X-ray-diffraction study of sulfur to 32 GPa: Amorphization at 25 GPa

The orthorhombic form of sulfur (${\mathrm{S}}_8$) has been studied in a diamond anvil cell to 32 GPa at room temperature by energy-dispersive x-ray-diffraction techniques using a synchrotron source. An orthorhombic-to-monoclinic phase transition was observed at about 5 GPa. The monoclinic phase could be followed until 24 GPa. Our previous optical results indicated a phase transition around this pressure. The diffraction patterns above 25 GPa showed the characteristics of an amorphous phase. This crystalline-amorphous phase change was found to be reversible with hysteresis upon decreasing pressure. The mechanism of this pressure-induced amorphization of sulfur is proposed. The equation of state was determined to 24 GPa.