Structural phase transitions and the equation of state in SnSe at high pressures up to $2$ Mbar

The crystal structure of tin selenide SnSe has been studied under quasihydrostatic compression at pressures up to $205$ GPa created in diamond anvil cells at room temperature. Two structural phase transitions have been detected at and $P\approx 2.5$ $32$ GPa. The former phase transition is continuous from the GeS-type structure (space group $Pbnm$) to the TlI-type structure (space group $Cmcm$). The phase transition to the CsCl-type cubic structure (space group $Pm\overline{3}m$) occurs at $32$ GPa and is accompanied by a stepwise decrease in the volume of the unit cell by $7\%$. The pressure dependence of the specific volume of the unit cell at room temperature has been constructed up to $205$ GPa.