Raman response of stage-1 graphite intercalation compounds revisited

We present a detailed in situ Raman analysis of stage-1 KC${}_8$, CaC${}_6$, and LiC${}_6$ graphite intercalation compounds to unravel their intrinsic fingerprint. Four main components were found between 1200 cm${}^{-1}$ and 1700 cm${}^{-1}$ and each of them were assigned to a corresponding vibrational mode. From a detailed line-shape analysis of the intrinsic Fano lines of the $G$- and $D$-line response we precisely determine the position (${\omega }_{\mathrm{ph}}$), line width (${\Gamma }_{\mathrm{ph}}$), and asymmetry ($q$) from each component. The comparison to the theoretical calculated line width and position of each component allows us to extract the electron-phonon coupling constant of these compounds. A coupling constant ${\lambda }_{\mathrm{ph}}<0.06$ was obtained. This highlights that Raman active modes alone are not sufficient to explain the superconductivity within the electron-phonon coupling mechanism in CaC${}_6$ and KC${}_8$.

Figure 1

(a) Optical modes of graphite. (b) Raman spectra from KC${}_8$, CaC${}_6$, and LiC${}_6$ taken with 568-nm laser at room temperature and low laser power of 0.25 mW.

Figure 2

$D$- and $G$-line analysis for stage-1 GICs and Raman response of their laser-induced deintercalated phases. The four components that can be identified in the $G$-line shape are: A${}_{1g}$ mode between 1250 and 1350 cm${}^{-1}$, E${}_{2g_2}$ mode $\sim$1510 cm${}^{-1}$, E${}_{2g_1}$ mode at $\sim$1547 cm${}^{-1}$, and stage-2 $G$ mode $\sim$1560 cm${}^{-1}$. In the upper panel (a) we can observe that KC${}_8$ exhibits a strong contribution from the E${}_{2g_2}$ with a broad Fano behavior, which is the fingerprint for the intrinsic line of a stage-1 compound.[27] In the lower panel (b) we present the same crystals analyzed in the upper panel but deintercalated. We can clearly observe the decrease of the E${}_{2g_2}$ mode, concomitant to a strong increase of the $G$ mode assigned to the XC${}_{24}$ graphitic face $\sim$1600 cm${}^{-1}$.

Figure 3

Calculated ${\gamma }^{\mathrm{EPC}}$ [Eq. (1)] for different GICs as function of their width ${\Gamma }_{\mathrm{ph}}$. Black stars ($★$) correspond to experimental values from Refs. 23, 37, and 38. The dashed line represents the approximation of ${\Gamma }_{\mathrm{ph}}\approx {\gamma }^{\mathrm{EPC}}$. The red dots show our calculated EPC, which are in better agreement to the expected approximation to the ${\Gamma }_{\mathrm{ph}}$ values.