X-ray diffraction study on YBa2Cu3O7−� with BaCuO2 additionSheng Xua�

Department of Physics, Nanjing University, Nanjing 210093, China and Zhangjiagang Campus, JiangsuUniversity of Science and Technology, Zhangjiagang 215600, China

Xiaoshan WuDepartment of Physics, Nanjing University, Nanjing 210093, China

Yanni GuZhangjiagang Campus, Jiangsu University of Science and Technology, Zhangjiagang 215600, China

�Received 9 July 2010; accepted 13 July 2010�

Two-phase polycrystalline powder mixtures of YBa2Cu3O7−� �YBCO� and xBaCuO2 �x=0, 0.05,0.1, 0.2, and 0.3� were prepared by solid reaction. The Rietveld refinements of X-ray powderdiffraction data show that BaCuO2 addition was successfully produced in superconductor YBCOand the unit-cell parameters of YBCO reach a maximum at x=0.05. The critical current density �Jc�also reaches a maximum at x=0.05 and then decreases sharply with increasing amount of BaCuO2.The change of Jc as a function of x was found to be similar to those of the unit-cell parameters. Thecharacteristic behavior of Jc may come from the structure-parameter change of YBCO caused byBaCuO2 addition. © 2010 International Centre for Diffraction Data. �DOI: 10.1154/1.3478854�

Key words: YBa2Cu3O7�� with BaCuO2 addition, X-ray diffraction, flux pining

I. INTRODUCTION

Among bulk superconductors, Y-123 type cuprates arethe most promising candidates for practical applications. Forthe purpose of promise of these applications, it is most im-portant to improve the pinning properties of these materials.By incorporating high-density extended defects to act as pin-ning centers for the magnetic flux lines, the vortex pinningforces can be enhanced.

Recently, there are many reports indicating that the in-corporation of nanosized particles or defects enhances thepinning force for YBa2Cu3O7−� �YBCO� �MacManus-Driscoll et al., 2004; Haugan et al., 2004; Wang et al., 2006�.MacManus-Driscoll et al. �2004� also reported that BaZrO3nanoparticles in YBCO films have improved the in-magnetic-field current densities. Wang et al. �2006� producedY2O3 nanoparticles in YBCO to improve the in-field Jc per-formances.

Besides the above-mentioned secondary phase of Y2O3,the addition of BaCuO2 can be another promising possibilityfor improving vortex pinning in YBCO. In the present study,we reported the X-ray diffraction �XRD� results on XRD andmagnetic measurement of Jc of YBCO and xBaCuO2 withx=0 to 0.3 prepared by solid-state reaction.

II. EXPERIMENTAL

Two-phase powder mixtures of YBCO and xBaCuO2with x=0, 0.1, 0.2, and 0.3 were prepared by solid-state re-action �Xu et al., 2004; Wu et al., 2004�. The structures ofthe samples were characterized at room temperature by XRDusing Cu Ka radiation. XRD experiments were performed atroom temperature using a Rigaku 12 kW Dmax-rB X-raypowder diffractometer with a rotating anode Cu K� X-ray

a�Author to whom correspondence should be addressed. Electronic mail:

xsheng1258@sina.com

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source in the Bragg-Brentano parafocusing geometry. XRDdata were collected from 20° to 90°2� by step scanning witha step size of 0.02° and a counting time of 2 s per step. Thewavelengths of Cu K�1 and Cu K�2 were chosen to be1.5406 and 1.5444 Å, respectively, with the intensity ratio of0.497.

The values of Jc were determined from the magnetiza-tion curves measured using a quantum design superconduct-ing quantum interference device magnetometer. According tothe Bean critical state model �Bean, 1962�, Jc is written asJc=�Mr / �2ag /3�, where ag is the average grain radius,which is set to be 4.9 �m �Harada and Yoshida, 2002�, and�Mr is the difference between the upper and lower branchesof a M�H� loop �Harada and Yoshida, 2002�.

III. RESULTS AND DISCUSSION

Typical X-ray diffraction patterns and the Rietveld-refinement curves for two-phase mixtures of YBCO andxBaCuO2 with x=0 and 0.3 are shown in Figure 1. TheX-ray diffraction data were analyzed in orthorhombic Pmmmspace group by the Rietveld structure refinement techniqueto obtain their structure parameters using the GSAS FORTRAN

program. Each diffraction profile was assumed to fit thepsedo-Voigt function. No absorption was taken into accountduring the refinements. Isotropic thermal parameters wereallowed to be modulated to reasonable values. At the initialstage of a refinement, full occupancy was assumed for allatoms except the partially occupied oxygen atoms located atthe positions O �1e� �0, 1/2, 0� and O �1b� �1/2, 0, 0�. Thetotal contents of oxygen atoms located at O �1e� and O �1b�were as fixed as 0.93, according the results reported by Wu etal. �1996�. The YBCO phase was first refined and then theBaCuO2 phase. All refined Rwp factors are between 7.9% and12.51%, which are reasonable to support the refined structuremodel. The Rietveld-refinement results show that BaCuO2

addition has successfully grown in YBCO superconductor.

S52156/2010/25�S1�/S52/3/$30.00 © 2010 JCPDS-ICDD

YBCO in all samples has the layered orthorhombic perov-skite structure with Pmmm symmetry. A summary of therefined unit cell and atomic parameters for two-phase mix-tures of YBCO and xBaCuO2 with x=0, 0.05, 0.1, 0.2, and0.3 is listed in Table I.

