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

The relative stabilities of the following phases: graphite-C3N4, alpha-C3N4 , beta-C3N4, cubic-C3N4 and pseudo-cubic-C3N4 have been determined using de nsity functional theory in its local density approximation. In particular t hree 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 serie s of C3N4 phases (alpha, beta, cubic and pseudo-cubic) as well as for the c ubic BN (c-BN) structure. To this purpose total energy calculations were pe rformed for different unit cell volumes and the resulting data were fitted to a polynomial function in order to determine the equilibrium lattice cons tants (a(eq) and c(eq)), bulk moduli (B-0) and pressure derivatives (B-0'). Even though the different methods do not produce comparable energy trends, all methods are in agreement in predicting equilibrium volume, bulk modulu s and pressure derivatives. Further, for the graphite-based structures the influence of hybridisation on the chemical bonding and stability is discuss ed in terms of the site projected densities of states as well as the crysta l orbital overlap population. For the hexagonal and orthorhombic phases the electronic properties are also discussed by means of a density of states a nalysis.