【作者】 钟燕；

【导师】 周怀营； 刘立斌；

【作者基本信息】 中南大学 ， 材料学， 2012， 博士

【摘要】 由轻质元素构成的复杂配位氢化物,如氨基／亚氨基化合物、硼氢化物、铝氢化物等由于具有较高的理论重量储氢密度和价格低廉等优点,被认为是最有应用前景的储氢材料之一,受到了广泛的关注。本文采用从头算进化结构预测与第一性原理计算相结合的方法,以现有实验或理论研究成果为基础,对几种配位氢化物的高压晶体结构、电子结构、晶格振动等特性进行了系统深入的理论研究,主要得到如下结论。(1)对LiNH2的高压结构稳定性进行了详细研究。随着压力的增加,发现高压β-LiNH2(正交晶系,空间群为Fddd)和γ-LiNH2(正交晶系,空间群为P21212)比基态α-LiNH2(正交晶系,空间群为I-4)更稳定。β→γ结构转变发生在10.7GPa,与实验观察完全吻合。进一步分析LiNH2所有竞争相的结构特性、电荷密度分布以及计算的声子态密度表明高压相LiNH2中存在着明显的N…H-N氢键作用。氢键的存在拉伸了N-H键长,使得N-H极性共价键被弱化,可能会有利于γ-LiNH2中氢的释放。(2) NaNH2的高压行为研究表明：在0-20GPa压力范围内NaNH2将经历两次压力诱导结构转变。基态α-NaNH2首先在2.2GPa转变为空间群为P21212的正交β-NaNH2,然后在9.4GPa进一步转变成空间群为C2/c的单斜γ-NaNH2,且分别伴随产生11.3和1.2%的体积收缩。预测的NaNH2高压结构转变顺序(Fddd→P21212→C2/c)与同族氨基化合物LiNH2的高压行为(I-4→Fddd→P21212)非常相似,表明压力诱导结构转变朝着对称性降低的方向进行可能是碱金属氨基化合物的一个共性。压力作用会使NaNH2中[NH2]-氨基离子的方向有序化且同时缩短相邻两个[NH2]-氨基离子分子间N…H距离,从而致使在β-和γ-NaNH2中出现N…H-N氢键。同时,计算结构分析表明β-ANaNH2中近线性排列的氢键作用应强于γ-NaNH2中弯曲排列的氢键作用。(3)探讨了Li2NH的高压晶体结构、电子结构和基态结构的晶格振动特性。总能计算表明：当压力上升到3.2和14.8GPa时,Li2NH将发生Pbca→Cmcm和Cmcm→P63mc结构转变,并且压力致使Li2NH结构有序化尤为明显。详细的电子态密度和电子局域化函数研究揭示Li2NH的三个稳定结构都具有非金属特性,Li+与[NH]2-亚氨基离子间是离子键作用,N与H之间则是共价键合作用。Li2NH的基态Pbca结构不存在虚频,动力学稳定。计算声子态密度主要由两个频率带构成,低于844cm-1的低频带来自于Li、N、H三种原子晶格振动模式的叠加,而高于3172cm-1的高频带则主要归结于N-H的伸缩振动。(4)进化模拟结构预测重现了Tekin等最新报道的LiBH4的基态结构(空间群为Pnma;晶格常数为α=8.564A,b=4.352A,c=5.753A)。总能计算发现尽管Tekin等理论预测的Pnma结构与Soulie等实验测定的Pnma结构之间存在显著差异,但两结构之间的总能差值仅有～1meV/f.u.,表明LiBH4的基态结构对能量变化并不敏感,这或许是实验或理论研究LiBH4基态晶体结构一直存在争议的重要原因之一。当压力上升至10GPa,LiBH4从基态的Pnma正交结构转变为四方的P-421c结构,并且预测的转变压力与最近的实验测定结果吻合较好。电子结构分析表明：LiBH4的基态结构和高压相都属于典型的离子型化合物,具有较大的能隙(>5eV),而其中[BH4]’四面体单元内的B与H之间存在较强的极化共价键合作用。更多还原

