Crystal structure of the superconducting phases of S and Se

Compressed S and Se are studied by x-ray diffraction with synchrotron radiation up to 160 GPa. Phase IV of S is shown to have a body-centered monoclinic structure and is stable between 83 and 153 GPa, where it transforms 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, the same as known for Se and Te. Our study on the stability range of the modulated phase of Se (Se-IV) shows that Se-IV transforms to the beta-Po phase Se-V at around 80 GPa. Pressure dependence of the structural parameters of these high-pressure phases in S and Se are discussed in relation to their superconducting behavior.