Changes of phonon dispersion in graphite at high pressure

Lattice dynamics of graphite at high pressures is studied by inelastic scattering of neutrons using anvils technique. Pressure dependence of frequencies for hexagonal-axis-polarized phonons in the strongly anisotropic layered crystal lattice of graphite has been measured up to 60 kbar. It is shown that the pressure effect, resulting in a contraction of interlayer distances, gives rise to a monotonous hardening of the measured frequencies. An investigated longitudinal acoustic branch does not change its sinusoidal shape under pressure while the continuous evolution from quasi-two-dimensional to three-dimensional behavior is found for a transverse acoustic branch. It is pointed out that the observed higher rate of pressure variations of the lattice dynamics parameters as compared to the structural anisotropy can reflect changes of the crystal potential in graphite related to an available high-pressure phase transformation from the layered to more isotropic lattice.