TlSr₂Ca_1-xY_xCu₂O₇の結晶構造と超電導性

抄録

We report the crystal structures and the superconductivity of the TlSr₂Ca_1-xY_xCu₂O₇ system. In the entire range of x=0-1, we could obtain single phase specimens. Each structure of TlSr₂Ca1-x Y_xCu₂O₇ (x=0-1) was similar to that of TlBa₂CaCu₂O₇ (so-called"1212"structure). With increas- ing x, the a-axis lengthened from 3.785A to 3.815Ä while the c-axis shortened from 12.104A to 11.978A, and the atom positions a little changed: i. e, oxygen atoms were drawn to Y/Ca atoms, however the positions of metal atoms did not change. The normal state conductivity decreases, with increasing x, the temperature dependencies of the resistivities of TlSr₂Ca_1-xY_xCu₂O₇ change from metal-like and not superconducting through metal-like and superconducting, semiconducting and superconducting to semiconducting and not superconducting. When the x values were 0.2, 0.4 and 0.6, they were superconductors with T_c (onset) of about 80K. In case of x=0, the mean Cu-valence is 2.5, and appears to be too large for superconductivity in this system (the mean Cu-valence was calculated assuming the oxygen and the all metal ion sites to be fully occupied). The valence of 2.2-2.4 (x=0.2-0.6) may be good for super conductivity, 2.0 or 2.1 (x=1.0, 0.8) is too small.