Carbon nitride:Ab initioinvestigation of carbon-rich phases

Abstract

We have examined the potential energy hypersurfaces for the carbon-rich phases of carbon nitride, CN and C 3 N, and discovered low-energy structures different from those reported previously. Trends in the preferred local bonding environments have been analyzed as a function of nitrogen content. For each composition, several structures with similar energies were found, but they have very different equilibrium volumes; the structure produced during synthesis will strongly depend on the preparation conditions. When low densities are favored, conjugated planar-ring structures with sp 2 hybridized carbon are most likely to be formed. These structures are similar to those suggested as potential photocatalytic materials. At high pressures, the preferred structures contain three-coordinate nitrogen and sp 3 hybridized carbon, including the ␤-InS structure, which we predict to be the thermodynamically preferred structure for CN under positive hydrostatic pressures. This structure has a moderately high bulk modulus with a lower formation energy than ␤-C 3 N 4 and so should be more easily synthesized.

Ternary silicon germanium nitrides: A class of tunable band gap materials

“…Neither the conduction nor the valence band energies change significantly. This decrease in the band gap becomes much greater for x > 1 3 when germanium occupies octahedral sites, with the energy of the conduction band decreasing significantly with increasing germanium content. For x = 0.17 with germanium in an octahedral rather than tetrahedral site, there is a large decrease in the band gap; there is also a significantly larger increase in the unit cell volume in this case, suggesting that germanium occupation of octahedral sites more significantly affects the degree of overlap of neighboring atomic orbitals and hence the conduction band energy compared with germanium occupation of tetrahedral sites.…”

“…We have recently investigated in detail the crystal structures of a range of binary group-14 nitrides and phosphides, [1][2][3] including predictions of many structures that are, as yet, unreported experimentally. We established that for many compositions a structure derived from the β-InS structure is low in energy for the 1:1 stoichiometry.…”

Enhanced tunnel transport in disordered carbon superlattice structures incorporated with nitrogen

“…Finally, at a very high N%, dt N reduces drastically although there is a small change of dt p , which explains that a stabilized C À N structure can be achieved in C-N films. 34,[37][38][39][40] There are proposals of formation of a stable (crystalline) structure of C À N for high N%. 34,[37][38][39][40] There could be a different hypothesis of dt N vs. dt p , but we wanted to include a general trend, i.e., dt N increasing with dt p and at some stage the behavior is just the opposite.…”

“…However, at a very high N concentration disorder decreases sharply as if the sp 2 C structures stabilize. 22,27,28,[37][38][39][40] To resolve this complicated problem, we propose a model of correlated disorder (case (B)) where the distortion of carbon sp 2 structures is correlated with the distortion of incorporated nitrogen. The N doping centers experience some distortion due to interaction with the C atoms.…”

Elastic anisotropy and shear-induced atomistic deformation of tetragonal silicon carbon nitride

“…1 The prediction of hexagonal b-C 3 N 4 2,3 with extraordinary hardness has led to a great deal of experimental works synthesizing this novel CN material with different approaches. [4][5][6][7][8][9][10] However, the growth of b-C 3 N 4 with crystal sizes large enough to enable measurement of its properties has not been achieved so far. In contrast to the b-C 3 N 4 , three different stable polymorphs of Si 3 N 4 (a-, b-, and c-Si 3 N 4 ) [11][12][13][14] have been well-characterized with properties that make them useful for applications in which wear resistance is required at high temperatures and in a corrosive environment.…”