Theoretical Study Of The Structure And Properties Of Several New Materials

With the rapid development of the high-performance computers and the theories of the computational technology,the computational materials science becomes an interdisciplinary subject which is independent of but cooperates with the experimental science.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:1.Ab initio study of the electronic and optical properties of the IVA-V and IVB-V group compounds.IVA-V compounds have attracted great attentions in the last decade due to their important applications.C₃N₄,Si₃N₄ and Ge₃N₄ are well known for their high bulk moduli and wide band gaps to be used as the high-performance engineering materials.Recently,some research groups investigated the structural properties and bulk moduli of C₃P₄,Si₃P₄,Ge₃P₄ and Sn₃P₄ using the first principles calculations,successfully to predict that the most stable phase of those compounds is the pseudocubic structure.On the other hand,the IVB group nitrides with the 3:4 stoichiometric ratio have the Th₃P₄ structure.In this thesis,the electronic structures and optical properties of pseudocubic A₃P₄(A=Si,Ge,and Sn) and cubic M₃N₄(M= Ti,Zr,and Hf) are studied based on the first-principles density functional theory. Results show that the pseudocubic A₃P₄ have indirect band gaps,and the electronic and optical properties of the A₃P₄ compounds with the pseudocubic phase are nearly isotropic.Meanwhile,the cubic M₃N₄ also have indirect band gaps.The complex dielectric and energy loss functions are also calculated based on the band structures. The effective number of the valence electrons of A₃P₄,which contributes to the interband optical transitions,decreases with increasing atomic numbers of atom A, while M₃N₄ have the opposite trends.Both compounds have large value of the static dielectric constants,rendering them the potential applications in the future microelectronic devices.2.Theoretical and experimental investigations of optical properties of Ge₂Sb₂Te₅ (GST) for the multi-state optical data storage.In present days,computers and numerous multimedia applications require denser and faster memories,and the phase-change materials become one of the most promising candidates for the re-writable optical disc productions.Among most phase-change materials,Ge₂Sb₂Te₅ is the most distinguished one owning to its good thermal stability,high crystalline efficiency under high temperature,and large number of re-writable times.The industry society takes advantage of the differences of the electric and optical properties between the amorphous and the cubic states of GST to store and read the data.In this thesis,the experiments and computations are performed to investigate the electric and optical properties of the amorphous,the cubic,and the hexagonal states of GST.The band structure and the dielectric functions predict that the cubic GST has properties of the semiconductor while the hexagonal one has higher conductivity.The optical properties studied by both the first-principles calculations and the experiments show that,the phase transition between the cubic and the hexagonal states is promising to be used in the phase-change optical disc as the multi-state storage media.3.Experimental and Theoretic studies of local structures of the liquid Al₆₀Cu₄₀ binary alloy.Amorphous metallic glasses and liquid alloys are in the cutting edge of the materials research.In contrast to the crystalline systems,the amorphous or liquid metals and alloys exhibit no long-range order.However,the short-range order or local structures behind this long disorder is expected to play an important role in the properties of the amorphous or liquid materials.Experiments and ab initio molecular dynamics simulations on the molten Al₆₀Cu₄₀ have been carried out between 973K and 1323K in this thesis.The results of our study show that,the models that calculated from the ab initio molecular dynamics simulations are consistent with the experimental samples.In addition,the local structures of the liquid models are analyzed using the geometric methods,which show that the structures of the system are evidently changed with the temperature.With the decreasing of the temperature,the number of types of the clusters is reduced while the local structures are more ordered,and the icosahedra may play an important role in constructing the local structures of the liquid Al₆₀Cu₄₀ alloy.