Simultaneous measurement of pressure evolution of crystal structure and superconductivity in Fese$_{0.8 }$ using designer diamonds

Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on (P4/nmm) PbO type $\alpha $-FeSe$_{0.89}$ compound to a pressure of 44 GPa and at low temperatures down to 4 K using a synchrotron source and designer diamond anvils technique. At ambient temperature, a structural phase transition from the tetragonal (I4/nmm) phase to orthorhombic (Pbnm) is observed at 11 GPa and persist up to 75 GPa. The superconducting transition temperature increases rapidly with pressure in a parabolic manner reaching a maximum of $\sim $40 K at $\sim $ 11GPa. It then decreases at higher pressures. We also performed a complimentary pressure dependence x-ray diffraction simultaneously with resistance measurement at low temperatures and observe superconductivity only in the low pressure orthorhombic phase (Cmma) of $\alpha $-FeSe$_{0.89}$ Upon increasing pressure at 10 K, structural phase change from a mixed phase of orthorhombic (Cmma) and hexagonal (P63/mmc) to a high pressure orthorhombic phase (Pbnm) is observed at around 12 GPa where Tc is maximum.