【作者】 杨峰；

【导师】 彭平；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2007， 硕士

【摘要】 采用第一原理的CASTEP软件,对顶点型Al₁₃I_n^m（n=1-12, m= -1,0,+1）团簇的成键特性和H在MgH₂表面的吸附与扩散特性进行了计算。第一原理的CASTEP软件是一种基于量子力学的专门为固体材料科学研究所设计的程序包,采用密度泛函理论（DFT）赝势平面波方法,能够很好地探索半导体、陶瓷、金属、矿物质和硅酸盐等材料的的晶体和表面性能,典型的应用包括研究材料的表面化学、结构性质、能带结构、态密度和光学性能。针对实验中发现的顶点型Al13In-团簇的稳定性因n的奇偶变化存在振荡现象,本文对顶点型Al₁₃I_n^m（n=1-12, m= -1,0,+1）团簇的能量、几何结构和电子机制进行了仔细地研究。结果表明:优化团簇结构后发现Al₁₃I_n^m团簇中的Al13部分不能保持优化前的正二十面体对称结构。结合能与能量二阶差分的计算结果表明,Al13In和Al13In-团簇在n为偶数时相对n为奇数稳定;而Al13In+团簇则在n为奇数时较稳定。垂直电子分离能（VDE）显示Al13In和Al13In-团簇在n为偶数时核外电子趋近于满壳层排布。电子态密度和电子密度差分图揭示了Al13In-团簇的稳定性变化不仅受核外的电子排布影响,而且与Al和I之间的成键特性有很大关系。在本文的第二部分,对H原子和H₂分子在MgH₂完整表面和缺陷表面的吸附以及H原子在表面层中的扩散进行了理论研究。结果表明:（1） H原子在表面的吸附分为物理吸附和化学吸附两种。与完整表面相比,缺陷表面对悬浮在其附近的H原子有更强的吸引作用。这种现象的原因是相对于前者表面上的H,后者表面上的Mg对悬浮H原子有更强的吸引。（2）H₂分子在表面的吸附只能是物理吸附。和H原子在两种表面的不同表现相似:在完整表面,和悬浮H₂分子相互作用的是表面上的H原子;而在缺陷表面,和悬浮H₂分子相互作用的是表面上的Mg原子。作用对象的不同,导致了悬浮H₂分子在真空层是否旋转。（3）对H₂分子被化学吸附到含有空位的缺陷表面进行过渡态计算,结果发现:只要克服一定的能垒,H₂分子可以以分子形式被吸附到表面,并使其中的一个H原子占据空位。但是,这种吸附结构是不稳定的。（4）表面上最容易形成空位;只要克服较小的能垒,空位可以在晶体中扩散,即H原子能够在晶体中的空位间迁移。这也使球磨中产生的空位能够迅速被H原子填充,吸氢量逐渐增大。更多还原

【Abstract】 Using CASTEP code based on first-principles, the bonding characters of on-top Al₁₃I_n^m（n=1-12, m=-1,0,+1） clusters and the sorption and diffusion of H on the surface of MgH₂ surface have been calculated. CASTEP is a state of the art quantum mechanics based program designed specifically for solid state materials science. CASTEP employs the Density Functional Theory （DFT） plane-wave pseudopotential method which allows you to perform first-principles quantum mechanics calculations that explore the properties of crystals and surfaces in materials such as semiconductors, ceramics, metals, minerals and zeolites. Typical applications involve studies of surface chemistry, structural properties, band structure, density of states and optical properties.In the first part of this paper, energetic and electronic structures of the on-top Al₁₃I_n^m （n = 1～12, m =–1,0,+1） clusters have been investigated by employing a first-principles pseudo-potential plane wave method. Several parameters such as binding energies, second differences of energy and vertical-electron detachment energies have been adopted to characterize and evaluate the structure stability of Al13In- （n = 1～12） clusters. The optimized models show that the Al13 moieties in the clusters cannot retain the original regular icosahedron structure. Results from binding energy and second difference of energy show that Al13In and Al13In- clusters with even n are more stable than those with odd n in contrast with Al13In+ clusters. The calculation of vertical-electron detachment energies （VDE） of Al13In clusters indicates that Al13In and Al13In- clusters with even n are closer to the closed shell of the Jellium model. Further analysis of electron density of states and electron density differences reveals that the enhanced stability of Al13In- clusters is not only associated with the closed shell of valence electrons but also with the bonding type between I and associated Al atoms.In the second part of this paper, the sorption of H atom and H₂ molecule on perfect and defect MgH₂ surface have been investigated by employing a first-principles pseudo-potential plane wave method. The following results are observed and interpreted: （1） According to the distance and the overlap of electron cloud, the physical sorption of H atom on the surface can be distinguished from the chemical sorption form. Compared with perfect MgH₂ surface, defect MgH₂ surface has a stronger adsorbability on the dissociative H atoms due to Mg has a stronger van der vaals forces to suspended H than H atom on the surface. （2） Unlike the sorption of H atoms on MgH₂ surface, there is only the physical sorption form for H₂ molecule. In the case of defect surface, the suspended H₂ molecule rotates to make the two H atom of it and the Mg on the surface are on a line, while the suspended H₂ molecule keep on vertical direction during the optimization of cases on perfect surface. （3） A transition state calculation of chemical sorption of H₂ molecule on defect MgH₂ surface leads to a conclusion that it is possible for the surface to trap H₂ molecule as a molecularly adsorbed states, in which H₂ molecule is forced to approach the surface and then is free to move along the surface to find the site of the defect. However, the state of the system is not stable and the H₂ molecule will dissociate from the surface automatically. （4） Closer to the surface, lower are the energies of defect formation Evan. The H atoms in MgH₂ slab can diffuse to the defects nearby just after conquering a little energy barrier.更多还原