First Principles Study Of The B-c-n System

New materials are of primary importance in modern science and technology. The research of super hard materials is an interesting research subject in the field of materials science. Owing to similarities between diamond and cubic boron nitride (c-BN), such as their closely lattice parameters, high melting temperatures, large bulk modul, and similar thermal-expansion coefficients, B-C-N ternary compounds have aroused the curiosity of the researchers, and they have many potential uses in the area of super hard materials and semiconductor materials. Recently, with the progress in density functional theory (DFT) and its numerical methods, the first-principles calculation based on DFT has become a routine method for condensed matter physics, quantum chemistry and material science. In this dissertation, the first-principle has been investigated about B-C-N ternary system, which has significant values of theory study and extensive foreground of applications.In the first chapter, the structures and properties of boron, carbon and nitrogen elements and their compounds of B-C-N systems have been introduced. Furthermore, the recent theory study and synthesized method have been summarized. In the second chapter, the basic concept of DFT and the method of quantum chemistry calculations have been introduced.Siesta (Spanish Initiative for Electronic Simulations with Thousands of Atoms) is both a method and its computer program implementation, to perform electronic structure calculations and ab initio molecular dynamics simulations of molecules and solids. In this dissertation, the structure of 64-atom diamond was studied by SIESTA. The energy of diamond was calculated by changing the lattice parameters and the stable structure of system was found. In addition, the bulk elasticity modul was calculated. It was found that the calculation result and the experiment value were consistent.In the fourth chapter, the first-principles calculation based on DFT have been carried out using the Generalized Gradient approximation (GGA). Theexchange-correlation functional of Perdew-Wang (PW91) was used. The ultrasoft pseudopotential was used to deal with the reciprocity of electron and ion. In the calculation process, the total energy and stress calculations used an eight-atom zinc-blende-structured unit cell, with a 310.00ev energy cutoff, and a 7x7x7 k-point grid. The results of first-principles calculations show that all seven kinds of BC2N have positive formation energy and tend to form diamond and c-BN phase structures. The I-BC2N and II-BC2N of tetragonal crystal system was comparatively stable due to the lowest total energy and least unit cell volume. In addition, it was found that the bulk and the shear modulus of I-BC2N> II-BC2N were less than diamond and more than c-BN, that meant their hardness was between them. All BC2N structures owned the speciality of semiconductor by analyzing their energy band and electrons density of state curves.