First-Principles Study of Structure and Electronic Property of Eu2MgH6and Rb2MgH4
|Course||Physical and chemical|
|Keywords||Mg-based hydrogen storage alloys Structures stability Electronic structures Elastic properties Thermodynamic properties First-principle calculations|
Magnesium-based hydrides are the most promising candidates as a hydrogen storage material due to their high hydrogen storage capacities, lightweight, low cost and high abundance. The present study shows that alloying magnesium hydrides with the Rare-earth elements and/or alkaline elements can effectively improve the hydrogenation/dehydrogenation performance and make to further improvement of desorption kinetics. In the present work, first-principles calculation has been performed to study the structural, elastic, thermodynamic and electronic properties of Mg-Eu-H and Mg-Rb-H hydrides. It will provide helpful theoretical basis to design new materials and predict the properties of materials. The main contents of the dissertation are as the following:Firstly, within framework of the density functional theory, the structural and electronic properties of EuMg2and its hydride EuMg2H6have been investigated. The calculated formation enthalpies indicate that stability of EuMg2is lower than that of EuMg2H6. The computed electronic structures show that the hydride exhibits primary ionic characteristic. Furthermore, three types of Mg-H bonding also display obvious ionic feature with somewhat covalent component. Especially, Eu-f electrons have important influence on structural stability and hydrogen storage behavior, and both compounds show a strong spin polarization feature near the Fermi level owing to the localized Eu-f electrons. The thermodynamics for hydrogenation/dehydrogenation process of this system was also studied, and the favorable hydrogenation and/or dehydrogenation pathways are determined.Then, within framework of the density functional theory, the structural, elastic, electronic and thermodynamic properties of Rb2MgH4has been investigated. The optimized structural parameters are in good agreement with the experimental data. The [MgH4]2-in Rb3MgH5may tend to be more stable than Rb2MgH4due to the better symmetry of the tetrahedron. The obtained formation enthalpies indicate that stability of Rb2MgH4is lower than that of Rb3MgH5. The reaction enthalpies along possible four reaction routes for hydrogenation/dehydrogenation process are also studied, and the favorable reaction pathways are further determined. The calculated elastic constants suggest that the structure of Rb2MgH4is mechanically stable. Bulk and shear moduli as well as young’s moduli for Rb2MgH4are lower, implying that the less pronounced directional bonding in the hydride. The electronic structures exhibit Rb2MgH4is insulator and the hydride contains mixed ionic and covalent contributions. Bader’s analysis reveals the covalence of Rb2MgH4is weaker.