Effect of Ba substitution on the crystal structure and superconducting properties of Tl(1212)

Abstract

Tl0.5Pb0.5Sr2−xBaxCaCu2O7−δ (x=0, 0.2, 0.4) samples were characterized by X-ray powder diffraction and AC susceptibility measurements. The 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, whereas the amount of 1223 phase increases. This is consistent with the observation that Ba enhances the formation of the 1223 phase. In addition to these two superconducting phases, (Ca, Sr)2CuO3 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. Unlike 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 content was raised. No correlation between critical temperature (≈68K) and structural changes was observed.