Theoretical strength and cleavage of diamond

The theoretical strength of diamond has been calculated for the [100], [110], and [111] directions using a first principles approach and is found to be strongly dependent on crystallographic direction. This elastic anisotropy, found at large strains, and particularly the pronounced minimum in cohesion in the {111} direction, is believed to be the reason for the remarkable dominance of the {111} cleavage plane when diamond is fractured. The extra energy required to cleave a crystal on planes other than {111} is discussed with reference to simple surface energy calculations and also the introduction of bond-bending terms.