Abstract
An x-ray scattering study is reported of the commensurate-incommensurate transition (CIT) in single-crystal ${\mathrm{C}}_{28}$${\mathrm{Br}}_{2}$ in which the ${\mathrm{Br}}_{2}$ stage-4 intercalant has an in-plane $\sqrt{3}\ifmmode\times\else\texttimes\fi{}7$ superlattice structure. The CIT, which occurs only in the sevenfold direction, is a solid-to-solid transition with incommensurability $\ensuremath{\epsilon}\ensuremath{\sim}{(T\ensuremath{-}342.20)}^{0.50\ifmmode\pm\else\textpm\fi{}0.02}$. The relative displacements of successive harmonics are accurately predicted by a sharp domain-wall model with $\frac{4\ensuremath{\pi}}{7}$ phase shifts. The in-plane coherent ${\mathrm{Br}}_{2}$ regions exceed 10 000 \AA{} in both commensurate and incommensurate phases.
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