Extended family of Sr free Bi-2201 copper oxides Bi2 + xLnyCa2 − x − yCuOz (La = La, Pr, Nd, Sm, Eu and Gd)

PhysicaC234 (1994) 307-310

ELSEVIER

Extended family of Sr free Bi-2201 copper oxides Bi2+xLnyCa2_x_yCuOz(Ln=La, Pr, Nd, Sm, Eu and Gd) Hiroyuki Sasakura a,., Shinnosuke Minamigawa b, Kiiehi Yoshiara c, Ken-ichi Yoshida a • Department of Physics, Hamamatsu UniversitySchool of Medicine, Handa-cho, Hamamatsu, 431-31, Japan b Department of Physics, Universityof Osaka Prefecture, SakaL Osaka 591, Japan c Materials andDevices Laboratory, Mitsubishi Electric Corporation, Sagamihara, Kanagawa, 229, Japan

Received 18 July 1994

Abstract

The extended family of St free Bi-2201 copper oxides has been synthesized in the Bi2+xLnyCa2_x_yCuOz(Ln=La, Pr, Nd, Sm, Eu and Gd) systems. The almost or nearly single-phase sample for each system was found to be obtained in a narrow composition region. Then, superconductivity was observed for the samples with Ln = La, Pr and Nd. Among them, the sample with L n f N d exhibited the highest superconducting onset at about 11 K and zero resistivity at about 9 K. 1. Introduction As is well known, three superconducting phases, 2201 phase, 2212 phase and 2223 phase, exist in the B i - S r - C a - C u - O system, and the ideal composition of each is Bi2Sr2CuOz, Bi2Sr2CaCu2Oz and Bi2Sr2Ca2Cu30~, respectively [ 1-3 ]. Among them, in the cases of the 2201 phase and the 2212 phase, without St, the isostruct~al samples have been synthesized. Inoue et al. found a superconducting phase showing the onset at about 52 K in the Sr free Bi-LaCa--Cu-O system [4 ]. The phase was concluded to be the 2212 phase, though the sample was not of single phase. Following the discovery, Sasalmra et al. successfully synthesized the extended family of Sr flee Bi-euprates in the Bi2LnxCa3_xCu20, systems (Ln=La, Pr, Nd, Sin, Eu and Gd) [5]. All the samples were almost single 2212 phase and exhibited superconductivity. Especially, the sample with Ln = Pr showed an onset of resistivity drop at about 70 K and * Correspondingauthor.

zero resistivity at about 45 K. As for the Sr-free Bi2201 samples, Bi2LaCaCuOz was first synthesized by Takemura et al. [6]. Then, Bi2LaBaCuOz was synthesized by Darriet et al. [ 7 ]. However, these Sr free Bi-2201 samples were semiconductors and showed no evidence of superconductivity. Recently, Sasakura et al. have first succeeded in the synthesis of Sr free superconducting 2201 phase in the Bi2PrxCa2_xCuO~ system [8]. The almost single-phase sample was attained at the nominal composition of Bi2Pro.7Cal.3CuOz, and the sample showed an onset of resistivity drop at about 12 K and zero resistivity at about 6 K. However, the interesting problems remain open, whether the Sr free 2201 samples can be synthesized by using Ln except for Pr in the Bi-LnCa-Cu-O systems (Ln: lanthanide elements) or not; if it is possible, whether the samples showing higher Tc than the sample with Ln = Pr exist or not. This paper reports the attempt to solve the problems. As a result, new Sr free 2201 members in addition to the B i 2 P r o . 7 C a l . 3 C u O z c a n be synthesized by using La, Nd, Sin, Eu and Gd for Ln in the Sr free

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H. Sasakura et al. / Physica C 234 (1994) 307-310

Bi2+xLnj,Ca2_x_yCuO=systems and the samples with Ln=La and Nd among the new 2201 members are found to show superconductivity.

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2. Experimental

Many polycrystalline samples were prepared by a solid-state reaction method in order to investigate whether the Sr free 2201 phase could be composed by using lanthanide elements except for Pr or not. The nominal composition was Bi2+~LnyCa2_x_yCuOz, where Ln represents a lanthanide element. The starting materials were high-purity ( > 99.99%) powders of Bi203, lanthanide oxides (La203 ,Pr60~, Nd203, Sm203, Eu203, Gd203, Dy203, Ho203), CaCO3 and CuO. First, the appropriate mixture of the powders was well ground and fired at 800"C for 15 h in air (first sintering). The resultant samples were thoroughly reground, cold-pressed into disk-shaped pellets, and sintered at 800-845"C for 15 h in air (second sintering). The second sintering was repeated twice to obtain homogeneous samples. The optimum sintering temperature for each system became lower for smaller lanthanide elements as noted in the caption of Fig. 1. After sintering, the samples were annealed at 800"C for about 15 h in 02 atmosphere, and cooled down to room temperature at a rate of 0.2"C/ min. The phases appearing in the resultant samples were determined by X-ray powder diffraction measurements using Cu Ka radiation monochromatized with a curved graphite single crystal. Diffraction intensity data were measured by step scanning at 0.05* intervals for 4 s in the range from 3* to 50*. The electrical resistivity was measured in the temperature range from 4.2 K to 273 K by a standard four-probe method. The DC magnetic susceptibility for powdered samples was measured with respect to temperature using a SQUID magnetometer. The applied field was 10 Oe.

