Crystal structure and phase transitions at high pressures in the superconductor FeSe0.89S0.11

• Structural phase transitions in the FeSe0.89S0.11 superconductor at high pressures up to 18.5 GPa. • Most sample part’s recrystallization into the hexagonal phase (sp. gr. P63/mmc) at decompression. • Strong hysteresis of the structural properties during a phase transition under pressure. We report on the structural phase transitions in the FeSe 0.89 S 0.11 superconductor with T C = 11 K observed by powder synchrotron X-ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. It was found that at ambient pressure and room temperature, FeSe 0.89 S 0.11 has a tetragonal structure (space group P4/n). Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure (sp. gr. Pnma) 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 (sp. gr. P6 3 /mmc). However, the other part of the sample remains in the high pressure orthorhombic phase (sp. gr. Pnma), while the tetragonal phase (sp. gr. P4/n) is not restored. These observations illustrate a strong hysteresis of the structural properties of FeSe 0.89 S 0.11 during a phase transition under pressure.