Thermal expansion and lattice anharmonicity of alkali-graphite intercalation compounds

We have measured the $c$-axis and $a$-axis thermal expansion of several alkalimetal—graphite intercalation compounds by means of x-ray scattering between 10 and 300 K. The $c$-axis thermal expansion depends strongly on the in-plane mass density of the alkali-metal atoms and the stage of the compound, and is largest for K${\mathrm{C}}_8$. The $a$-axis thermal expansion of K${\mathrm{C}}_8$ and K${\mathrm{C}}_{24}$ is negative in the considered temperature region and the room-temperature coefficient is twice as large as for pristine graphite. We have analyzed the $c$-axis data in terms of a one-dimensional quasiharmonic approximation, which yields Grüneisen parameters 2 or 3 times as large as those for graphite.