Amorphous FeAs-free SmFeAsO_1-xF_x using low temperature sintering with slow cooling

We obtained the highest superconducting transition temperature of SmFeAsO_1-xF_x at 58.1 K by using low temperature synthesis with slow cooling. We found that the low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces a high level of fluorine into oxygen site. By using this method, impurity phases are not formed up to x 0.16, and they gradually increase above x 0.16. However, the resistivity of the normal state continues to decrease and T_c onset gradually increases with increasing fluorine concentration even when x > 0.16. Therefore, it is suggested that the fluorine is gradually introduced into oxygen site above x 0.16.