A model study for the breaking of N2 from CNx within DFT

A hypothetical CN2 structure was investigated as a model to study the release of N2 from the octahedral hole of 3D carbon based ultra hard compounds, which is the most important drawback in the attempts to synthesize ultra hard compounds like C3N4 and C11N4. Full structure relaxations using DFT methods led to a structure at the energy minimum showing a significantly enlarged N---N distance of 1.34 Å compared to the molecular N2 (1.09 Å). While for small volume changes a high hardness for CN2 of 405 GPa is calculated, we found that enlargements of the cell constant lead to the release of N2 that could be followed calculating the ELF and the charge transfer within the AIM theory. The whole procedure simulates an inverted “harpoon mechanism”

In search of new candidates for ultra-hard materials: the ternary BC₃N₃ stoichiometry

Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Investigations of the geometry optimisation and of the electronic properties were carried out using pseudo potential and full potential computations in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They lead to propose a precursor (2D), a β-structure and a new ultra hard rhombohedral compound with a hardness (B0358 GPa) that reaches the range of formerly studied BC2N structures built from hexagonal and cubic diamond

Potential new candidates for hard materials within the ternary XC₃N₃ (X = B, Al, Ga) stoichiometry

Starting from formerly investigated graphitic like C3N4, selective substitution of nitrogen with boron led to model structures for the experimentally observed BC3N3 stoichiometry. Similar investigations were extended to the 2nd- and 3rd-period elements Al and Ga. Geometry optimisation and studies of the electronic properties were carried out using the pseudo-potential (VASP) method in the framework of the local density functional theory for the two and three dimensional structures (2D and 3D). They respectively lead to propose a precursor (2D), a β-structure and new ultra hard materials (3D), with hardness (B0 358 GPa) for BC3N3 and (B0 325 GPa) for AlC3N3 for the high-pressure phases

Composition-dependent structural and electronic properties of a-(Si1-xCx)3N4

The highly unusual structural and electronic properties of the α-phase of (Si1-xCx)3N4 are determined by density functional theory (DFT) calculations using the Generalized Gradient Approximation (GGA). The electronic properties of α-(Si 1-xCx)3N4 are found to be very close to those of α-C3N4. The bandgap of α-(Si 1-xCx)3N4 significantly decreases as C atoms are substituted by Si atoms (in most cases, smaller than that of either α-Si3N4 or α-C3N4) and attains a minimum when the ratio of C to Si is close to 2. On the other hand, the bulk modulus of α-(Si1-xCx)3N 4 is found to be closer to that of α-Si3N 4 than of α-C3N4. Plasma-assisted synthesis experiments of CNx and SiCN films are performed to verify the accuracy of the DFT calculations. TEM measurements confirm the calculated lattice constants, and FT-IR/XPS analysis confirms the formation and lengths of C-N and Si-N bonds. The results of DFT calculations are also in a remarkable agreement with the experiments of other authors