  We report on the structural phase transitions in the S doped FeSe
superconductor by powder synchrotron X ray diffraction at high pressures up to
18.5 GPa under compression and decompression modes. In order to create high
quasi hydrostatic pressures, diamond anvil cells filled with helium as a
pressure transmitting medium were used. It was found that at ambient pressure
and room temperature, S doped FeSe has a tetragonal structure. Under
compression, in the region of 10 GPa, a phase transition from the tetragonal
into the orthorhombic structure is observed, which persists up to 18.5 GPa. Our
results strongly suggest that, at decompression, as the applied pressure
decreases to 6 GPa and then is completely removed, most of the sample
recrystallizes into the hexagonal phase of the structural type NiAs. However,
the other part of the sample remains in the high pressure orthorhombic phase,
while the tetragonal phase is not restored. These observations illustrate a
strong hysteresis of the structural properties of S doped FeSe during a phase
transition under pressure.
