Study Of Basic Properties Of Polymeric Nitrogen And Micromolecule Silicon Nitride Under Ultra High Pressure By First Principles Method

Nitrogen is the seventh element at the pe riodic table. N itrogen molecule is the simple diatomic molecule. High pressure s can greatly modify the structures, chemical bonds, properties of the nitrogen. High external pressure destabilizes the triple molecular bonds and leads to various covalent polymeric nitrogen forms. The polymeric nitrogen will be a high energy density material(HEDM). Many researches for polymeric nitrogen under high pressure have been carried out. Furthermore, the researches for micromolecule nitrogen-rich compounds under high pressures are very interesting. Silicon nitrogen(Si3N4) is a high-performance engineering material with high mechanical strength and hardness in combination with high thermal stability and good resistance to corrosion and wear.The structural, electronic, thermodynamical, mechanical and dynamical properties of polymeric nitrogen and micromolecule silicon nitride compound under different pressures are investigated using first-principles calculations based on the density functional theory, and the module CASTEP of the software Materials Studio is used.(1) The structures and the stabilities of the polymeric nitrogen. Six new metastable polymeric nitrogen structures P21/m, C2/c, P212121-500, Pnnm, P21 and C mc21 are predicted by the USPEX at a pressure range from 100 to 600 GPa. The thermodynamical, mechanical and dynamical stabilities of polymeric nitride for the predicted structures have been extensively investigated. The results suggest that the thermodynamic and dynamic properties of C mc21 are both stable, showing C mc21 is competitive structure. And the electronic property show Cmc21 does not have metal ic property.(2) The structures and the stabilities of the silicon nitride. The paper chooses α-Si3N4(P31c), β-Si3N4(P63/m), c-Si3N4(Fd-3m), δ-Si3N4(P3), wII-Si3N4(I-43d), P6 and P ’6. The thermodynamical, mechanical and dynamical stabilities of silicon nitride for the proposed structures have been extensively investigated under 0-100 GPa. The results suggest that Fd-3m, I-43 d and P 6 satisfy the conditions of stabilities and are competitive structures. And the electronic properties show these seven structures are all insulators.(3) The mechanical properties of the silicon nitride. P31 c, P63/m, Fd-3m, I-43 d and P 6 are chosen to study the mechanical properties of the silicon nitride.The results suggest the five Si3N4 polymorphs are all not superhard materials, the c-Si3N4 polymorph is the hardest material and its Vickers hardness is 38.85 GPa, lower than 40 GPa. The stronger bond covalent Si-N bonds existing in wII-Si3N4 result in a harder wII-Si3N4 than α-, β-, and P 6-Si3N4. And the largest valence electron density and iconicity of c-Si3N4 lead to the hardest c-Si3N4 phase among the five phases.The results are helpful to understand the pressure- induced metallization, molecular dissociation and phase transition from high-symmetric structure to complex structure in the other molecular systems.