Physica C 235-240 (1994)365-366 North-Holland
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Synthesis and Superconducting Properties of Pr Doped Y2Ba4CuTO~ N. Seiji, S. Adachi and H. Yamauchi Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135, Japan We have synthesized (Y~. Pr )2Ba4CuTOz compounds and investigated their superconducting properties. The samples are prepared by means of an O2-HIP technique. Nearly single phase samples of the Y2Ba4Cu70~ structure are obtained for x=0"--0.8. All the lattice parameters, a, b, and c, increase with increasing Pr content. The superconducting transition temperature T decreases monotonically from 93K at x--0 to 38K at x=0.6. The sample with x=0.8 does not show superconductivity above 4.2K. Compared with T's which were previously reported for (Yz. Pr )Ba2Cu30 and (Y~.xPr)Ba2Cu4Os, the T values for (Y~.~Pr~)zBa4Cu~O~ samples are found to be closer to those for (Yv Pr )Ba2Cu30 for x<:0.2 but to those for (Yv Pr)Ba2Cu4Os for x>0.2. 1. INTRODUCTION Y2Ba4Cu70~ ("247") consists of an ordered intergrowth structure comprising alternating blocks of YBazCu30z ("123") and YBazCu408 ("124") [1]. A number of studies on the suppression of superconductivity by partial substitution of Pr for Y in "123" [2-4] and "124" [5-7] compounds have been reported. However, similar studies on the "247" compounds have been scarcely reported. In this paper, we describe the preparation of (YvxPrx)zBa4Cu7Oz compounds and their superconducting properties in comparison with those of (YvPr)Ba2Cu30 and (Y v~Pr~)Ba2Cu,Os. 2. EXPERIMENTAL Samples were prepared by a solid state reaction method using an O2-HIP (hot isostatic pressing) technique. High purity (>99.9%) powders of Y203, Pr6Oll, BaCO 3 and CuO were used as starting materials. These powders were mixed to nominal compositions of (Yz.xPrx)2Ba4CUTO z (with x=0, 0.2, 0.4, 0.6, 0.8 and 1.0). The powder mixtures were calcined at 860--890°C in air for 90h with several intermediate grindings. The calcined powder was pressed into parallelepiped bars, which were then sintered at 900°C in flowing oxygen gas of 1g/min for 36h. Finally the samples were heat-treated at 1000°C in a gas mixture of Ar-20%O 2 of 50atm for 40h using an Oz-HIP apparatus. The crystal structure of the sample was characterized by powder x-ray diffraction (XRD) using C u K radiation. The dc magnetic susceptibility
was measured using a SQUID magnetometer by cooling the sample in an applied field of 20 Oe. 3. RESULTS AND DISCUSSION From the XRD patterns for samples with x<0.8, the samples are nearly single phase of the "247" structure containing small amounts of BaCuO2 and PrBaO 3 for x=0.2 --0.8. The sample with x=l.0 was melted as the result of O2-HIP-treatment under the condition previous described. When heat-treated at lower temperatures, samples of the same composition are not melted but do not contain the "247" phase. The lattice parameters a , b and c increase with increasing the Pr content as shown in Fig. 1. Similar tendency was reported for (Y1. Pr)Ba2Cu30 [3] and (Yl.xPr~)Ba2Cu4Os [5-7]. The ionic radius [8] of y+3 (0.102nm) is smaller than that of Pr÷3(0.113nm) but larger than that of Pr+4 (0.096rim). If all the doped Pr ions were in the +4 state, it is considered that the electrostatic attraction between the oxygen ions in the Cu-O2 plane and a Pr ion is stronger than that between the oxygen ions and a y+3 ion, and subsequently not only the c axis but also a and b axes through some distortion of the Cu-O2plane would decrease with increasing x. Therefore the observed changes in the lattice parameters imply that the doped Pr ions are partially in the +3 valence state. Figures 2 shows the temperature dependences of dc magnetic susceptibility. The transition temperature, Tc, which was determined from the onset of diamagnetic signal is plotted in fig. 3 as a function
0921-4534/94/S07.00 © 1994 - Elsevier Science FIN. All rights reserved. SSDI 0921-4534(94)00746-2
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SUMMARY We have successfully synthesized nearly single phase samples of (Y~.xPr)zBa4Cu7Oz with x%0.8. The T for the sample decreases with increasing the Pr content. The sample with x=0.8 does not show superconductivity at temperatures above 4.2K. It is found that, for each x, T for (YvxPr,)2Ba, CuTO is close to the higher value of the two Tc's for the "123" and "124" phases with the same x.
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ACKNOWLEDGMENT This work was supported by NEDO for the R&D of Industrial Science and Technology Frontier Program. REFERENCES
of x. Data previously reported for (YI_ Prx)BazCu30~ [3,4] and (Y,_ Pr )BazCu408 [5-71 are also plotted for comparison. T for (YvxPr)zBa4CuvOz decreases monotonically from 93K at x=0 to 38K at x=0.6 and the sample with x=0.8 does not show any diamagnetic signal above 4.2K. It is likely that, for each value of x, T for (Y v Pr~)zBa4Cu70~is close to the higher one of the two values for T's of (Yx. Pr,)Ba2CusO~ and (YvxPr)BazCu, Os.
]l] [21 [31 [4] [5] [6] [7] 181
P. Bordetet al., Nature 334(1988)596. H.B. Radousky, J. Mater. Res., 7(1992)1917. J.L. Peng et al., Phys. Rev. B 40(1989)4517. J. J, Neumeier, Ph. D. thesis (1990), cited in the referance [2]. N. Yamada etal., Physica C 185-189(1991)809. S. Adachi et al., Physica C 207(1993)127. K. Koyama et al., Physica C 185-189(1991)771. R. D. Shannon, Acta Crystallogr. A 32(1976)751.