Intercalate structure, melting, and the commensurate-incommensurate transition in bromine-intercalated graphite

Abstract

In situ high-resolution x-ray scattering experiments have been carried out to study in-plane intercalate structure and phase transitions as a function of temperature in a single crystal graphite host. For the case of bromine-intercalated graphite the intercalate plane has three sublattices and each sublattice has a centered (3×7) rectangular structure with four Br2 molecules per two-dimensional unit cell in the commensurate phase. The coherently ordered in-plane bromine regions exceed 10 000 in size. Above the commensurate-incommensurate transition (342.20 0.05 K), a stripe domain phase becomes established in a single domain of a sublattice along the sevenfold direction. The incommensurability as a function of reduced temperature exhibits a power law with an exponent of 0.50 0.02, confirming the existing theories. The relative shifts observed for the various harmonics are accurately predicted by a sharp-domain-wall model with 47 phase shifts. A power-law line shape is observed for the incommensurate intercalate layer, yielding values for the exponent consistent with model calculations. Results on the temperature dependence of the intensities, linewidths, and line shapes of several Bragg peaks around the melting transition are presented. The intercalate layer exhibits a continuous melting transition from a two-dimensional solid phase to an anisotropic fluid phase, occurring at 373.41 0.10 K for a stage-4 compound. © 1983 The American Physical Society.