Pressure Tuning of Bromine Ionic States in Double-Walled Carbon Nanotubes

The energetic, vibrational, and electronic properties of bromine polyanions intercalated in double-walled carbon nanotubes (DWCNTs) were investigated by resonance Raman and X-ray absorption spectroscopies under high pressure conditions up to values close to 25 GPa. The mechanical resistance of the bromine intercalated DWCNTs is known to be affected by the presence of bromine polyanions, which induces uniaxial constraints in the interstitial regions of DWCNT bundles, thus leading to lower collapse pressure as compared with pristine DWCNTs. An upshift of bromine Raman frequencies concomitant to changes in the local structure of bromine atoms takes place at about 15 GPa, which is the pressure where the studied DWCNT intercalated bundles collapse. This suggests a differentiation of bromine polyanions interaction depending on the local curvature and the arrangement of the collapsed carbon structures. Supported by atomistic calculations, we suggest that those chains of Br2_- and Br5_- tend to dissociate to form Br-Br₃-Br complexes or elongated Br5_- polyanions in the interstitial regions of DWCNTs bundles after the nanotube collapse phase transition takes place. Chains of Br3_- could be found stable even after DWCNT collapse. Those transitions appear to be reversible.