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 α-S₈ (monoclinic Se-I) and α-S₈ (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 14₁/acd, which is made up of 16 atoms in the unit cell. The structure consisted of unique spiral chains with both 4₁ and 4₃ 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.