EFFECTIVE AND DEBYE TEMPERATURES OF ALKALI-METAL ATOMS IN GRAPHITE-INTERCALATION COMPOUNDS

The vibrational energies of the alkali-metal atoms in graphite intercalation compounds have been analyzed to yield effective temperatures T parallel-to and T perpendicular-to and the corresponding Debye temperatures Kelvin parallel-to and Kelvin perpendicular-to parallel and perpendicular to the graphite planes. In this context, T parallel-to and T perpendicular-to differ from the thermodynamic temperature as they include the kinetic energy of the zero-point vibrational motion of the alkali-metal atoms. It was found that the value of T perpendicular-to at 0 K is independent on the stage and that a universal linear relation of T perpendicular-to as a function of (M(x)d)-1/2 exists (where M(x), the atomic mass of the alkali-metal atom and d, the distance between two graphene layers sandwiching an alkali-metal layer). This demonstrates that the effective force constant between the alkali-metal and the binding graphene layers, calculated per alkali-metal atom, is always the same for all compounds. By applying an "infinite-mass" correction to T parallel-to a similar linear relation between the corrected values T' parallel-to Kelvin' parallel-to, and (M(x)d)-1/2 has been found. These relations can be used for predicting T perpendicular-to, Kelvin perpendicular-to, T' parallel-to and Kelvin' parallel-to for cases for which no experimental data are reported.