The variations of unit-cell parameters of YBCO vs theBaCuO2 content x are plotted in Figure 2. The unit-cell pa-rameters along the a, b, and c axes and the unit-cell volumeV of YBCO initially increase with increasing x from 0 to0.05, then decrease after x=0.05, and finally increase afterx=0.1. The unit-cell parameters and the unit-cell volume Veach has a maximum at x=0.05, suggesting maximal tensilestrains in YBCO crystalline grains of the x=0.05 sample.

Figure 3 shows the magnetic field dependence of Jc mea-sured at 65 K for samples with different values of x. For allsamples, Jc decreases monotonically with the increasingmagnetic field, and a shoulderlike feature appears in each

TABLE I. The atomic parameters for two-phase samples of YBCO and xB

xSpace group

Unit-cell parameters

Pmmma=3.824 81�4� Åc=11.663 38�3� Å

Atom Position x

Y 1h 1/2Ba 2t 1/2Cu 1a 0Cu 2q 0O 2q 0O 2s 1/2O 2r 0O 1e 0O 1b 1/2R values Rp=7.88%

Pmmm

0.1Z=1

Pmmma=3.816 66�2� Å b=3.886 06�2� Å a=3.824 41�5� Åc=11.662 06�2� Å V=172.969�1� Å3 c=11.674 81�6� Åx y z x

0.5 1/2 1/2 1/20.5 1/2 0.1867�9� 1/20 0 0 00 0 0.3390�1� 00 0 0.1855�4� 00.5 0 0.3549�6� 1/20 1/2 0.3637�5� 00 1/2 0 00.5 0 0 1/2Rp=6% Rwp=7.9% Rp=7.97%

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curve. This feature may indicate the effect of magnetic fieldon the pinning centers. It should be noted that the sample ofx=0.05 has the highest values of Jc under all magnetic fields.

To investigate these features further, we also plotted thedata of Jc at T=65 K as a function of x for four differentvalues of magnetic field �see Figure 4�. For each of the fourJc curves shown in Figure 4, the value Jc reaches a maximumat x=0.05 and then decreases sharply with increasing x.

The characteristic behavior of Jc may come from thechanges in unit-cell parameters of YBCO caused by BaCuO2

addition, as the changes of Jc were found to be similar tothose of the unit-cell parameters a, b, and c and unit-cellvolume V. The value of Jc and unit-cell parameters reach amaximum at x=0.05. The sample with x=0.05 may inducemaximal tensile strains in YBCO crystalline grains. There-fore, x=0.05 is the most effective concentration to obtain a

Figure 1. The X-ray diffraction patterns and refinedcurves for two-phase mixtures of YBCO and xBaCuO2:�a� x=0 and �b� x=0.3. Open circles are the measuredX-ray diffraction data, the solid lines denote the calcu-lated profiles, and the vertical short lines � � � representthe positions of the Bragg reflections. The differencecurves were plotted at the bottom.

2 �x=0, 0.05, 0.1, 0.2, and 0.3� determined by Rietveld refinement.

b=3.880 68�1� ÅV=173.118�2� Å3

Pmmma=3.827 95�3� Åc=11.695 71�5� Å

0.05Z=1

b=3.890 23�4� ÅV=174.168�3� Å3

z x y z

1/2 1/2 1/2 1/20.1871�7� 1/2 1/2 0.1786�9�

0 0 0 00.3594�8� 0 0 0.3592�8�0.1608�6� 0 0 0.1841�6�0.3791�5� 1/2 0 0.3635�9�0.3740�8� 0 1/2 0.3892�4�

0 0 1/2 00 1/2 0 0

Rwp=11.79% Rp=6.39% Rwp=8.12%

Pmmm

0.3Z=1

b=3.880 99�7� Å a=3.831 31�5� Å b=3.892 14�5� ÅV=173.283�9� Å3 c=11.693 58�8� Å V=174.375�1� Å3

z x y z

1/2 1/2 1/2 1/20.1807�6� 1/2 1/2 0.1846�4�

0 0 0 00.3418�1� 0 0 0.3424�4�0.1513�1� 0 0 0.1712�3�0.3580�7� 1/2 0 0.3610�1�0.3586�3� 0 1/2 0.3780�8�

0 0 1/2 00 1/2 0 0

Rwp=10.27% Rp=9.1% Rwp=12.51%

aCuO

0Z=1

y

1/21/20000

1/21/20

0.2Z=1

y

1/21/20000

1/21/20

S53-ray diffraction study on YBa2Cu3O7−� with BaCuO2 addition

maximum value of Jc in a magnetic field. The changes of theunit-cell parameters due to BaCuO2 addition have importanteffect on the magnetic property of YBCO.

IV. CONCLUSION

We have reported on XRD and Jc of the two-phase mix-tures of YBCO and xBaCuO2. Our results clearly show thevalue of Jc by BaCuO2 addition in magnetic field measuredat 65 K. Jc reaches a maximum at x=0.05 and then decreaseswith increasing BaCuO2 content x. The change of Jc corre-lates with those of the unit-cell parameters a, b, c, and V.

Figure 2. Plots of unit-cell parameters a, b, c /3, and V of YBCO vs BaCuO2

content x.

Figure 3. Field dependences of Jc at 65 K for two-phase mixtures of YBCOand xBaCuO .

2

S54 Powder Diffr. Suppl., Vol. 25, No. S1, September 2010

The characteristic behavior of Jc may come from the changesin the unit-cell parameters of YBCO caused by BaCuO2 ad-dition.

ACKNOWLEDGMENTS

This work has been supported by the NNSFC �Grant No.10974081�. The authors would like to thank the financialaids from NCET-04-0463, NKPBRC �Grant No.2006CB921802�, and JSKJDXBSJJ �Grant No. 35270802�.

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Figure 4. Plots of Jc at 65 K vs BaCuO2 content x in applied magnetic fields.

S54Xu, Wu, and Gu