Investigation of Structural, Electronic, Elastic, Vibrational, Thermodynamic, and Optical Properties of Mg2NiH4 And Mg2RuH4 Compounds used in Hydrogen Storage

Abstract

Structural, electronic, elastic, vibrational, thermodynamic, and optical properties of Mg2XH4 (X = Ni, Ru) compounds were investigated using density functional theory in the presence of zero temperature and pressure. Under ambient conditions, the Mg2NiH4 crystallizes in an orthorhombic type structure with space group Pccn and there are 56 atoms in its unit cell. Mg2RuH4 crystallizes in an orthorhombic type structure with space group Cmcm and contains 28 atoms in its unit cell. In addition, electronic band structure calculations were made for the orthorhombic structures of these two compounds. Mg2NiH4 is a direct band gap semiconductor of 1.3479 eV, while Mg2RuH4 is a semiconductor with an indirect band gap of 0.8768 eV. To understand whether the studied structures of Mg2XH4 are dynamically stable, phonon calculation was performed and both structures were found to be stable. In addition, the second-order independent elastic constant values, which give information about the hardness of Mg2XH4 and whether it is mechanically stable or not, were also calculated. From the elastic constant results obtained, Mg2XH4 was found to be mechanically stable in both structures. Some stiffness constants such as Bulk modulus (B), Shear modulus (G), Young's Modulus (E), and Poisson's ratio (upsilon) were also calculated from elastic constant values. According to the B/G ratio, the material was found to be brittle in both structures. It was also observed that the atoms forming the structures according to the Poisson ratio were connected by ionic bonds. In addition, thermo-physical and optical calculations of the compounds were also examined to have more information about the physical properties of Mg2XH4.