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

An incommensurate composite phase with a monoclinic host-guest structure, designated $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{IV}$, is found between the previously known $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{I}$ and $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{II}$ phases, and an incommensurate to incommensurate transition between phases II and IV is described. The monoclinic $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{IV}$ phase is observed on both pressure increase between 8.2 and $9.0\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and on pressure decrease between 8.0 and $6.9\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The full structure of $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{IV}$, 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, $\mathrm{As}\text{\ensuremath{-}}\mathrm{III}$, is shown to have the same monoclinic composite structure.