Graphite under pressure: Equation of state and first-order Raman modes

Abstract

We have measured lattice parameters and Raman spectra of hexagonal graphite at pressures up to the structural phase transition near 14 GPa (T=300 K). The frequencies of the $E_{2g}$(1) rigid-layer shear mode at 44 ${\mathrm{cm}}^{\mathrm{-}1}$ and the $E_{2g}$(2) in-plane mode at 1579 ${\mathrm{cm}}^{\mathrm{-}1}$ increase sublinearly under pressure with initial pressure coefficients of 4.8(5) and 4.7(3) ${\mathrm{cm}}^{\mathrm{-}1}$/GPa, respectively. Using the a- and c-axis compression data, we determine the dependence of phonon frequencies on intralayer and interlayer separation. For the high-frequency $E_{2g}$(2) mode we find d ln ω/3 dlna=1.06(10), which is in the range of bond-length scaling parameters of phonon frequencies in three-dimensional covalent structures. For the low-frequency $E_{2g}$(1) mode we obtain d ln ω/3d ln c=1.4(1), which indicates a stronger anharmonicity of this mode.