  We investigated the magnetization and crystal structure of FeSe
polycrystalline samples with various grain sizes. For the samples with large
grains, a large magnetic critical current density Jc was observed. For the
samples with small grains, superconductivity signals were not observed;
instead, magnetization hysteresis, which is not a result of superconductivity,
was observed. In the X-ray diffraction pattern for the samples with small
grains, broad additional peaks were observed, corresponding to the formation of
the non-superconducting (monoclinic) Fe-Se phase at the FeSe grain surface. The
crystal structure instability at the grain surface would be the reason for the
low superconducting properties of the Fe-chalcogenide polycrystalline wires
investigated thus far.
