  Tetragonal beta-FeSe obtained by hydrothermal reaction is not superconducting
and transforms to a triclinic structure at 60 K unlike superconducting FeSe
from solid state synthesis, which becomes orthorhombic at 90 K. In contrast,
tetragonal iron sulphide FeS from hydrothermal synthesis is superconducting at
4.8 K but undergoes no structural transition. Our results suggest that the
absence of superconductivity in hydrothermally synthesized FeSe may be
associated to the low-temperature structure with zigzag chains of iron atoms,
which is different from the known orthorhombic Cmme structure of
superconducting FeSe.
