Spiral chain structure of high pressure selenium-II′ and sulfur-II from powder x-ray diffraction

The structure of high pressure phases, selenium-II' (Se-II') and sulfur-II (S-II), for alpha-Se-8 (monoclinic Se-I) and alpha-S-8 (orthorhombic S-I) was studied by powder x-ray diffraction experiments. Se-II' and S-II were found to be isostructural and to belong to the tetragonal space group I4(1)/acd, which is made up of 16 atoms in the unit cell. The structure consisted of unique spiral chains with both 4(1) and 4(3) screws. The results confirmed that the structure sequence of the pressure-induced phase transitions for the group VIb elements depended on the initial molecular form. The chemical bonds of the phases are also discussed from the interatomic distances that were obtained.