Suppression of superconductivity in the Y1−xPrxBa2Cu2.95Li0.05O7-δ system

~ ~

El..')EVIER

PHYSICA ® Physica C 282-287 (1997) 757-758

Suppression of Superconductivity in the Yl-xPrxBa1Cul.9sl.io.oS07-S System Hongqing Song, Guanghan Cao, Yabin Yu, Zhengkuan Jiao , Qirui Zhang Department of Physics, Zhejiang University, Hangzhou 310027, P.RChina Yl-xPrxBa2Cu2.9SLio.oS07-S and Yl-xPrxBa2Cu307-S samples were prepared and studied. It is found that the normalized Tc (Tctrc(max» ofYl-xPrxBa2Cu2.9sLio.oS07-S is higher than that ofYl_xPrxBa2Cu307-S for a fixed Pr content, indicating that the effect of the superconductivity suppression by Pr doping becomes weaker in the Li-doped system. Considering the Li substitution effect on the structural details, we explain the results in terms ofPr4f-02p hybridization.

1. INTRODUCTION

structure within the experimental error. The XRD reflections can be indexed with the "123" structure.

The Yl_xPrxBa2Cu307-S system has attracted much attention [1,2). Previous study [3] on Li

The Tc for the Yl_xPrxBa2Cu307-S sample is very close to previous report [2).

substitution for Cu in YBa2CU3_yLiy07-S showed that small amount of Li substitution leads to increasing the angles of Cu(2)-O(2,3)-Cu(2) and the average Y -0 bond length. It has been pointed out [4,5] that the absence of superconductivity in PrBa2Cu307-S is due to the Pr4f-02p hybridization. Considering that small amount of Li substitution may change the effect of Tc depression by Pr doping, it is meaningful to study on the superconductivity in Yl_xPrxBa2CU3_yLiy07-S (y=O, 0.05).

Fig. 1

shows

that

Tc

of

Yl-xPrxBa2Cu2.9sLio.oS07-S decreases monotonically with increasing Pr content and superconductivity is completely,suppressed at x=O.6. It is also noted that the room-temperature resistivity of the samples increases with the Pr content, and a metal-to-semiconductor transition takes place at x=O.5. 5,,-------------,

Y PrBaCu

2. EXPERIMENTAL

the

1-x

4

"

2

Li

0

2.S5 0Jl5 7-1

x-o.eo

The samples were prepared by a solid state reaction method. They are calcined at 1203K for

x-O.55

I

20h in air and annealed at 773K for 20h under flowing oxygen. The resistivity of the samples was measured by the standard four-probe method. The oxygen content was determined by iodometry. In all

x-O.5O x-O.4D x"O.20 X=

the samples, the oxygen content was in the range of 6.92-6.95, independent of x and y.

50

100

150

200

250

300

Temprature (1<)

3. RESULTS AND DISCUSSION

Figure 1. R-T curves for Yl-xPrxBa2Cu2.9sLio.oS07-S

X-ray powder diffraction shows that each sample is a single phase of the orthorhombic

We use a normalized Tc (Tctrc(max» to compare these two systems, which is shown in

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758

H. Song et al.lPhysica C 282-287 (1997) 757-758

Fig.2. It can be found that the normalized Tc of YI_xPrxBa2Cu29SLioos07-1l is higher than that of Y I-x PrxBa2Cu307-1l for a fixed Pr content, implying that the effect of Tc depression by Pr doping becomes weaker in the Li-doped system. 1~----------------------~ Y1...PrBaCu UGIll0 7.. I I _

shifts from the CU02 plane to the chains. According to the structural data, we calculated the average bond length of Y -0(2) and Y -0(3) in YBa2Cu3_xLix07-1l, and we found that the average Y-O bond length increases from 2.392A at x=o to 2.39SA at x=O.05. Because of the small amount of Li doping (only 1.67%) and the similar structure, it is reasonable to think that the effect of small amount of Li substitution in YI-xPrxBa2Cu2.9sLio.oS07-1l is similar to that in YBa2Cu3_xLix07_8. So we can explain the of the superconductivity in change

Y PrBaCuO

I-x

a

x

2

3

7-1

~~~~~~~~~~~~

a

0.1

0.2

0.3

0.4

0.5

0.6

Pr content x

Figure 2. Normalized Tc (T/fc(max» versus Pr content x of YI-xPrxBa2Cu2.9sLioos07-1i and YI_xPrxBa2Cu307_1i samples (here Tc refers to Tc(mid». The above result suggests that Li substitutes for of Cu and enters into the lattice YI_xPrxBa2Cu2.9SLiooS07-1l. There are several reasons. First, the ionic radii of Cu2+ and Lt is similar (six coordinate: 0.73A versus 0.76A; four coordinate: 0.57 A versus 0.59A, respectively) [6]. Second, the work of Feduzi et al.[7] proved that small amount of Li can substitute for Cu in YBa2Cu3_xLix07-1l without much loss of Li in the process of the solid state reaction. Finally, there is an obvious change of the superconductivity. The neutron powder diffraction study on YBa2(Cul_xLixh07-1l [3] points out that when Li doping is less than 5%, Li substitutes for Cu at the chain site, which causes the following results: (1) 0(4) shifts to the chains; (2) the angles of Cu(2)-O(2,3)-Cu(2) becomes larger; (3) charge

YI_xPrxBa2Cu2.9SLioos07-1l: Li ions substitute for Cu ions and enter into the chain site, which causes that the angles of Cu(2)-O(2,3)-Cu(2) and the average Pr-O bond length become larger. Therefore, the Pr4f-02p hybridization becomes weaker, and the effect of the superconductivity suppression by Pr doping decreases.

4. CONCLUSIONS In summary, Li ions can substitute for Cu ions of and enter into the lattice YI-xPrxBa2Cu2.9SLio.oS07-8, and this substitution causes that the effect of Tc depression by Pr doping becomes weaker. Our study supports that Pr4f-02p hybridization is responsible for the superconductivity suppression by Pr doping. REFERENCES 1.

H. B. Radousky, J.Mater.Res., 7 (1992) 1915.

2.

M. B. Maple et al., J.Alloys Compounds, lSI (1992) 135. George H. Kwei et al., Physica C, 194 (1992) 307.

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X. X. Tang et al., Physica C, 161 (19S9) 574. R. Fehrenbacher and T. M. Rice,

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R. D.Shannon, Acta Cryst. A, 32 (1976) 751. R. Feduzi et al., Physica C, 195 (1992) ISO.

Phys.Rev.Lett., 70 (1993) 3471.