Compressibility and phonon spectra of the ternary graphite compound KHgC4

Room-temperature measurements of the $c$-axis compressibility and longitudinal phonon dispersion ($\overrightarrow{\mathrm{q}}\parallel c$) are reported for the stage-1 ternary graphite compound KHg${\mathrm{C}}_4$. Both the direct measurement and the small-$q$ slope of the LA branch give $K=4.4\mathrm{}\pm 0.1\times {10}^{-12\mathrm{}}$ ${\mathrm{cm}}^2$/dyne, twice the value for the stage-1 binary K${\mathrm{C}}_8$. Phonon energies are generally lower than in K${\mathrm{C}}_8$, and the dispersion is well represented by a second-neighbor force-constant model based on the Herold four-layer repeat unit, although the experimental evidence for one of the predicted optic branches is tenuous. All of the results point to much softer interlayer interactions in KHg${\mathrm{C}}_4$ relative to K${\mathrm{C}}_8$, consistent with other experiments which indicate that the $c$-axis charge distribution in the ternary is more metallic (or less ionic) relative to the corresponding binary compound.