【Abstract】 Complex hydrides composed of light weight elements such as amides/imides, borohydrides and alanates have attracted great attention as one of the most promising materials for hydrogen storage due to their high theoretical gravimetric density and low price. Base on the previous experimental or theoretical results, we have investigated systemically and deeply the high-pressure crystal structure, electronic structure, and vibrational properties of some light complex hydrides using first-principles calculations and the ab-initio evolutionary structure prediction simulations. The main conclusions obtained from our theoretical calculations are given as below:(1) A detailed study of the high-pressure structural stability in LiNH2is performed. Two high-pressure polymorphs,β-LiNH2(orthorhombic, Fddd) and;γ-LiNH2(orthorhombic, P21212), are found to be more stable than the ground-state a-LiNH2(orthorhombic,1-4) with increasing pressure. The β→γstructural transition occurs at10.7GPa, which is in good agreement with experimental observation. Further analysis of the structural properties, charge density distribution, and calculated phonon density of states for all the competing phases shows the N-H…N hydrogen bond obviously emerges in the high-pressure γ-LiNH2. The existence of hydrogen bond elongates the N-H bond length and weakens the N-H polar covalent bonds within NH2groups, which may be beneficial to the hydrogen release from γ-LiNH2.(2) The investigations on the high-pressure behavior of NaNH2show that NaNH2undergoes two pressure-induced structural transitions in the pressure range from0to20GPa. The ground-state a-NaNH2first transforms into an orthorhombic β-NaNH2with space group P21212at2.2GPa and then into a monoclinic y-NaNH2with space group C2/c at9.4GPa, accompanied by the volume reductions of11.3%and1.2%, respectively. The predicted transition sequence(Fddd→P21212→C2/c) of NaNH2is very similar to the high-pressure behavior of its counterpart LiNH2(I-4→Fddd→P21212), suggesting that the trend of lowing symmetry induced by high-pressure may be common in alkali metal amides. Pressure not only leads to an increase in the orientational order of the [NH2]-amides anions in NaNH2but also simultaneously shortens the intermolecular N-H distances between the two neighboring [NH2]-amides anions, which yields the emergence of the N-H…N hydrogen bond in β-and γ-NaNH2. Our calculated results also show that the strength of the approximately linear N-H… bond in β-NaNH2is expected to be stronger than that of the bent N-H…N bond in γ-NaNH2.(3) The high-pressure crystal structure, electronic structure, and lattice dynamical properties of Li2NH are investigated systematically. The total-energy calculations show that the Pbca→Cmcm and Cmcm→P63mc structural transitions occur respectively at3.2and14.8GPa and the obvious pressure-induced ordering appears under compressing Li2NH. A detailed study on the density of states and electron localization function reveals that all the three stable structures of Li2NH are nonmetallic with the typical ionic bonding between Li+cations and [NH]2-imide anions and the strong N-H covalent bonding prevails in each [NH]2-unit. No imaginary phonon frequencies are found in the ground-state Pbca phase, indicating this structure is dynamically stable. The calculated phonon density of states for the Pbca phase consists of two frequencies bands:the low-frequency band below844cm-1being ascribed to the motion of the Li, N and H atoms and the high-pressure band above3172cm-1resulting from the N-H bond stretching.(4) The latest ground-state structure of LiBH4(space group:Pnma; lattice parameters:a=8.564A, b=4.352A, c=5.753A) reported by Tekin et al. has been reproduced by our evolutionary structure prediction simulations. However, our total-energy calculations show that although the Pnma structure predicted by Tekin et al. exhibits a significant structural difference in comparison with the experimental Pnma structure determined by Soulie et al., the difference in their calculated total energies is only about1meV/f.u.. The ground-state structure of LiBH4is insensitive to the variation of energy, which may be one of the important reasons why the structural data of the ground-state phase of LiBH4still remains controversial in experimental or theoretical studies. The ground-state orthorhombic Pnma structure transforms into the tetragonal P-421c structure with increasing pressure to10GPa, which is in good agreement with the recent experiment. The further analysis on electronic structure shows that the ground-state Pnma and high-pressure P-421c structures belong to the typical ionic compounds with a large band gap (>5eV) and the strong polarized covalent exists between B and H atoms in the tetrahedal [BH4]-units.更多还原