Ammonia intercalation of stage-3 ${\mathrm{KC}}_{36}$ is found to produce a predominantly single-phase stage-2 graphite intercalation compound with stoichiometry K(${\mathrm{NH}}_3$${)}_{4.5}$${\mathrm{C}}_{36}$. As a result of the ammonia intercalation, the free-carrier plasma frequency ${\omega }_p$ is observed to decrease by 0.28 eV to a value of ${\omega }_p$=3.12 eV. This downshift in ${\omega }_p$ is consistent with the backdonation of 30% of the carbon ${\pi }^{\mathrm{*}}$ electrons to states in the intercalate layer. This result demonstrates an interesting stage dependence for this backdonation, as previous NMR and optical studies of the stage-1 K(${\mathrm{NH}}_3$${)}_{\mathrm{x}}$${\mathrm{C}}_{24}$ (x=4.1,4.4) compounds reported values in the range 15–20 %, about one-half the value found in the present study for the stage-2 compound. A narrow absorption band is observed near 1.7 eV, similar to that observed at 1.85 eV in the stage-1 K(${\mathrm{NH}}_3$) GIC’s, and identified with the presence of solvated electrons in the K(${\mathrm{NH}}_3$${)}_{4.5}$ intercalate layers.

• Received 13 July 1988

DOI:https://doi.org/10.1103/PhysRevB.38.10878