Relative stabilities, bulk moduli and electronic structure properties of different ultra-hard materials investigated within the local spin density functional approximation

Abstract

The relative stabilities of the following phases: graphite-C₃N₄, α-C₃N₄, β-C₃N₄, cubic-C₃N₄ and pseudo-cubic-C₃N₄ have been determined using density functional theory in its local density approximation. In particular three calculational methods were imployed: augmented spherical wave, linear muffin-tin orbitals and full-potential linearized augmented plane-wave. The main objective of this work was the prediction of the hardness for a series of C₃N₄ phases (α, β, cubic and pseudo-cubic) as well as for the cubic BN (c-BN) structure. To this purpose total energy calculations were performed for different unit cell volumes and the resulting data were fitted to a polynomial function in order to determine the equilibrium lattice constants (a_eq and c_eq), bulk moduli (B₀) and pressure derivatives (B₀′). Even though the different methods do not produce comparable energy trends, all methods are in agreement in predicting equilibrium volume, bulk modulus and pressure derivatives. Further, for the graphite-based structures the influence of hybridisation on the chemical bonding and stability is discussed in terms of the site projected densities of states as well as the crystal orbital overlap population. For the hexagonal and orthorhombic phases the electronic properties are also discussed by means of a density of states analysis.