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α-FeAs-free SmFeAsO1-xFx by low temperature sintering with slow cooling

We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 hours 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 SmFeAsO1-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 °C, 900 °C, 850 °C, 800 °C, 750 °C 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 SmFeAsO1-xFx with high fluorine concentration.