Crystal and electronic structure of FeSe at high pressure and low temperature.

We have investigated the high-pressure crystal and electronic structures of superconducting FeSe by high-resolution synchrotron powder X-ray diffraction and density functional theory (DFT) calculations at ambient and at low temperatures down to 8 K. Ambient nuclear resonant inelastic X-ray scattering (NRIXS) experiments were performed on FeSe to understand the partial phonon density of states (PDOS) of the high-pressure phases. On the basis of our experimental results and DFT calculations, we demonstrate a pressure-induced distortion of the low-temperature Cmma phase at around 1.6 GPa and the appearance of a high-pressure Pbnm phase. Upon increasing the pressure above 9 GPa, the orthorhombic phase becomes the major phase, and a mixed-phase region exists up to 26 GPa. The pressure-induced structural changes in this system and its connection to T(c) enhancement are discussed.