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 amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S-8 molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of alpha-S-8. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30 mu W/mu m(2) at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. 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 low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.