Abstract
Nominal samples TlSr 2(Sr 0.5R 0.5)Cu 2O x with R= Sc, Y and lanthanides, have been prepared, and investigated by X-ray diffraction analysis and resistance measurement. The sample with R= Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu consists mainly of a tetragonal 1212-type phase TlSr 2(Sr 0.5R 0.5)Cu 2O 7- γ , and exhibits a superconducting transition at about 90 K. The sample with R= Ce forms a very pure 1212 phase, but is a semiconductor. The sample with R= Sc forms essentially a 1201 phase, and is a semiconductor. The sample of R= La consists of 1201 phase as its major phase and a 1212 phase as its minor phase, and shows two-step superconducting transitions at 40 K and 90 K. The results are discussed from the point of view of the average valence of Cu and the ionic size of the rare earth.
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