Influence of the Partial Substitution of Small Rare Earths for Ca in the Superconductive Bi-Sr-Ca-Cu-O Compound
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INFLUENCE OF THE PARTIAL SUBSTITUTION OF SMALL RARE EARTHS FOR Ca IN THE SUPERCONDUCTIVE Bi-Sr-Ca-Cu-O COMPOUND M. J. CASANOVE, P. BAULES, E. SNOECK, C. ROUCAU, P. MILLET, R. ENJALBERT and J. GALY CEMES-LOE / CNRS, B.P. 4347, 31055 TOULOUSE Cedex, FRANCE.
ABSTRACT Superconductive 2:2:1:2 phases of the type Bi2Sr2Cal-yLnyCu2O8+x+y/2, where Ln stands for Yb or Er, have been synthesized and characterised by X-ray and electron diffraction. The small rare earth atoms prove to substitute preferentially in the Ca site. The electrical properties as well as the structural characteristics of these compounds are affected by the rare earth substitution rate. It is shown, in particular, that the long axis parameter of the subcell and the incommensurate period of the modulation decrease with increasing y. The substitution of trivalent cations for Ca requires the presence of additional oxygen atoms in the structure, the location of which is discussed.
INTRODUCTION Chemical substitutions have already provided some useful information about the part played by the different elements in the remarkable properties of new high Tc superconductors [12]. The results reported in the present paper concern the influence of the substitution of small rare earths for Ca on the structure and electrical properties of the 2:2:1:2 phase in the Bi-Sr system. Samples of composition Bi2Sr2Cal-yLnyCu208+x+y/2, where x corresponds to an excess of oxygen related to the possible oxidation of copper (II) and 0 •_y _
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