3. Results and discussion

Numerous samples were prepared, varying the compositions and varying the sintering temperatures. From the X-ray diffraction analysis for all the samples, we found that the 2201 phase samples in the

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present Sr free Bi2+xLnyCa2_x_yCuOzsystems could be obtained when La, Pr, Nd, Sm, Eu and Gd was used for Ln, whereas this was not the case for Dy and Ho, and the almost or nearly single phase samples for each system existed in a narrow composition region. Moreover, we found that it became very difficult to obtain the almost single-phase samples, as the Ln became the smaller element. Especially, in the case of the smallest Ln = Gd, the quality of the samples was very sensitive to the preparation condition such as the sintering temperature, therefore, the best resultant samples were of nearly single phase. Moreover, we found that in the case of the almost or nearly single phase samples for Ln = Sm, Eu and Gd, the Bicontents interestingly increased from 2.0 as the Ln became the smaller element. Fig. 1 shows X-ray patterns for the almost or nearly single phase samples of

H. Sasakura et ol. / Physica C 234 (1994) 307- 310

Ln = La, Pr, Nd, Sm, Eu and Gd from the t o p in the present Bi2+xLnyCa2_x_yCuOzsystems. The nominal compositions and the lattice parameters of a and c for each sample are summarized in Table 1. In the case for the sample with Ln = I_a, the diffraction peaks are indexed on the basis on a pseudotetragonal unit cell. Figs. 2 (a) and (b) show the temperature dependence of the resistivity (p--T dependence) for the same samples as shown in Fig. 1. As seen from Fig. 2 (a), the samples with Ln = La, Pr and Nd each show resistivity drops. The inset shows an enlargement of the temperature range near the resistivity drops. From the inset, we can see that the starting temperatures of the resistivity drop become higher in the order of La, Pr and Nd, and in the case of Ln = Nd, the sample shows a resistivity drop starting from the highest temperature of about 15 K and shows zero resistivity at the highest temperature of about 9 K. When the lanthanide element change s from Sin, Eu and Gd passing by Nd, the p-T dependence becomes more semiconductor-like as seen from Fig. 2(b). In these cases of Ln = Sm, Eu and Gd, none of the almost or nearly single phase samples showed resistivity drops down to 4.2 K. However, it is worth mentioning that some samples with Ln = Sm also showed a resistivity drop with an onset near 8 K, though the phenomenon was not shown in Fig. 2 (b). Such a phenomenon was interestingly observed for multiphase samples consisting of mainly 2201 phase, not for almost singlephase samples. The same situation was also observed for the 2212 phase in the Sr free Bi2Gd~Ca3_xCu20= system [5 ]. That is, the almost single 2212 phase sample with the nominal composition of Bi2Gdo.~Sr2.4Cu20= in this system was also a semiconductor without a superconducting behavior, but Table 1 Nominal compositions, lattice parameters with pseudotetragonal unit cell and superconducting onset temperatures (Tm ) for almost or nearly single-phase samples in the present Bi2+~J.nyCa=_z_yCuO= (Ln =La, Pr, Nd, Sm, Eu and Gd) systems

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T m (K)

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T (K) Fig. 2. p-Tdependence for the almost or nearly single 2201 phase samples in the Bi2+xLn~Ca2±x_yCuO= systems, (a) Ln=La, Pr and Nd, (b) Ln=Sm, Eu and Gd. Each of the samples is the same as the one shown in Fig. 1.

a multiphase sample showed a resistivity drop at about 60 K and zero resistivity at about 26 K. Recently, we discovered that the almost single phase samples existed in a fairly wide region of x and y in the nominal composition of Bi2+xGdj, Ca3_x_yCu20, and that an almost single-phase sample with the nominal composition of BizosGdo.5oCaz45Cu20, showed a resistivity drop starting from about 56 K and zero resistivity at about 21 K originating from superconductivity [ 9 ]. Therefore, it may be also possible for the present Sr free 2201 system with Ln = Sm to obtain the almost single-phase sample showing not only the resistivity drop but also the zero resistivity originating from superconductivity, if samples are prepared over a wide composition range by varying not only x and y but also even the composition of Cu. Finally, from Figs. 2(a) and (b), we can interest-

310

H. Sasakura et aL / Physica C 234 (1994) 307- 310

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ingly see that the value of the resistivity at 273 K is the minimum one in the case ofLn = Nd showing both the resistivity dropping onset and the zero resistivity at the highest temperature. Fig. 3 shows the temperature dependence of the DC magnetic susceptibility for the powdered samples with Ln = La and Nd, which showed a resistivity drop as shown in Fig. 2 (a). The inset shows the temperature dependence of the AC magnetic susceptibility for the powdered sample with Ln = Pr, which was already reported [ 8 ]. All the samples show the diamagnetic signal. Therefore, the phenomenon of the resistivity drop and zero resistivity is confirmed to originate from superconductivity. The starting temperature of the diamagnetic signal for each sample, that is, the superconducting onset temperature T m was decided by magnetic susceptibility; the value is about 9 K for Ln = La, about 10 K for Ln = Pr and about 11 K for Nd as given in Table 1, becoming higher in order of La, Pr and Nd. This result is consistent with that of the resistivity measurement. Superconducting volume fractions for the samples with Ln = La and Nd at 3 K are about 6.5% and about 30%, respectively. The result shows that the superconductivity for each sample is a bulk feature.

4. Conclusions The extended family of Sr free Bi-2201 copper oxides has been synthesized in the

Acknowledgements The authors would like to thank K. Nakahigashi and M. Kogachi of University of Osaka Prefecture for allowing us the use of X-ray diffraction apparatus and Y. Akagi of University of Osaka Prefecture for his assistance in the sample preparation. They also would like to thank Y. Hayashi of University of Osaka Prefecture and A. Minakata of Hamamatsu University School of Medicine for their continuous encouragements throughout this study.

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