Fluorine substitution in high temperature superconductors

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconductin g phases was studied. We obtained superconducting structures, never observe d before, of the Bi-based superconductors by a low temperature (200-400 deg rees C) fluorination process. Fluorine substitutes completely the oxygen si tes in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and anions were ind uced, especially in the Bi and partially in the Sr layer. F-doped Tl(1223) has been prepared in the same way as Bi(2223) and Bi(2212) (low temperature fluorination), but also starting from precursor containing fluorides of di fferent elements. No significant differences in the crystal structure have been observed between the TI-based samples with F inclusions and without. T he critical temperature (116 K) remains unchanged but a significant increas e of the irreversibility field at low temperature was found.