Enhanced superconductivity of SmFeAsO co-doped by Scandium and Fluorine to increase chemical inner pressure

Sm1−x/3Scx/3FeAsO1−xFx (x=0.09–0.27) were synthesized by the mechanical alloying and subsequent low temperature rapid sintering (2 h at 950 °C). The superconducting transition temperature (Tc) increased with the doping level of x. The optimal doping achieved a Tc up to 53.5 K in Sm0.93Sc0.07FeAsO0.79F0.21. The higher Tc value was attributed to the increased chemical inner pressure from local lattice distortion induced by smaller-size dopants, which was further confirmed by Sm1−xScxFeAsO0.88F0.12 (x=0.04, 0.08, 0.12). Accordingly, larger lattice distortion can enhance the superconductivity below the doping limit. Similar phenomenon was also observed in the La1−xYxFeAsO0.8F0.2 (x=0.4, 0.5, 0.6).