THE CRYSTAL STRUCTURES AND SUPERCONDUCTIVITY OF A NEW SERIES OF SUPERCONDUCTING PHASES

We synthesized new superconducting phases in TlBa2Can-1CunO2n+2.5 by solid state reaction. The crystal structures of TlBa2CaCu2O6.5 and TlBa2Ca2Cu3O8.5 were determined by means of X-ray powder diffraction method. The two compounds belong to primary lattice with space group D4h1-P4/mmm. The lattice constants of TlBa2CaCu2O6.5 are α= 3.847 ?, c = 12.73 ?. The lattice constants of TlBa2Ca2Cu3O8.5 are α = 3.847?, c= 15.89?. Each cell contains one chemical formula unit. The cations are distributed alternately at positions of (0,0,z) and (1/2, 1/2, z) along Z axis. TlBa2CaCu2O6.5 shows zero resistance at 101.4K. TlBa2Ca2Cu3O8.5) shows zero resistance at 112K. The oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n+1 is separated by single Tl-O layer in the crystal structure of TlBa2Can-1CunO2n+2.5, while the oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n is separated by bilayer of Tl-O in the crystal structure of Tl2Ba2Can-1CunO2n+2. At the same n value, Tc of Tl2Ba2Can-1CunO2n+4 is about 10K higher than Tc of TlBa2Can-1CunO2n+2.5. This indicates that Tc depends on the number of Tl-O layer as well as the number of [CuO5] and [CuO4] in oxygen deficient pseudo-perovskite unit Ba2Can-1CunO2n.