La₃MgNi₁₄电子结构及（110）面吸氢的第一性的理研究

【作者】 邵爽；

【导师】 张敏刚；

【作者基本信息】 太原科技大学 ， 材料学， 2012， 硕士

【摘要】 新型La-Mg-Ni系储氢合金具有优异的储氢特性，较高的放电容量使其成为改善负极充放电容量材料的希望，本课题对La-Mg-Ni系储氢合金的一个主要相La₃MgNi₁₄进行了系统的研究，为更好的改善La-Mg-Ni系储氢合金的储氢特性、对其储氢进行微观机理的研究提供了帮助。本课题利用基于第一性原理平面波赝势的计算法的CASTEP软件模块，从电子结构的研究入手，首先根据La占据4f位置的不同，建立了不同的结构模型，对各个结构模型进行结构的优化，并对优化后的模型计算了能量以及生产热，从而得到了体系的最稳定结构。后对稳定结构的性质如能带结构、态密度和布居分析等加以研究，探讨Ce₂Ni7型La₃MgNi₁₄储氢合金的微观电子结构，结果表明：Ce₂Ni7型La₃MgNi₁₄的结构更趋向于形成La分散的S1结构；对体系的能带结构和态密度研究表明，体系具有典型的金属性，在结合中以金属键为主；体系在费米能级附近的能态主要由Ni的₃d轨道贡献。La原子的贡献主要在－16eV的5p轨道及－₃₂eV的6s轨道。体系的最低能态位于费米面以下－4₂eV，由Mg原子的₂p轨道提供；布居分析为原子间的成键提供了一个客观判据。由键布居及原子布居分析表明，La₃MgNi₁₄中Ni-La键为离子键，Mg-Ni以共价键为主兼有离子键，Ni-Ni键为共价键。四种结构中Ni-Ni键均为共价键，Ni-La键均为离子键。当结构为中心对称结构（S₂、C₂结构）时Mg-Ni键既有共价键也有离子键，由于离子键的存在使结构的稳定性降低。当结构为轴对称结构（S1、C1结构）时Mg-Ni键只有共价键存在，从而使结构的稳定性高于中心对称结构，且S1结构的共价键平均键长₂.6767小于C1结构的₂.69₃4，从而使S1结构更加的稳定。研究氢在金属表面的吸附过程能够为研究氢在金属表面吸附机理提供帮助。本课题利用基于第一性原理平面波赝势的计算法的CASTEP软件模块，对氢原子在La₃MgNi₁₄晶体的（110）面吸附进行了研究。研究发现： La₃MgNi₁₄晶体的（110）面态密度与体相态密度的在大体上较为相似，但（110）面的态密度带隙宽度均有所减小，导带在费米能级以上部分有向低能级漂移的倾向；表面处的活性也要高于体相的活性；氢原子在各吸附位置上的稳定距离在₃^<sub>3.5之间；当氢原子在La₃MgNi_（14）晶体的（110）面吸附时，氢原子吸附在La-Ni长桥位时的结构最为稳定，Mg-Ni长桥位次之，Mg-La短桥位的稳定性最低。更多还原

【Abstract】 The new La-Mg-Ni system hydrogen storage alloy has excellent hydrogenstorage properties, It is hopeful to improve the cathode charge and dischargecapacity with its higher capacity. In this paper, a major phase La₃MgNi₁₄of theLa-Mg-Ni hydrogen storage alloy have been investigated, that provide help toimprove hydrogen storage properties of La-Mg-Ni hydrogen storage alloy andresearch hydrogen storage microscopic mechanism.In this paper, the CASTEP module which first-principles plane wavepseudo-potential method based on density functional theory was used, it wasstudied from electronic structure, Firstly, build different structures according toLa occupy the4f position, optimize the structures and calculate energy andformation heat, so as to get the most stable structure of the system. After that,investigate band structure, density of states and population of the stable structureas well as the microscopic electronic structure of Ce₂Ni7-type La₃MgNi₁₄hydrogen storage alloy. The results show that: Ce₂Ni7-type La₃MgNi₁₄tends tobe S1structure, in which lanthanums are scattered to each other; Band structureand density of states shows that the system have typical metallic, in combinationwith metallic bonds. The energy state of the system near Fermi level was mainlycontributed by Ni₃d orbital. The contribution of the La atoms provided by-16eV5p orbital and-₃₂eV6s orbital. The lowest energy state of the system wasobserved at the-4₂ev below the Fermi level, which contributed by Mg₂p orbital.Population analysis provides an objective criterion for the bonding between atoms. The bond population and atomic population analysis showed that: InLa₃MgNi₁₄system, The bond of Ni-La is ionic bond, Mg-Ni dominated both theionic bond, covalent bond. Ni-Ni is covalent bond. In all of the four structuresNi-Ni is covalent bond, Ni-La is ionic bond. When the structure is centrosymmetric structure （S₂, C₂）, Mg-Ni bond is both covalent key and ionic bond,the stability of the structure was reduced because of the presence of ionic bonds.When the structure is an axisymmetrical structure （S1, C1）, Mg-Ni bond is onlycovalent bond exists, so the stability of the structure higher than thecentrosymmetric structure, S1structure with the average bond length of covalentbond average₂.6767is less than the C1structure of₂.69₃4, so that the S1structure is more stable.Research on metal surfaces of the hydrogen adsorption process is to be ableto study the hydrogen adsorption on metal surfaces. In this paper, the CASTEPmodule which first-principles plane wave pseudo-potential method based ondensity functional theory was used, the hydrogen atom in La₃MgNi₁₄ Crystal（110） surface adsorption was studied. The results show that: The DOS（density ofstates） of La₃MgNi₁₄（110） surface is similar with bulk DOS, however, theband gap width of （110） surface DOS has reduced, the conduction band aboveFermi level have a tendency to drift to the low energy level; Surface activity isalso higher than the activity of the bulk; The stability of distance of hydrogenatom adsorbs at various position between₃to₃.5; when the hydrogen atomsadsorbed in the crystal La₃MgNi₁₄（110） surface, the most stable structure is hydrogen atoms adsorbed on the La-Ni Longbridge, with the Mg-Ni long bridgesite next and Mg-La short bridge site minimum.更多还原