Calculations Of BC₇, SiCN And CaSi₂ Based On First-principles At High Pressure

Discovering more new semiconductor materials has become a very important part in material science and technology because of their essential functions in fundamental science and technological applications. The semiconductor materials are widely used in the national defense, manufacturing, and even everyday life. Superhard material is an important kind. As we know about the materials field that we have found some kinds of natural existing superhard material though experiments. Diamond is the hardest material ever known. However its function is limited because it is easy to react with oxygen and ferrous metals. Another reason is that it is very expensive to synthesize this kind of material. c-BN is also superhard material, but it is very hard to be synthesized in normal environment, it needs high temperature and pressure. So to find out more superhard material is essential.In this paper, we used the method of first-principles calculating the mechanical properties of BC₇ 、 Si CN and CaSi₂ structures（including lattice constant, crystal structure, thermodynamic properties, etc.）, and then analyzed their propertities at high pressure. Finally, electrical characteristics were analyzed to obtain the conclusions of the materials, it is useful for the development of materials and provides a more complete theoretical basis for the scientists. This helps us find out the suitable candidate materials and provide some theoretical help to the scientists. We first introduce the relevant research background and theoretical basics about what we calculated. Most of the work we do is on the basis of first-principles calculation method. In our paper, the Castep module of Material Studio software was applied to calculate the structural, mechanical and electronic properties of the structures.In this paper, we mainly studied BC₇ which belongs to superhard materials. Then we also studied the three structures of Si CN which including t-Si CN, h-Si CN and o-Si CN. In the last chapter, we analyzed the propertities of 2H-CaSi₂, EuGe₂-CaSi₂. The first step is to calculate the elastic constants. Then according to the judgment we could know the thermal stability and mechanical stability of the material. The value of G/B determines whether the material is brittle. The information of its bonding and covalent bond is also revealed, Next we calculated their anisotropy. BC₇ is brittle and has a big anisotropy The related parameters include bulk modulus, shear modulus, Poisson’s ratio and Young’s modulus. We also draw the curves of different parameters which changes with increasing pressure. t-Si CNand h-Si CN is easy to compress along a axis than c axis. GIBBS software is used to obtain the thermodynamic properties of the structures, such as the Debye temperature, thermal expansion coefficient and so on. Finally we calculated the band structure and density of states, according to the graph it is found that the new superhard material has a certain metallicity and strong covalent bond. In the last chapter, the development and prospects of materials science computing is summarized.