Pressure-induced incommensurate-to-incommensurate phase transition in antimony

An incommensurate composite phase with a monoclinic host-guest structure, designated Sb-IV, is found between the previously known Sb-I and Sb-II phases, and an incommensurate to incommensurate transition between phases II and IV is described. The monoclinic Sb-IV phase is observed on both pressure increase between 8.2 and 9.0 GPa and on pressure decrease between 8.0 and 6.9 GPa. The full structure of Sb-IV, including a determination of the structural modulations of the host and guest components, is reported from x-ray powder diffraction data. The high-pressure phase of arsenic, As-III, is shown to have the same monoclinic composite structure.