First-Principles Studies Of High Pressure Phase Of Solid Nitrogen

The study of homonuclear diatomic molecular solids such as H2,N2,O2,I2,Br2,and others under high pressure, is one of the fundamental problems of condensed matter physics. The high-pressure behavior of N2 is of fundamental importance to physics, chemistry, life science and so on. Studies of the structure of nitrogen are also of general theoretical importance in giving insights into the common regularities of structure and polymorphism of various substances featuring molecular crystals. Nitrogen is the unique elemental diatomic molecule with triple covalent bonds. At low pressures, atoms are strongly covalently bonded in the molecules which, in turn, weakly interact with each other. At high pressures, intermolecular and intramolecular interactions become comparable, ultimately leading to dissociation of molecules.In this paper, we systematically study the nitrogen molecular crystal and atomic crystal structure property and the phase transformations based on the density functional theory for first principle calculation.Firstly we calculate the high-pressure molecular phasesεandζ. Found that all the parameters can describe theεphase and all the calculation results accord with the experimental results. The X-ray diffraction spectra of theεphase we calculated very accord with the Eremets'result in the intense peak, the number of peaks and the position of peaks. The structural parameters and the X-ray diffraction spectra are in good agreement with experimental results from several research groups. All this indicate theεphase parameter maybe not correct. The simulation research in theζphase has been done, but the cell parameters and the X-ray diffraction spectra are very different with the experimental results, so the conclusion is theζphase structure is wrong, to fully understand this phase, further experimental research and theoretical study from other aspects should be performed.In the second part investigation, we introduced what we can found in the literature about the polymeric structure of nitrogen in the high pressure. The methods of geometry optimization, molecular dynamics simulation, uspex and the random-search method have been used to obtain these structures.A new phase of nonmolecular nitrogen has been suggested by first-principles density functional theory simulations at high pressures. This phase has zigzag chains similar to the BP, A7 and LB phases, but the zigzag chains in the PP phase are connected by two atoms with double bonds with Pmna symmetry, which we designated as the PP phase. Properties of PP are presented in comparison with other polymeric phases of nitrogen discussed previously. The calculated enthalpy versus pressure reveals a similarity between the PP and CH phases. The structure of PP, initially obtained at high pressures, remained qualitatively unchanged when reoptimized at different pressures in the range 0–360 GPa, indicating that, along with other polymeric phases of nitrogen, PP is also metastable for a wide range of pressures. In order to explore the stability of this phase, we have performed two tests similar to those used by Mattson et al, Both tests show that the structure returns to the initial PP structure after optimization.Secondly, we predicted another new phase of nonmolecular nitrogen using density functional theory and the plane-wave pseudopotential method. This phase was come from the molecular phaseε, they all have the same symmetry R-3C,so we designated as the RE(related epsilon) phase. This phase has 48 atoms in the unit cell, the atom connect the neighbor one with the single-bond. 8 atoms gathered together to form one cluster.The isosurface figure of RE andεphase shows the difference of molecular crystal and the atomic crystal. We carry out detailed studies on the electronic structure of the RE phase similarity ofεphase, viewing from the spatial charge density distributions of these two structure, we confirm the same conclusion about molecular crystal and the atomic crystal. In addition, the study of band structure of RE indicates that this structure is insulators with indirect band gap. The occupied states consist of two major groups. The lowest-energy states consist of hybridized covalent bonding states. Integration over these states shows that there are only three bonds of this type per atom. The second lowest group of occupied states, consists of nonbonding states, which relate to a lone-pair orbital. Integration of these non-bonding states yields two electrons per atom.