Crystal structure of sulfur and selenium at pressures up to 160 GPa

Using advanced in situ X-ray diffraction techniques at high pressures and temperatures, we have resolved the long-standing problem of the phase transition sequence of sulfur in its non-metallic state. Our data show that there are only two phases of sulfur stable between 1.5 GPa and pressure of metallization of 86GPa, S-II with triangular chain structure and S-III with novel squared chain structure. The same squared chain structure is formed in the heavier group-VI element Se at pressures of 20 GPa and temperatures of 450 K. Our X-ray diffraction data on metallic phases of sulfur above 83 GPa show that the S-IV phase has an incommensurately modulated monoclinic structure, the same as recently reported modulated structures of Te-III and Se-IV. S-IV is shown to transform to primitive rhombohedral beta-Po phase at 153(3) GPa, the same transition is found in Se at pressure of around 80 GPa.