高温超電導体の結晶化学

概要

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The high temperature superconductors discovered so far (La-, Y-, Bi-, Tl-systems) were classified according to their crystal structures or the number of perovskite blocks in the unit cell. It is empirically found that the critical temperature is higher with a higher number of perovskite blocks in unit cell. Of particular interests are two series of Tl-systems, namely Tl-Ba-Ca-Cu-O and Tl-Sr-Ca-Cu-O. The Tl-Ba-Ca-Cu-O forms (2223), (2212), and (2201) and Tl-Sr-Ca-Cu-O forms (1223), (1212) superconducting states. They are isostructual except that the former has double Tl-O plane and the latter single Tl-O plane. The Tl/Ba-system always gives higher Tc's than the Tl/Sr system in the respective crystal structures. The Tl-Sr-Ca-Cu-O system is closer related to the Bi-Sr-Ca-Cu-O system from the view point of Tc. Comparing the Tc's and crystal structures of the Tl and Bi systems, it is suggested that Tc in mainly determined by the combination of two alkaline elements, i. e. Sr/Ca vs Ba/Ca, when the repetition unit in the layered perovskites is equal.
九州大学の論文
1991-03-30