Electronic contribution to the c-axis elastic constants of hexagonal crystals

The effect of c-axis elastic strain on the first Brillouin zone for hexagonal crystals is determined. The electronic contribution to c-axis elastic moduli is estimated for graphite, a semimetal, and boron nitride, an insulator, using the parabolic and other band models. It is found that a large contribution to the elastic constants can arise from the effect of strain on the electronic energy, arising from a redistribution of electrons within the strained Brillouin zone, or from changes in the bulk electronic properties. It is shown that experimental c-axis moduli for graphite can be described empirically if both ion-ion and electron band energies are considered. (7 fig, 29 references)