Dimensional change as a function of charge injection in graphite intercalation compounds: A density functional theory study

Charge-induced dimensional changes as a function of charge injection are predicted for graphite intercalation compounds by the means of geometry optimization with density functional theory. A plane wave basis set with periodic boundary condition and ultrasoft pseudopotentials are used in the generalized gradient approximation. Agreement with experiment is obtained for the calculated strain-charge relationship for the graphite sheet and for the calculated inter-layer distances of KC8, XC6 (X=K, Li, Ca, and Ba), C6Cl, C18PF6, and C24AsF6. When the interlayer distances are increased from the optimized values up to 10 A, the charge transfer and basal plane strain in KC6 decrease whereas they remain unchanged in C6Cl. The results of the full calculations, which include specific counterions, are consistent with the results from jellium calculations (in which the counterions are represented by a uniformly distributed background charge). This shows that hybridization between the ions and the graphite sheets is not significant. All of the investigated models provide calculated charge-strain relationships for graphite that are asymmetric with respect to the sign of charge transfer. Expansion occurs for negative charge injection and contraction occurs for positive charge injection, due to the electron-hole symmetry breaking effect of second neighbor antibonding interactions.

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