Crystal structures of superconducting phases of S and Se

Compressed S and Se are studied by x-ray diffraction with synchrotron radiation up to 160 GPa. The S-IV phase is shown to have a body-centered monoclinic structure and to be stable between 83 and 150 GPa on pressure increase, where it transform to S-V with a primitive rhombohedral $\beta$-Po structure. Observation of the modulation reflections in S-IV up to 135 GPa shows that its crystal structure is incommensurately modulated, as recently reported for Se and Te. The modulated body-centered monoclinic phase of Se, Se-IV, is shown to transform to the $\beta$-Po phase Se-V at around 80 GPa. Se-V transforms to a body-centered cubic phase at 140 GPa in accordance with previous studies. Pressure dependence of the structural parameters of these high-pressure phases in S and Se are discussed in relation to their superconducting behaviour.