First-principles Study Of The Single-bonded Cubic Phase Of Nitrogen

In this paper, we systematically study a new class of single-bonded nitrogen materials—single-bonded cubic form of nitrogen, namely cubic gauche nitrogen (cg-N), by first-principles calculations with density functional theory implemented in the CASTEP code. The single-bonded cubic form of nitrogen was synthesized experimentally at the temperature above 2000K and pressure above 115GPa until recently. Single-bonded cubic form of nitrogen has unique properties and enormous potential applications. However, the basic physical properties of cg-N, such as mechanical and optical properties, still remains much less understood in theory and experiment. So, in this paper, we study the basic physical properties including three parties, i.e. mechanical properties, optical properties, lattice dynamics and correlative thermal properties of cg-N.In the first party, we aimed at study the mechanical properties of single-bonded cubic form of nitrogen. Firstly, we discussed the stability of single-bonded cubic form of nitrogen at different pressure. We found that single-bonded cubic form of nitrogen is a meta-stability structure at pressure of 0 Pa, while it's stable at pressure above 60 GPa. Then, the internal parameter x and band gap are discussed at the pressure range from 0 to 600 GPa. With increasing pressure, internal parameter x of cg-N is decreasing. Perhaps, it's implying that the cg-N will transform to single-cubic form at extremely high pressure. Interestingly, the band gap is also decreased with increasing pressure. We find that a pressure-induced phase transformation from nonmetal to metal takes place at the pressure of 680 GPa. Finally, we systematically study the mechanical properties of cg-N and attainted some basic mechanical constants, such as bulk modulus, Young modulus etc. Additionally, the elastic constant of cg-N was calculated and the pressure dependence of the elastic anisotropy A=2C44 / (C11?C12) was discussed. Compared to diamond and cubic boron nitride, we think cg-N is a stiff solid.In the second party, we investigated the band structure and optical properties of cg-N. Band structure of cg-N and corresponding density of states are discussed at firstly. We find that conduction bands originate mainly from the contribution of 2p orbits of the N, while the major contributions to valence bands between -25.61 and...