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

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 E2g(1) rigid-layer shear mode at 44 cm^-1 and the E2g(2) in-plane mode at 1579 cm^-1 increase sublinearly under pressure with initial pressure coefficients of 4.8(5) and 4.7(3) cm^-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 E2g(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 E2g(1) mode we obtain d ln {ω}/3d ln c=1.4(1), which indicates a stronger anharmonicity of this mode.