Fluorine substitution in high temperature superconductors

Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. The authors obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200--400 C) fluorination process. Fluorine substitutes completely the oxygen sites 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 induced, 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 different elements. No significant differences in the crystal structure have been observed between the Tl-based samples with F inclusions and without. The critical temperature (116 K) remains unchanged but a significant increase of the irreversibility field at low temperature was found