Concentration dependence of optical reflectivity and 2γ-angular correlation distribution of positron annihilation in donor- and acceptor-intercalated graphite

Abstract

Optical reflectivity studies of interhalogen- and alkali-intercalated graphite have shown different behavior of the interband transitions at energies jv ∼ 5 eV in donor and acceptor compounds. The corresponding structure in the reflectivity curve is shifted to lower energies compared with pristine graphite, with increasing donor concentration. By contrast, no shift occurs in acceptor compounds, reflecting the rigidity of the graphitic π-bands. Lithium-graphite intercalation compounds exhibit additional features in the reflectivity spectra which were not observed in heavy alkali compounds, indicating a more complex electronic structure. In particular, shoulders appear in the metallic reflectivity edge. Measurements of the 2γ-Angular Correlation Distribution (ACD) of the positron annihilation radiation in donor and acceptor graphite intercalation compounds are presented. In donor compounds, the ACD is strongly dependent on the stage number n, approaching the ACD of pure graphite for large n. In contrast to that, the ACD of acceptor intercalated graphite is independent of n. The Coulomb repulsion of positrons from donor intercalant layers offers opportunities to study the charge distribution in these materials. Since, in acceptor compounds, the positrons are found to be localized at anion sites, ACD studies may prove useful in identifying the anionic species in acceptor intercalants.