Superconducting properties of 1212 phase in (Tl, M) Sr2(Ca1-xCrx)Cu2O7 (M=Zr and Ce)

Samples with nominal starting compositions (Tl0.8M0.2)Sr2(Ca1-xCrx)Cu2O7 (1212) where M=Ce and Zr have been prepared and investigated by the d.c. electrical resistance method and powder X-ray diffraction analyses. Most of the samples showed the 1212 as the major phase and 1201 as the minor phase. Both series of samples showed the highest superconducting onset temperature (Tc) at x=0.2. Ce was found to be more favourable for the formation of the 1212 phase compared to Zr. The valence state of Zr, estimated by employing previous results of optimum average Cu valence for superconductivity in the 1212 superconductors, is 4+. The importance of ionic radius of the substituted elements and the effect of starting composition on Tc and phase formation are discussed. © 1998 Chapman & Hall