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

Systematic pressure-dependence studies have been performed on MC₈ and MC₁₂ₙ (M=Rb,0ex0exCs and $n=2, 3, 4$) graphite intercalation compounds. Pressure-induced staging phase transitions similar to that which had been previously reported for KC₂₄ by Clarke et al. were found in these high-stage graphite intercalation compounds (n≥2). The →q⊥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 RbC₁₂ₙ ($n=2, 3, 4$) gave clear evidence for the pressure-induced staging transition by showing a change in the Raman intensities of the bounding (1605 cm^-1) and interior (1582 cm^-1) E2g-like phonon peaks with pressure. In addition, the compressibilities kd of CsC₈ and RbC₈ were measured as (1.56±{}0.05) and (2.42±{}0.12)×{}10^-12 cm²/dyn, respectively. The compressibility results indicate that the interlayer-coupling strength varies drastically among alkali-graphite intercalation compounds. The pressure-dependent Raman spectra of RbC₈ showed an appreciable change in the Fano resonance shape at {~} 580 cm^-1 with pressure. The mode-Gr\"uneisen constant γ_0⊥ of pristine graphite for the for the interlayer mode E2g was found to be 1.66±{}0.13. The rates of Raman frequency shifts with respect to pressure, ∂ω₀∂P, were measured to be (0.70±{}0.12), (0.51±{}0.09), (0.44±{}0.05), (0.54±{}0.08) cm^-1 kbar^-1 for the bounding modes of CsC₂₄ and RbC₁₂ₙ ($n=2, 3, 4$), and (0.78±{}0.08), (0.32±{}0.12) cm^-1 kbar^-1 for the interior modes of RbC₁₂ₙ ($n=3, 4$), respectively.