T-ASTERISK PHASE (BI0.5SR1.5)(Y2-XCEX)CU2O8-DELTA SUPERCONDUCTORS - THE EFFECT OF IONIC RADIUS ON STRUCTURE

We have successfully synthesized the superconductor (Sr1.5Bi0.5)(Y2-xC ex)Cu2O8-delta, the crystal structure of which is similar to that of T -phase (Nd,Sr,Ce)(2)CuO4. This structure is also believed to be witho ut the double Bi-O layer as in Bi2Sr(L,Ce)(2)Cu2Oy, and the coupling b etween layers in (Sr,Bi)(2)(Y,Ce)(2)CU2O8-delta is stronger than in Bi 2Sr2(L,Ce)(2)Cu2Oy. The transition temperature is actually increased b y about 8 K. This result seems to be different from that in Tl-based s uperconductors. It is also found that the ionic radius of the A site i n the Cu perovskite unit ACuO(3-delta) is critical to the formation of the T, T', T phases.