First Principles Study Of γ-Si₃N₄ Materials

Density function theory is applicable for the many-electron system by considering a single-electron system. The parameters such as binding energy,lattice constants,bulk modules calculated by density function theory of semiconductor and metal materials have shown to be in good agreement with the experimental results. As such, the first principles study of the condensed matter materials based on density function theory is being paid more and more attention. To date, a great deal of arithmetic based on density function theory, including LCAO-TB,OPW,PWP,LAPW and LMTO, etc. has been developed. With the development of the large,high-speed computers in recent years and report of a lot of experimental data, the first principles calculation has played a more and more important role in studying physical properties of materials and designing new types of materials.Si₃N₄ is an important structural ceramic with many applications because of its high decomposition temperature, outstanding oxidation properties, good thermal stress resistance, mechanical properties and optical properties. It has been extensively applied to micro-electronics, photoelectronics, mechanics, automobile, solar cells, and tools for ceramic cutting and processing. Two stable phases of Si₃N₄,αandβ, have thus far been synthesized. The third phase of Si₃N₄ in the cubic spinel phase (γ-Si₃N₄) could be synthesized under high pressure above 15 GPa and at a temperature exceeding 2000 K. To date, much work has been focused on computer simulation to study structure and properties of such structure. Especially in recent years, first-principles calculations based on density-functional theory (DFT) have been successful in predicting crystal structures and properties of material. As such, the first-principles calculation is a good method to investigate the materials difficultly synthesized in experimentIn this thesis, we calculated the electrical structure and physical properties ofγ- Si₃N₄ at zero pressure,high pressure and with doping by the first principles calculation based on PWP. The main research work is divided into three parts:1. Electronic structures, optical and mechanical properties ofγ-Si₃N₄ have been calculated by means of plane-wave pseudo-potential method (PWP) with generalized gradient approximation (GGA) and local density approximation (LDA). The calculated values were in good agreement with experimental results. The comparative investigation revealed that the band gap calculated by GGA agreed better with the experimental value than by the other methods, while the calculated bulk modulus by LDA is comparable with that by orthogonalized linear combinations of atomic orbitals method (OLCAO). Our results indicated that the satisfactory calculation on the structures and properties ofγ-Si₃N₄ could be given by combining GGA and LDA methods.2. The pressure-dependent electronic structures and physical properties ofγ- Si₃N₄ have been calculated by means of plane wave pseudo-potential method (PWP) with GGA-PW91. Based on the calculations above, we analyzed the influence of pressure on the optical and mechanical propertiesγ-Si₃N₄, which indicates thatγ-Si₃N₄ is quite suitable for the application under high pressure.3. The crystal structures, electronic structures, and optical properties ofγ-Si₃N₄ doped with La are studied using the plane wave pseudo-potential method (PWP) based upon the density functional theory with Perdew-Wang 91 (PW91) generalized gradient approximation (GGA). The calculated band gap of dopingγ-Si₃N₄ was found to significantly decrease after introduction of La. The band structures ofγ-Si₃N₄ behave like semiconductor for low La concentration and metal for high La concentration. The calculated optical properties showed that the static dielectric constant ofγ-Si₃N₄ doped with La is much higher than that of undopedγ-Si₃N₄, so it may act as new dielectric and refractive material.