Fluorine substitution for oxygen in the high Tc superconducting ceramic oxides is not only fundamentally but also technologically interesting. We have previously reported the observation of superconducting zero resistance transition at 155–168K, subsequently confirmed by several other groups. We have also observed diamagnetic signals and flux trapping at temperatures as high as 305K. The samples were made from solid-state reaction in a pressed pellet form. Because of the BaF2 formation and the small volume fraction, we have not yet been able to identify the specific high Tc phases.
In the course of elucidating and optimizing the fluorination effect, we have carried out other means of low-temperature fluorination experiments. The zero resistance at 154K was repeated.
In addition to the observation of these high 1c results, we found that a dopant amount of fluorine promotes the oriented crystal growth. We also found that the replacement of weakly bonded oxygen by fluorine can greatly enhance the material's stability by eliminating the serious problem of oxygen diffusion.