for the x = 0.5 sample at 5 K areshown in Fig. 3. The final R factors, including the weightedpattern R factor (Rwp) and the pattern R factor (Rp), as wellas the atomic coordinates, the isotropic thermal parameters(B) and the occupancies for all samples are listed in Table 1.

Magnetic ordering temperatures of Ru-1222 compoundsare known to depend upon both the R/Ce ratio and the oxy-gen content [14]. However, the locations of the oxygen va-cancies reported in the literature appear to be controver-

Fig. 3 Neutron diffraction data and Rietveld refinement pattern forthe (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ (x = 0.5) sam-ple. The observed intensities are shown by circles and the calculatedones by the solid line. The positions of the Bragg reflections are shownby the small vertical lines below the pattern. The lines at the bottom in-dicate the intensity difference between the experimental and the refinedpattern

sial. Knee et al. [13] suggested that oxygen vacancies inRuSr2(Gd1.3Ce0.7)Cu2O10−δ were located on both the O(4)site within the Gd2−xCexO2−δ block and the O(1) site ofthe RuO2 basal plane. Lynn et al. [7] reported that oxy-gen vacancies originated from oxygen sites [O(1) and O(2)]surrounding the Ru ion in RuSr2(Eu1.2Ce0.8)Cu2O10−δ . Onthe other hand, Kuz’micheva et al. [9] reported that oxy-gen vacancies were located on the apical oxygen site O(2)in RuSr2(Nd,Ce)Cu2O10−δ . Table 1 shows that the oxygenvacancies are located on both the O(4) site of the fluorite-type (Nd,Ce)O2−δ block and the disordered (Ru,Cu)O2−δ

basal site O(1) in consistency with the report of Knee etal. [13]. The overall oxygen content, as determined fromoxygen occupancies, was 9.6(1) for the x = 0.25 sampleand 9.2(2) for the x = 0.5 sample. Considering the factthat the x = 0.25 sample was annealed in N2 atmosphereat 400 °C, the results in Table 1 indicate that the overalloxygen content decreases as the Cu content x increasesfrom x = 0.25 to 0.5 and the decrease of the oxygen con-tent is mainly caused by the decrease in oxygen contentlocated on the O(1) site of the RuO2 basal plane. Table 1also shows that in contrast to the negligible change of thetilt angle, the rotation angle of the RuO6 octahedra awayfrom the crystallographic c-axis increases as the Cu content

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Table 1 Refined structural parameters of (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe) Cu2O10−δ with x = 0.25 at 6 K and x = 0.5 at 5 K (upper and,where different, lower values, respectively). The space group used is I4/mmm

Sample a (Å) c (Å) Rwp(%)

Rp (%) Rotationangle (°) ofRuO6

Tilt angle (°) ofRuO6

x = 0.25 3.86414(8) 28.5654(7) 6.00 4.47 12.98 6.79

x = 0.50 3.85760(8) 28.5514(7) 7.10 5.17 13.54 6.83

Atom Position x y z B (Å2) Occupancy

Nd/Ce 4e 1/2 1/2 0.20438(8)0.20443(9)

0.45(8)0.35(9)

0.58(2)/0.42(2)0.55(3)/0.45(3)

Sr/Ba/Nd(1) 4e 1/2 1/2 0.07673(8)0.07686(9)

0.70(5)0.95(6)

0.735/0.1/0.165

Cu 4c 0 0 0.14303(7)0.14243(8)

0.45(4)0.55(5)

1.0

Ru/Cu(1) 2a 0 0 0 0.93(6)1.63(8)

0.75/0.250.5/0.5

O(1) 8j 0.1153(10)0.1204(15)

1/2 0 2.2(2)1.9(3)

0.43(1)0.33(1)

O(2) 16n 0.0582(24)0.0580(29)

0 0.06613(9)0.06549(11)

0.8(2)1.2(2)

0.25

O(3) 8g 0 1/2 0.14901(6)0.14887(7)

0.73(4)0.69(5)

1.0

O(4) 4d 0 1/2 1/41/4

0.38(8)0.39(10)

0.95(1)0.95(1)

x increases. In addition, it is notable that our recent studyon the Rietveld refinement of the X-ray diffraction data fora (Ru0.5Cu0.5)(Sr1.47Ba0.2Nd0.33)(Nd1.34Ce0.66)Cu2O10−δ

sample [15] showed that the rotation angle of RuO6 octa-hedra increases after the same N2 treatment as mentionedabove. Consequently, the results in Table 1 imply that therotation angle of the RuO6 octahedra away from the crystal-lographic c-axis increases as the Cu content x increases.

The weak ferromagnetism observed in the Ru-1222 sys-tem was previously proposed to originate from cantingof the Ru moment [1]. However, we did not find anyferromagnetic-like magnetic ordering of Ru moments in thex = 0.25 and x = 0.5 samples in both magnetization andneutron diffraction measurements. Kuz’micheva et al. alsoreported the absence of magnetic ordering of the Ru mo-ments in RuSr2(Nd,Ce)2Cu2O10−δ phases. Therefore, it isnot clear whether the absence of ferromagnetic-like mag-netic ordering of Ru moments in the present samples is dueto either an intrinsic property of the Nd-based Ru-1222 com-pound or the Cu substitution effects accompanying an in-crease in the oxygen vacancies and dilution of Ru which canbreak the correlation of the rotated RuO6 octahedra. Fur-ther detailed structural studies on samples with reduced Cu-doping content will be helpful to clarify the reason for theabsence of ferromagnetic-like ordering in the Nd-based Ru-1222 system.

4 Conclusion

We have investigated the effect of Cu substitution for Ruon phase formation and structure of samples prepared withnominal composition of (Ru1−xCux )(Sr1.47Ba0.2Nd0.33)(NdCe)Cu2O10−δ (x = 0 ∼ 0.5). In contrast to the Cu-freesample, the phase purity of the samples increased as theCu content increased and single-phase samples could beobtained for the samples from x = 0.25 to x = 0.5. Thestructural parameters refined by the Rietveld method forthe x = 0.25 and x = 0.5 samples around 5 K showed thatthe oxygen vacancies were located on both the O(1) site ofthe RuO2 basal plane and the O(4) site of the fluorite-type(Nd,Ce)O2−δ block, and the overall oxygen content is foundto decrease with increasing Cu content x from x = 0.25 tox = 0.5, mainly caused by the increase in oxygen vacancieslocated on the O(1) site. It is also observed that the rotationangle of the RuO6 octahedra away from the crystallographicc-axis increases as the Cu content x increases. We did notfind any ferromagnetic-like ordering of Ru moments around5 K in both the x = 0.25 and x = 0.5 samples.

Acknowledgements This work was supported by Korea ResearchFoundation Grant (KRF-2009-0075747). We express our thanks to theKorea Basic Science Institute for help with SQUID measurements andto the central laboratory of Kangwon National University for help withXRD measurements. The authors wish to thank Prof. C. Wolfe for hisvaluable comments on the manuscript.

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