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SYNTHESIS AND CHARACTERIZATION OF Tl2Ba 2CaCu20g AND TI2Ba2Ca2Cu3O10 Lauren E. H. McMills, Shu Li, Zhen Zhang and Martha Greenblatt*, Department of Chemistry, Rutgers The State University of New Jersey, Piscataway, NJ 08855-0939

ABSTRACT Single-phase samples of Tl2Ba2CaCu2O8 and T12Ba2Ca2Cu3O1O have been prepared using a reliable and simple synthetic method. Samples were characterized by x-ray powder diffraction, magnetic susceptibility and four probe resistivity methods. The TcR=O values for T12Ba 2 CaCu2O8 ranged from 100 to 105K, whereas those for T12Ba2Ca2Cu 3Olo ranged from 96 to 107K. The relationship between the superconducting properties and the various preparation conditions are discussed. INTRODUCTION The discovery of high temperature superconductivity in the TI-Ba-Ca-Cu-O system by Sheng and Hermann has generated much interest in the scientific community.[1,2] There are four known phases in the T12Ba2Can-lCunO2n+4 (n=1-4) in which the values of Tc are correlated with the number of CuO2 layers (n) in the structure. We have focused our attention on two of these superconducting phases, Tl2Ba 2 CaCu 2O 8 (2212) and T12Ba2Ca2Cu 3OlO (2223), which possess high Tc values of 108K and 125K, respectively. As these materials have an extremely complex structure it is imperative to be able to synthesize single phase material in order to further probe the relationship between structure and superconductivity in these compounds.

EXPERIMENTAL Stoichiometric amounts of T12 0 3 , BaO2 , CaO2 and CuO were ground in an agate mortar, pressed into pellets, wrapped with very thin (0.025 mm thick) silver or gold foil and sintered in air. The samples were then either furnace cooled or quenched to room temperature. X-ray powder diffraction data were recorded on a SCINTAG PAD V diffractometer with monochromatized CuKa radiation using fluorophlogopite mica (NBS primary standard) as a primary standard. Lattice parameters were calculated by fitting the observed diffraction data by a least-squares program. Electrical resistivity was measured from 300 to 15K using the four probe technique. Magnetic susceptibility measurements were done on a Quantum Design SQUID magnetometer. The magnetic fields applied were 50 or 100 Gauss.

RESULTS AND DISCUSSION A. 2212 A variety of temperatures and heating times have been used to optimize the preparation conditions for 2212. It was found that very thin silver or gold foil can be used as a seal to protect

Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society

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against severe thallium loss. The optimum sintering temperature range needed to obtain single phase materials of 2212 is 860-870'C. A study of both the effect of sintering temperature and sintering time was undertaken. Table 1 shows that the Tc value remains relatively constant with inc