Statics and dynamics of interlayer interactions in the dense high-pressure graphite compound ${\mathrm{LiC}}_2$

Reaction of Li with graphite at high pressures yields ${\mathrm{LiC}}_2$, three times as dense in Li than the ambient-pressure graphite intercalation compound ${\mathrm{LiC}}_6$. We study the stability of this unusually high Li density against Coulomb repulsion by neutron and x-ray scattering. Surprisingly, parameters which should be sensitive to interlayer interactions are quite similar to the ${\mathrm{LiC}}_6$ values: c-axis compressibility ${\kappa }_c=1.43\mathrm{}\times {10}^{-12}\hspace{0.5em}{\mathrm{cm}}^2/\mathrm{dyn}$, sound velocity $v_s=5.1\mathrm{}\times {10}^5$ cm/s, zone-boundary acoustic-phonon energy = 18.5 eV, and slightly higher thermal expansion ${\alpha }_c=66\mathrm{}\times {10}^{-6}$/K (300–450 K). Moreover, both compounds are yellow in reflection, implying that the delocalized charge densities are comparable. These and other results are consistent with partially covalent in-plane Li-Li bonds, i.e., partial charge transfer to the graphene layers, as opposed to the more conventional ionic picture which applies to most alkali-graphite intercalation compounds.