EFFECT OF BA SUBSTITUTION ON THE CRYSTAL-STRUCTURE AND SUPERCONDUCTING PROPERTIES OF TL(1212)

Tl0.5Pb0.5Sr2-xBaxCaCu2O7-delta (x=0, 0.2, 0.4) samples were character ized by X-ray powder diffraction and AC susceptibility measurements. T he highest weight fraction of the 1212 phase, 96.4%, was found for the Ba-free sample. On adding Ba, the amount of 1212 phase decreases, whe reas the amount of 1223 phase increases. This is consistent with the o bservation that Ba enhances the formation of the 1223 phase. In additi on to these two superconducting phases, (Ca,Sr)(2)CuO3 and (Sr,Ca)CuO2 were detected More than 10 at.% Tl on the Ca site and less than 2 at. % Ca on the Sr site were refined in the structure of the 1212 phase. U nlike for the 1223 phase, the distance from the Cu atom to the apical O atom of the square pyramid increased only slightly when the Ba conte nt was raised. No correlation between critical temperature (similar to 68 K) and structural changes was observed.