(TL,BI)SR2CACU2O7-1212 SUPERCONDUCTING SINGLE-CRYSTAL - A STRUCTURAL AND MAGNETIC STUDY

Single crystals of Tl2/3Bi1/3Sr2CaCu2O7 with T(c) = 86 K have been gro wn and their structure has been refined from a single crystal X-ray di ffraction study in the space group P4/mmm with a = 3.7922(6) angstrom and c = 12.0661(25) angstrom. The structure of this oxide is very simi lar to those of (Tl, Pb)Sr2CaCu2O7 and TlSr2CaCu2O7. Nevertheless, the oxygen O(3) at the level of the thallium layer is found to be in a xx 0 site which is unusual for such compounds. The superconducting proper ties have been studied by performing magnetic measurements in the H pa rallel-to c geometry. The critical current densities have been extract ed and the J(cH) = 0(t) have been found to be similar to those of Tl0. 5Pb0.5Sr2CaCu2O7. The comparison of the irreversibility line location for the (Tl, Bi)-1212 with those of other 1212 thallium crystals suppo rts the model which predicts that, in this series, the H(t) location depends essentially on the thickness of the rock salt type layer and i s not greatly affected by the presence of Bi or Pb in the TlO layer.