A theoretical study of the electronic and optical properties of the graphite intercalation compound K(NH3)4C24

The electronic properties of the ternary potassium-ammonia graphite intercalation compound K(NH3)(4)C-24 are studied using generalized gradient-corrected density functional theory, following recent theoretical and experimental studies on the microscopic structure and dynamics of intercalation compounds of similar composition. Localized electronic states in the intercalant K-NH3 layer, whose existence has been postulated in order to explain peculiar features in the optical absorption of K(NH3)(x)C-24 compounds, with x similar or equal to 4 and, ultimately, the occurrence of a 2D metal-nonmetal transition at x similar or equal to 4.3, are shown to originate from the overlap of diffuse K-NH3 hybrid orbitals enveloping discrete K(NH3)(4) clusters. This gives rise to a highly inhomogeneous conduction band extending in the inter-cluster region, which percolates throughout the crystal in narrow winding channels bounded by H atoms. The estimated frequency-dependent complex dielectric function is found to reproduce with remarkable accuracy the experimental spectra. In particular, we can establish a direct link between the intercalate state and the occurrence of the 1.85 eV peak in the epsilon(2)(omega) spectrum, a well-known feature specific to K-NH3 graphite intercalation compounds. Issues related to the actual occupation of the intercalate state (depending on the degree of charge back-transfer from the C sheets to the K-NH3 intercalate) are discussed within the limitations of a conventional electronic structure density functional approach.