Investigation of the phase diagram of selenium by means of Raman spectroscopy

We report a detailed and systematic Raman scattering investigation of the high pressure - high temperature (350–620 K, 0–20 GPa) phase diagram of elemental selenium starting from its trigonal structure (Se-I). Phase transitions to the tetragonal Se-VII phase were identified by the measured Raman pattern along several isotherms, upon pressurization and depressurization. A wide hysteresis region (∼10–15 GPa) has been observed at 350 K, gradually reducing to about 3 GPa at 620 K. These results extend previous scattered determinations of the phase diagram and shifts the Se-I Se-VII transition line up to about 3 GPa at lower pressures and ∼150 K at lower temperatures.