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.