Superconductivity in the triple-perovskite La-Ca-Ba-Cu-O system

Abstract

Three series of single-phase La-Ca-Ba-Cu-O compounds with tetragonal triple-perovskite structure and respective nominal compositions of La4-xCaxBa3Cu7Oy (LC series, 0.50<or=x<or=1.15), La3CaxBa4-xCu7Oy (CB series, 0<or=x<or=1.25) and La3.5-0.5xCaxBa3.5-0.5xCu7Oy (LCB series, 0<or=x<or=1.25) were prepared by either a solid state reaction method or a polymeric citrate precursor method. All the single-phase materials were superconducting except for samples containing small x in the LCB series. Unit-cell parameters, such as a axes, c axes and cell volume, decreased with increasing Ca substitution. In general, the Tc and hole concentration (p) increased 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 reason for the increase of the Tc from 0 to 80 K. The optimal Tc was observed in a region close to the formula of La3CaBa3Cu7Oy which had a formula per unit celt of La1.29Ca0.43Ba1.29Cu3Oz where z=7.104, p=0.308, a axis =0.3879 nm and Tc(zero)=80 K.