FORMATION AND SUPERCONDUCTIVITY OF 1212-TYPE PHASE TLSR2(SR0.5R0.5)CU2O7-DELTA WITH R = SC, Y AND LANTHANIDES

Nominal samples TlSr2(Sr0.5R0.5)Cu2Ox 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 TlSr2(Sr0.5R0.5)Cu2O7-delta, 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.