Studies Of Phase Transition In Solid Nitrogen Under High Pressure And Theoretical Prediction Of The Superhard Carbon Nitride Materials

As we all know that nitrogen is the simple element at the fifth group elements at the periodic table. Nitrogen molecules are the simple diatomic molecule, to research the properties of it can make helpful to understand the other simple molecular systems. High pressure can greatly modify the structures, chemical bonds, properties of the nitrogen at the normal pressure. Furthermore, nitrogen molecules exist in nature widely, the properties of nitrogen molecules under high pressure is very interesting for it can provide understanding to planetary physics and interior earth.Nitrogen usually consists of molecules in which two atoms are strongly triple-bonded under ambient conditions, but high external pressure destabilizes the triple molecular bonds and leads to various covalent polymeric nitrogen forms, in which each nitrogen atom is bonded to three nearest neighbours by single or double covalent bonds. Due to the uniquely large energy difference between single and triple bonds, the polymeric nitrogen will be a high energy density material (HEDM). Owning to the intriguing properties of the polymeric nitrogen in HEDM, plenty of structures have been suggested. Besides cubic gauche (cg) which was synthesized by Eremets et al at high pressure (110 GPa) and high temperature (2000 K), none of the theoretically proposed structures have been obtained experimentally. Up to now, almost all of the candidate polymeric nitrogen structures are proposed from the aspect of thermodynamics, their mechanical and the dynamical stabilities are not studied yet. The metallic nitrogen has attracted a lot of interests. Therefore, we detailed study the pressure induced molecular dissociation, metallization of nitrogen and superhard carbon nitrides.(1) The structures and the stabilities of the polymeric nitrogen. In this study, the stabilities of polymeric nitrogen for the previously proposed structures have been extensively investigated using ab initio calculations based on density functional theory. We present a comparison between the recently found phases and other proposed nitrogen phases. Our results indicate that at pressure lower than 47 GPa, the molecular phasesε-N₂ remain as the lowest-enthalpy structures for solid nitrogen. Consideration of the stable criteria and enthalpies, our studies show that Cmcm, rcg, cg, BP, Pba2 and P2₁2₁2₁ are competitive structures. Finally, we suggest the new phase transition sequence with increasing pressure from the molecular phaseε-N₂to cg at 47 GPa, to Pba2 at 170 GPa, and then to P2₁2₁2₁ at 307GPa.(2) Prediction of metallic nitrogen at high pressure. The pressure-induced transformation of molecular crystals into nonmolecular states, which is expected to be accompanied by a decrease in the band gap and eventually metallization, has attracted a lot of interests. Pressure-induced metallization has been observed in many systems, especially, in some elemental solids, such as sulfur, oxygen and iodine etc., which are very important for understanding and researching metallic hydrogen. Nitrogen, one of the most important elements, is also expected to become metallic states when compression is sufficiently strong. However, metallic nitrogen at high pressure is still not found yet. Two new metallic polymeric structures of nitrogen, Pnnm and Cccm, are found by means of first-principles density functional theory and a random structure-searching method. It is firstly shown that the transition behavior of nitrogen from insulator to metal starts at a pressure of approximately 450 GPa at 0 K. The Pnnm phase becomes energetically favorite with respect to cubic gauche (cg) at 363 GPa, and then transforms to the Cccm structure at 884 GPa. Electron - phonon coupling calculations suggest that the Pnnm crystal possesses superconductivity. The stability of these two phases is explored, showing for the first time that they are stable structures of nitrogen exhibiting metallic properties. Electron-phonon coupling calculations suggest that Pnnm is a superconductor and with a superconducting critical temperature of 0.089 K at 600 GPa. We hope that this study will stimulate the search for new metallic nitrogen.(3) Design the superhard carbon nitride materials. Materials with high hardness (H_v≥40 GPa) are of considerable fundamental interest and practical importance because of their excellent mechanical and thermal properties, such as great hardness, wear resistance and high melting point. We suggest a novel potential superhard material, single-crystals of a new carbon nitride phase with all-sp³ bonds, possessing a cubic P2+13 symmetry (8 atoms/cell, labeled by cg-CN) which is similar to cubic gauche nitrogen (cg-N) by first-principles calculations. This compound is metallic, different with most of the other superhard insulators or semiconductors. The Vickers hardness of cg-CN is 82.56 GPa, if we considered the negative effect of metallic component on hardness, the hardness is also 54.7 GPa, very harder than any other metallic materials. It is found that a three-dimensional C-N network is mainly responsible for the high hardness. Both elastic constant and phonon-dispersion calculations show that this structure remains dynamically and mechanically stable in the ranges from 0 to 100 GPa. Furthermore, all the proposed candidate structures of carbon nitride with 1:1 stoichiometry were explored and found that only cg-CN is the most favorable stability crystal structure. Formation enthalpies calculations demonstrate that this material can be synthesizable at high pressure (12.7 GPa ³6.4 GPa).The stabilities, metallic and superconductivity of polymeric nitrogen have been extensively investigated using ab initio calculations based on density functional theory. From the candidate structures of polymeric nitrogen we suggest a novel potential superhard material. Our 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.