Abstract
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 ${\ensuremath{\kappa}}_{c}=1.43\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} {\mathrm{cm}}^{2}/\mathrm{dyn}$, sound velocity ${v}_{s}=5.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm/s, zone-boundary acoustic-phonon energy = 18.5 eV, and slightly higher thermal expansion ${\ensuremath{\alpha}}_{c}=66\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$/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.
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