【作者】 王晓丽；

【导师】 崔田；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2011， 博士

【摘要】 氮元素是元素周期表中第七号元素,氮分子属于简单双原子小分子体系,对它的研究可以促进人们对其他小分子体系的认识。在压力作用下氮分子在常压下的结构、性质可以被大大的改变,会发生复杂的结构转变,呈现出一系列性质迥异的新相,从而带来很多新奇的现象和性质。另外由于氮元素在地球内部和其他星体内部也存在,所以研究其在压力下的结构和性质具有非常重要的实际意义。压力能够改变氮原子与原子之间的距离,当分子间的距离与分子内的距离变得可以相比拟的时候,就会由双原子分子转化成由单键或双键键合的聚合氮的形式。有研究表明这将是一种能量密度非常高的能源材料,从而引起科学家的广泛兴趣,建议了大量的聚合氮的候选结构,对聚合氮结构的研究目前仍然是人们对氮元素研究的热点问题之一。但这些候选结构的结构特点以及稳定性等等还没有做过统一的系统研究。氮的高压金属相也是近年来的研究热点,另外,探索新型超硬材料一直是科研工作者的研究热点。本文采用基于密度泛函理论的第一性原理计算方法对高压下的固体氮的聚合氮的稳定性、金属特性以及超硬碳氮材料进行了研究,获得了如下研究结果:（1）聚合氮的结构以及稳定性研究。我们收集了目前所有的聚合氮的候选结构,细致总结了结构特点,在不同压力点下进行了几何结构优化,给出了所有候选结构在100GPa时候的结构参数和原子位置。并对每个结构在不同压力点下的热力学稳定性、力学稳定性以及动力学稳定性进行了研究。通过计算发现,这些候选结构中sc和ch由于焓比较高,而且动力学不稳定,首先被排除在外;cw、A7和lb虽然从能量的角度看和其他结构差距不大,但动力学依然不稳定,所以这两个结构也可以排除。其余的结构cg、cmcm、bp、rcg、pba2和p2₁2₁2₁都是比较理想的候选结构。（2）超高压下金属氮结构的预测。高压除了能缩短原子间的距离还可以促进相邻电子轨道的重合程度的提高,使物质由绝缘体到金属的转化,压力导致金属化在很多元素中都曾经观察到,比如在S、O₂以及I₂中,但是氮的金属化问题目前尚不清楚。我们通过随机搜索的方法预测了两个力学稳定、动力学稳定的聚合氮的结构Pnnm和Cccm。这两个结构都是正交层状结构,每个氮原子与周围3个氮原子相连接。Pnnm相在890 GPa的时候会转化成Cccm结构。我们通过研究Pnnm的电子结构发现在450 GPa的时候会转化成金属氮,这是首次在氮元素中出现了稳定的具有金属特性的氮结构,这个结果对研究简单元素氮具有非常重要的意义,同时对于研究其他简单小分子体系的压制金属化的机理也是一个借鉴作用。进一步计算Pnnm在600 GPa时的超导转变温度,计算结果表明只有0.089 K,要比其他氮化物低很多,虽然超导转变温度比较低,但足可以说明氮在高压下也可以转变成超导体,这与同一主族的P、As、Sb等非常类似。（3）新型碳氮超硬材料的理论设计。探索新型超硬材料一直是实验和理论工作者的研究热点之一,其中最常用的方法就是替代法,我们通过对众多聚合氮的候选结构进行替代研究得到了一个具有超硬特性以及金属特性的多功能碳氮材料,它具有cubic gauche非常相似的结构,碳氮化学计量比为1:1,通过与其他5个化学计量比都是1:1的碳氮材料的对比研究发现这个结构是最稳定的。如果考虑金属特性对对硬度的负面影响,硬度仍然能够达到54.7 GPa,远远超过了其他过渡金属氮化物,与立方氮化硼的硬度相当。这是目前具有金属特性的超硬材料中硬度最大的。同时我们还分析了这个材料的超硬机理,并计算了合成条件。我们采用基于密度泛函理论的第一性原理计算方法系统研究了固态氮在高压下的聚合氮的结构特点、稳定性以及氮的金属化和超导电性的问题,并由聚合氮候选结构出发成功预测了一种具有金属特性的多功能超硬碳氮材料。这些研究结果为其他分子晶体在高压下分子解离、由简单结构向复杂结构的压致相变、压制金属化、超导转变以及超硬金属的设计、超硬机理的研究提供了重要的参考价值。更多还原

【Abstract】 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.更多还原