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

TL;DR

This study investigates the structural phase transitions of S-doped FeSe superconductor under high pressure using synchrotron X-ray diffraction, revealing a tetragonal-to-orthorhombic transition and hysteresis effects during decompression.

Contribution

It provides detailed high-pressure structural data on S-doped FeSe, highlighting phase transition behaviors and hysteresis not previously characterized.

Findings

Tetragonal structure at ambient pressure

Orthorhombic phase appears around 10 GPa

Recrystallization into hexagonal phase during decompression

Abstract

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.