Preparation of REFeAsO1−xFx (RE=Sm and Gd) superconductors at a relatively low temperature

A series of the SmFeAsO_1−xF_x and GdFeAsO_1−xF_x (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF₃ as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO_1−xF_x and GdFeAsO_1−xF_x, respectively. These temperatures are at least 50-60° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in T_c proved that this low temperature process was more effective to introduce fluorine into R_EFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO_1−xF_x with x=0.05 and at 22 K for GdFeAsO_1−xF_x with x=0.1. The highest T_c was detected to be 54 K in SmFeAsO_0.8F_0.2 and 40.2 K in GdFeAsO_0.75F_0.25. The use of the nano-scaled ReF₃ compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.