Structures and Intergrowths in Thallium Superconductors
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STRUCTURES AND INTERGROWTHS IN THALLIUM SUPERCONDUCTORS J. P. ZHANG, D. J. LI AND L. D. MARKS Materials Research Center, Northwestern University, IL 60208
Evanston,
ABSTRACT The superconducting phases of TI Ba 2 CaCu 2 0 8 (2212) and TI2Ba a 2 Cu 3 iO (2223) have been identified by means of electron diffraction analysis and high resolution electron microscopy. The 2212 phase is essentially perfect, with the structure previously determined by x-ray studies but with a different c/a ratio, while the 2223 phase shows structural variations which may be due to oxygen vacancies. In addition, superstructures based on intergrowths of 2212 and 2223 and the block structures TI 2 Ba PaCu,+O2,+6 (n=l, 3,4,5) intergrown in 2223 were observed. INTRODUCTION Recently, new high Tc single crystals (124K) (1] and thin films (106K) (2) have been developed based upon thallium containing phases (3,4] with the composition TI Ba2 Ca CLu+ ,O6 1 4, or TlBa 2 Ca.Cu ,1 02, which superconduct at somewhat higher temperatures and may well carry larger critical currents. In this paper we report results on the structure of 2212 (n=l) and 2223 (n=2) superconductors and observation of intergrowths with their structural variants of n=3,4,5 and of (2212)i(2223)j superstructures (i,j integers). EXPERIMENTAL METHOD The specimens that we have examined were prepared as described elsewhere by Hazen et al [5]. For electron microscopy these specimens were crushed and then mounted onto well pyrolised holey carbon films and examined in either a Hitachi H-700U for tilting experiments or a Hitachi H-9000 for high resolution studies. To assist in interpreting the results image simulations were performed using the NUMIS programs developed at Northwestern University. RESULTS There has been some controversy in the literature to date (4-61 as to the exact unit cell of the 2223 superconductor. Figure 1 (a) and (b) show diffraction patterns taken from grains of 2223 and 2221 respectively indicating a body centered structure with 14mm space group. As a rule there are systematic absences when h+k+l is odd, but an exception is found occasionally where weak (100) and (010) spots are observed. They are in fact due to intergrowths, as will be shown below, which destroy the local (1/2,1/2,1/2) translational symmetry. The 2212 superconductor showed an essentially perfect structure, see for instance Figure 2(a-d). Image simulations (see inset) indicated that the material had the basic structure Mat. Res. Soc.Symp. Proc. Vol.139. 41989 Materials Research Society
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320 ''31 110 Figure 1. Diffraction patterns from different single grains of the thallium superconductors: (a) 2122; (b) 2223. Systematic absences occur when h+k+l is odd indicating a body centered tetragonal unit cell with axes a=.385 and c approximately 2.9 and 3.6 nm respectively. previously determined by x-ray studies [7],
with a Cu-Ca-Cu
perovskite unit sandwiched between two Ba-Tl-Tl-Ba units with a rocksalt structur
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