Structural transition of La.Ba.Cu.O doped with iron

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PHYSICA ® Physica C 282-287 (1997) 753-754

Structural transition of La.Ba.Cu.O doped with iron W. M. Chen·· b, J. Chen', N. L. Chen a.b, X. Zhang", X. S. Wu·, X. Jin", and S. S. Jiang! IOepartment of Physics and National Laboratory of Solid State Microstructure, Analysis, Nanjing University, Nanjing 210093, P. R. China

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Crystal structures of LaBa2CuJ.xFe.Oy with 0.0 ~ x ~ 0.25 show an anomalous structural phase transition depending on Fe contents. With Fe content increasing the structure varies in the form of orthorhombic-totetragonal phases and then regeneration of orthorhombicity from tetragonal phase,which is observed from the separation or overlap of (0 2 0) and (2 0 0) lines in the X-ray diffractograms. Variation of lattice constants a, band c with Fe content also supports by the anomalous phase transition.

l. INTRODUCTION

3. RESULTS AND DISCUSSION

In order to investigate the superconductivity of YBa2Cu30y(YBCO), it is admitted that the method of iron's substituting for copper is useful [1]. Although compound YBCO is isostumtured to LaBa2Cu3 .• Fe.Oy (LBCO), behaviour of ferric ion in LBCO and YBCO is different. For exmloe, iron as it substitutes for copper in YBCO occupies the Cu (I) sites instead of Cu (2). Whereas, iron replacing copper in LBCO prefers to occupy Cu (2) sites [2]. For Fe substituted LBCO, some problems relating to properties and structure need te be investigated further. We studied them and report the results below.

The variations of oxygen content(y) and superconducting transition temperature (Tc) vs iron content (x) were shown in Fig. lA and Fig.1 B, respectively. In Fig. I A, with x increasing y decreases slightly first and then increases significantly. Fig.1 B shows that Tc of Fe doped LBCO varies inversely as iron content x. Tc with zero resistivity is 70.34 K. At x = 0.03 and 0.06, Tc are 55.23 K and 35.90 K, respectively. For x = 0.10, its Tc is lower than 30 K. The ten perature den pence of electrical resistivities for samples with x > 0.15 semiconducting behavior.

2. EXPERIMENT

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The perparation of LBCO with 0.0 ~ x ~ 0.25 was described in ReD. Copper valence, 2+p, and oxygen content were determined by an improved double iodometric titration, including experiment I and 2 [4,5]. An improved calculating equation [6] was employed to calculate copper valence p. Their structures were characterized by powder Xray diffraction analysis as orthorhombic or tetragonal phases. Measurements of electrical resistivity were carried out on samples with different iron content, employing a four end leads method from room temperature down to 30 K. 0921-4534/97/$17.00 © Elsevier Science B.V. All rights reserved. PH S0921-4534(97)00391-2

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W.M. Chen et al.lPhysica C 282-287 (1997) 753-754

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Rg. 2. An anomalous structrual transitions of Fe doped lBCO depend on Fe contert, which Is in the form of orthorhombio-to-tetragonal and tetragonaH<>-OrthorhombIc phases. It Is observed from the separation or overlap of (020) and (200) Moes In theX-ray clffraction patterns.

X-ray diffraction patterns are put in Fig. 2. As Fe content increasing, the structure varies from orthorhombic phase to tetragonal and again a tetragonal-to-orthorhombic structural transition occurs. Behaviour of structural transitions can be observed in Fig. 2, by analysing the separation or overlap of the (0 2 0) and (2 0 0) lines in the Xray diffractograms of the Fe doped samples. Samples with small Fe contents, in the region of 0.00 :s; x :s; 0.03, is of a orthorhombic phase; in the region of 0.06 :s; x :s; 0.15, a tetragonal structure appears; For sample with x == 0.15, the 200 line is weak, and in the sample tetragonal phase which is a major structure coexists with orthorhombic phase; and as x ~ 0.25, the tetragonal structure disappears and the orthorhombic phase reappears. The changes of lattice constants a, band c of Fe

doped LBCO system vs iron content are shown in Fig. 3. The variation in the lattice constants also supports the anomalous structural transitions in the Fe doped LBCO superconducting system. ACKNOWLEDGEMENT This work was supported by the Chinese National Centre for Research and Development on Superconductivity. REFERENCES I. Y. Xu, M. Suenaga, J. Tafto, R. L. Sabatini, and A. R. Moodenbaugh, Phys. Rev. B 39 (1989) 6667. 2. P. R. Slater, A. 1. Wright and C. Greaves, Physica C 183 (1991) III. 3. W. M. Chen, et al. To be published in J. Superconductivity. 4. W. M. Chen, C. C. Lam, L. Y. Li, J. F. Geng, F. M.Wu, K. C. Hung, and X. Jin, J. Superconductivity, 9, 553( 1996). 5. W. M. Chen, W. Hong, J. F. Geng, X. S. Wu, W. Ji, L. Y. Li, L. Qiu, X. Jin, Physica C 270, 349(1996). 6. W. M. Chen, C. C. Lam, J. F. Geng, L. Y. Li, K. C.Hung, X. Jin, Physica C 270, 155(1996).