PRESSURE-INDUCED STRUCTURAL PHASE-TRANSITION IN SULFUR AT 83-GPA

The pressure-induced phase sequence of sulfur up to 90 GPa has been studied by an angle-dispersive x-ray-diffraction method. The orthorhombic sulfur (S-I) with S8 cyclic molecules transforms to a high-pressure crystal phase (S-II) between 6 and 55 GPa. The second structural phase transition takes place at 83+/-2 GPa. The highest-pressure phase (S-III) can be explained as a base-centered orthorhombic lattice. This lattice is isomorphous to that in the metallic high-pressure phase of selenium. Metallization for sulfur reported by Luo et al. is ascribed to the second transition.