Structural Phase Transition Of Layered Van Der Waals Material In₂Se₃ Under High Pressure

The high-pressure structure and the phase transitions of the In₂Se₃with three different structures（the 3R-αstructure with rhombohedral R3m,theβ′structure with monoclinic C2/m and the 2H-αstructure with hexagonal P6₃mc）are investigated by the high-pressure X-ray diffraction（XRD）and the high-pressure Raman spectra.The key results are listed as follows:（1）The Raman spractra show that the the 3R-αphase transforms toβ’phase at 0.6 GPa.The Raman peak for A_g⁴mode shifts toward the low frequency significantly at 11.2 GPa,and A_g³merge with the peak for B_g²at 12.6 GPa,which implys that a phase transition occurs at11.2 GPa and finishes at 12.6 GPa.The XRD results confirm the phase transition from 3R-αphase toβ’phase at 1.2 GPa and the phase transition from theβ’phase to the rhombohedral R-3mβstructure.The volume collapse only 0.44%during theβ’toβphase transition,suggesting a second order phase transition.（2）The high-pressrue XRD spectra show a phase transition from the monoclinic C2/mβ’phase to the rhombohedral R-3mβstructure at 11.5 GPa,with the 1.6%volume collapsion.The high-pressure Raman spectra show the red shift of A_g⁴Raman mode shifts at pressure ranging from 8.8 GPa up to 15.2 GPa,confirm this phase transition initialing at 8.8 GPa and finishing at 15.2 GPa.（3）The high-pressure XRD spectra indicate a phase transition from the hexagonal P6₃mc2H-αphase to the hexagonal P6₃/mmc 2H-βphase at 1.1 GPa,with a 6.4%volume collapsion during the transition.The Raman spectra confirm this phase transition at 0.7 GPa.It is found that the high pressure 2H-βphase cannot maintain when the sample is pressed upto 9 GPa and released the pressure to the normal atomospheric pressure.However the high pressure 2H-βphase become stable at ambient condition when it is pressed upto 15 GPa.