Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

The phase transition of orthorhombic sulphur alpha-S-8 to a high pressure a morphous sulphur allotrope (a-S) has been investigated by Raman spectroscop y. The conversion is found to be induced by the absorption of laser light a nd can be discussed in terms of ring opening followed by cis-trans conversi on of the dihedral angle of S-8 molecules. Laser energy and transition pres sure are correlated due to the pressure tuned red shift of the absorption e dge of alpha-S-8. The amorphous (a-S) phase is observed up to 15 GPa at las er intensities below 30 mu W/mu m(2) at 514.5 and 488.0 nm. Above this thre shold power a-S transforms into a second photo-induced phase (p-S), whose d iscrete Raman spectrum implies an ordered molecular and crystalline structu re. By further increasing pressure crystalline S-6 can be created which is found to be the dominant molecular species at pressures above 10 GPa and lo w temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is al so presented.