SUPERCONDUCTIVITY IN THE TRIPLE-PEROVSKITE LA-CA-BA-CU-O SYSTEM

Three series of single-phase La-Ca-Ba-Cu-O compounds with tetragonal t riple-perovskite structure and respective nominal compositions of La4- xCaxBa3Cu7Oy (LC series, 0.50 less than or equal to x less than or equ al to 1.15), La3CaxBa4-xCu7Oy (CB series, 0 less than or equal to x le ss than or equal to 1.25) and YLa3.5-0.5xCaxBa3.5-0.5xCu7Oy (LCB serie s, 0 less than or equal to x less than or equal to 1.25) were prepared by either a solid state reaction method or a polymeric citrate precur sor method. All the single-phase materials were superconducting except for samples containing small x in the LCB series. Unit-cell parameter s, such as a axes, c axes and cell volume, decreased with increasing C a substitution. In general, the T-c and hole concentration (p) increas ed with increasing x from x = 0 to x = 1.00. In other words, evidence of hole doping was observed during Ca substitution that was the main r eason for the increase of the T-c from 0 to 80 K. The optimal T-c was observed in a region close to the formula of La3CaBa3Cu7Oy which had a formula per unit cell of La1.29Ca0.43Ba1.29Cu3Oz where z = 7.104, p = 0.308, a axis = 0.3879 nm and T-c(zero) = 80 K.