High-pressure x-ray and Raman studies of rubidium and cesium graphite intercalation compounds

Systematic pressure-dependence studies have been performed on $M{\mathrm{C}}_8$ and $M{\mathrm{C}}_{12n}$ ($M=\mathrm{R}\mathrm{b},\mathrm{C}\mathrm{s}$ and $n=2, 3, 4$) graphite intercalation compounds. Pressure-induced staging phase transitions similar to that which had been previously reported for K${\mathrm{C}}_{24}$ by Clarke et al. were found in these high-stage graphite intercalation compounds ($n\ge 2$). The $\overrightarrow{q}\perp c$ x-ray experiments revealed the formation of a 2×2 superlattice under hydrostatic pressure from the disordered intercalant layers at ambient conditions. The Raman spectra from $\mathrm{Rb}{\mathrm{C}}_{12n}$ ($n=2, 3, 4$) gave clear evidence for the pressure-induced staging transition by showing a change in the Raman intensities of the bounding (1605 ${\mathrm{cm}}^{-1\mathrm{}}$) and interior (1582 ${\mathrm{cm}}^{-1\mathrm{}}$) $E_{2g}$-like phonon peaks with pressure. In addition, the compressibilities $k_d$ of Cs${\mathrm{C}}_8$ and Rb${\mathrm{C}}_8$ were measured as (1.56±0.05) and (2.42±0.12)×${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^2$/dyn, respectively. The compressibility results indicate that the interlayer-coupling strength varies drastically among alkali-graphite intercalation compounds. The pressure-dependent Raman spectra of Rb${\mathrm{C}}_8$ showed an appreciable change in the Fano resonance shape at ∼ 580 ${\mathrm{cm}}^{-1\mathrm{}}$ with pressure. The mode-Grüneisen constant ${\gamma }_{0\perp }$ of pristine graphite for the for the interlayer mode $E_{2g}$ was found to be 1.66±0.13. The rates of Raman frequency shifts with respect to pressure, $\frac{\partial {\omega }_0}{\partial P}$, were measured to be (0.70±0.12), (0.51±0.09), (0.44±0.05), (0.54±0.08) ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$ for the bounding modes of Cs${\mathrm{C}}_{24}$ and $\mathrm{Rb}{\mathrm{C}}_{12n}$ ($n=2, 3, 4$), and (0.78±0.08), (0.32±0.12) ${\mathrm{cm}}^{-1\mathrm{}}$ ${\mathrm{kbar}}^{-1\mathrm{}}$ for the interior modes of $\mathrm{Rb}{\mathrm{C}}_{12n}$ ($n=3, 4$), respectively.