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

Abstract

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 86 GPa, 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 β-Po phase at 153(3) GPa, the same transition is found in Se at pressure of around 80 GPa.

Keywords:

• High pressure
• Crystal structure
• Elements
• Incommensurate modulation
• Covalent bonding

Acknowledgements

We acknowledge our colleagues M. Somayazulu and P. Dera for help in conducting experiments and discussions. We also thank V. F. Degtyareva for helpful discussions. This work and HPCAT is supported by DOE-BES, DOE-NNSA, DOD-TAKOM, NSF, NASA, and the W.M. Keck foundation. The authors acknowledge financial support from NSF through grant EAR-0217389. The Advanced Photon Source is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. W-31-109-Eng-38.