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 $\sqrt{3}\times 7$ superlattice structure. The CIT, which occurs only in the sevenfold direction, is a solid-to-solid transition with incommensurability $\epsilon \sim {\mathrm{}(T-342.20\mathrm{})\mathrm{}}^{0.50\pm 0.02}$. The relative displacements of successive harmonics are accurately predicted by a sharp domain-wall model with $\frac{4\pi }{7}$ phase shifts. The in-plane coherent ${\mathrm{Br}}_2$ regions exceed 10 000 Å in both commensurate and incommensurate phases.

• Received 30 July 1982

DOI:https://doi.org/10.1103/PhysRevLett.49.1427