Phase transitions in graphite-halogens

X-ray diffraction and differential scanning calorimetry (DSC) were used to study the phase transitions in graphite-halogens (Br₂, ICl). In graphite-Br₂, a reversible transition associated with the melting of the ordered intercalate layers into liquidlike layers was observed at 373.7 K, and a transition corresponding to the change of streaks to spots in the (0001) zone electron diffraction pattern on cooling was observed at 226 K. Moreover, a transition at 277 K has been tentatively interpreted as being due to a phase transition associated with the stacking order of the intercalate layers. In addition, in graphite-Br₂, the first observation was made of a phase transition involving the shifting of various X-ray diffraction peaks in various directions at ∼340 K. This transition is tentatively interpreted as being due to a commensurate-incommensurate phase transition. In graphite-ICl, reversible transitions were observed at 307 and 314 K. The 314 K peak is associated with the melting of the ordered intercalate layers into liquidlike layers; the 307 K peak is due to a phase transition associated with the stacking order of the intercalate layers.