Two Infrared Detection Materials And Of Mg <sub> 2 </ Sub> The Feh <sub> 6 </ Sub> Electronic Structure And Optical Properties Of The Theory

This thesis presents the fundamental of the computational materials science and its primary research methods, including the ab initio computations and the molecular dynamics simulations, as well as the theoretical way to obtain the structural properties, the mechanic properties, and the optical properties of the crystal and the non-crystal materials. The thesis is arranged as the following parts:First, the electronic structures and optical properties of the direct-band-gap alloys Hg1-xCdxTe (x=0.250,0.375,0.500,0.625,0.875 and 1.000) were calculated with local spin density approximation (LSDA) based on density functional theory (DFT). MCT alloys are the leading material on infrared detection and it has been the critical point for research because of its unique advantages of tunable band gaps, high electronic mobility, and wide temperature adaptation. The lattice optimization was performed at first and the results satisfy the Vergard's law. With the increase of Cd, the lattice constant decreases linearly. There are similar band structures with only different band gaps in different Cd density alloys and the valence top and conduction bottom are both onΓpoint. There are three main absorption peaks for the dielectric function making blue-shift with the increase of Cd, indicating that the conduction band of the alloy lifting as Cd density increases leading to the band gap increasing. The static dielectric function decreases satisfying a parabolic equation with the increase of Cd. The calculation results of the energy loss function and effective electronic density indicate that there are only few electrons taking part in the interband transition.SnxGe1-x is another material with tunable band gap and high electronic mobility. Therefore it is a prospective candidate to replace theⅢ-Ⅴcompounds in the realm of optoelectronics. Moreover since the material can be compatible to current Si based technology, it is widely interested. In the second part of the paper, the electronic structures and optical properties of the direct band alloys SnxGe1-x (r=0.000,0.042, 0.083,0.125,0.167, and 0.208) were calculated using the generalized gradient approximation (GGA) based on the density functional theory (DFT). The theoretical lattice constants that increase with the increase of Sn are in consistence with the Vergard's law and the experimental values. The band structures show that the materials are the direct band gap semiconductor for the specific five Sn intensities and the band gap decreases with the increase of Sn satisfying the experimental parabolic equation. Then the optical properties of the alloys were calculated and the dielectric function has two main peaks that can be interpreted with the interband transition. The results of the energy loss function tell that the bulk plasma frequency decreases with the increase of Sn but the static dielectric constant increases as Sn increases. The results give conduction to the application of the material.Mg2FeH6 is the hydride with the highest hydrogen volume storage and the lowest cost amongst all hydrides. The third part of the paper introduces its electronic structure, optical and elastic properties results performed using the first-principles calculations. The lattice optimization was performed at first in consistence with the experimental one. It indicated that the crystal is the direct-band-gap material with the band gap of 1.94eV in accordance with the experiment. After that, the dielectric function was calculated and analyzed based on the electronic structure. There is a sharp peak at 57.35eV on the energy loss function, where is the bulk plasma frequency. The elastic constants, Young's modulus, and Poisson's ratio were calculated at the end of the part.