Elasticity of carbon allotropes. III. Hexagonal graphite: Review of data, previous calculations, and a fit to a modified anharmonic Keating model

The experimental data relating to the second- and third-order elasticity and the zone-center optic modes of hexagonal graphite are reviewed and some amendments proposed. A modified Keating model involving three sets of interactions, one planar and two interlayer, has been developed. The harmonic parameters, four planar and seven interlayer, have been fitted by least-squares procedures to five second-order elastic constants, five zone-center optic-mode frequencies and two assumptions relating to internal strain. The anharmonic parameters comprise three planar and three interlayer ones. They have been fitted to the pressure derivatives of the five second-order constants and of three of the optic-mode frequencies. The full spectrum of inner elastic constants and internal strain tensors is given, the composition of the second- and third-order elastic constants is exposed, and the corresponding elastic compliances calculated. A pressure-induced phase transition is correctly predicted at around 16 GPa.