Possible phase separation in Pb-doped Bi-2223 superconductors

PHYSICA® ELSEVIER

Physica C 341-348 (2000) 663-664 www.elsevier.nl/Iocate/physc

Possible Phase Separation in P b - d o p e d Bi-2223 S u p e r c o n d u c t o r s L.W.Zhang

Z.J.Chen

B.S.Cao

M.H.Zhu

and

H.S.Huang

Physics Department, Tsinghua Universi.ty, Beijing 100084, P.R.China Materials Research Center, Tsinghua University, Beijing 100084, ER.China Upon annealing in N2 at 730°C, the resistivity of the Pb-doped Bi-2223 superconductors was initially increased, then slowly decreased. There exists a resistivity, peak during the subsequent heat cycling. The relaxation process below and above the peak temperature is much different. In addition, many transition steps were observed in the ac susceptibili.ty. Phase separation and phase transition can explain our results. 1. I N T R O D U C T I O N

Phase segregation was widely discovered in the under-doped high temperature superconductorsIt31. But few reports were presented on Bi-2223 superconductors. In this paper, the high temperature resistivity, of Pb-doped Bi-2223 superconductors was studied. The results were explained by phase segregation mad phase transition.

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100

200 Time (min)

2. E X P E R I M E N T A L

Bi~6Pb04Sr2Ca2Cu3Ox superconductors were heat treated at different temperatures under flowing (200nfl/min) 02 (99.9%) or N2 (99.99%), and the process was monitored by in situ resistivity measurements. Ac susceptibility, was measured by a lock-in technique. 3. RESULTS AND DISCUSSIONS

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Fig. 1 Time dependent resistivity, for Bi-2223 sample annealed at 730°C in N2

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Fig. 1 shows the resistivity variation with time for the Bi-2223 superconductor isothermally annealed at 730°C in flowing N2. The resistivity increase from a to b was obviously attributed to the loss of oxygenI41. But the decrease in resistivity from b to c was not observed for both Pb-free and Pb-doped Bi2212 superconductors. Starting from state c, the Bi-2223 superconductor was thermally cycled in N2. A resistivity, peak appeared on both warming and cooling p -T curves as shown in fig.2.

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Fig.2 p - T curve for the BPSCCO 2223 sample after • amlealed in N2 to state c All rights reserved.

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L. W Zhang et al./Physica C 341 348 (2000) 663-664

During the above cooling process if the sample was kept isothermally annealing at temperatures above and below the peak temperature TpD, the resistivity behavior was much different as seen from fig.3

quenched sample, two of which, phase A and phase B, were respectively related to the states above and below TpD(fig.2). By isothermally annealing in N2 (fig.3) or 02 (fig.4), phase A could be changed to phase B.

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During the cooling process in fig.2 if the sample was kept at temperatures above and below TpD with the atmosphere suddenly switched from N 2 to 02, the p -t curves were as in rigA. In contrast with the annealing below TpD (see insert), there appeared a resistivity shoulder on the p -t curve when annealed at temperatures above TpD

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Fig.3 Variation of resistivity with time annealed at temperatures above TpD (insert: annealed below TpD ) in N2

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Fig.5

ac susceptibility on the sample quenched from state c shown in fig. 1

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