α-feas-free SmFeAsO_1-xF_x by low temperature sintering with slow cooling

We obtained amorphous-FeAs-free SmFeAsO_1-xF_x using a low temperature sintering with slow cooling. SmFeAsO_1-xF_x is sintered at 980 °C for 40h and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO_1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96%. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950, 900, 850, 800, 750, and 700 °C. It is found that fluorine is substituted not only at the maximum heating temperature but also during the cooling process. The low temperature sintering with slow cooling is very effective to obtain a homogeneous SmFeAsO_1-xF_x with high fluorine concentration.