Phase diagram and superconductivity at 58.1 K in α-FeAs-free SmFeAsO_1-xF_x

The phase diagram of SmFeAsO_1-xF_x in terms of x is exhibited in this study. Specimens of SmFeAsO_1-xF_x from x = 0 to x = 0.3 were prepared by low-temperature sintering with slow cooling. The low-temperature sintering suppresses the formation of the amorphous FeAs, which is inevitably produced as an impurity when using high-temperature sintering. Moreover, slow cooling is effective in obtaining a high fluorine concentration. The compositional change from feedstock composition is quite small after this synthesis. We can reproducibly observe a record superconducting transition for an iron-based superconductor at 58.1 K. This achievement of a high superconducting transition is due to the success in substituting a large amount of fluorine. A shrinking of the a lattice parameter caused by fluorine substitution is observed and the substitutional rate of fluorine changes at x = 0.16.