Superconductivity on FeSe synthesized by various sintering temperatures

We report on the superconductivity in FeSe samples, which were prepared from Fe and Se powder, synthesized by different sintering temperatures (Ts) from 573 K to 1173 K at intervals of 100 K. According to observation of the surface of the resulting samples using a scanning electron microscope (SEM), the grain-size increases from ∼1 μm (573 K) to ∼50 μm (1173 K) with raising Ts. These polycrystalline samples with Ts ≥ 773 K, composed almost entirely of the tetragonal FeSe phase, although small amounts of the hexagonal FeSe phase were detected by X-ray diffraction (XRD) measurements. These samples indicated the negative susceptibility in magnetization, which gave estimates of the onset Tcon∼7-9K, respectively. The sample for Ts = 873 K showed the maximum value of Tcon.