Structure Design Of Several Typical B-C-N Superhard Materials

The design and synthesis of novel superhard materials have always fascinated humans for their extensive industrial applications. Since boron (B), carbon (C), nitrogen (N), and oxygen (O) can easily form short and strong covalent bonds, these light elements are still the best candidates for designing novel superhard materials. Recent years, with the developments of experimental conditions, the synthesis of superhard materials have achieved great success. However, the experimental conditions for synthesizing superhard materials are still difficult, and the blind synthesis always require a lot of time, manpower and raw materials. More importantly, the possibility of successfully synthesizing novel superhard materials is very low. Therefore, designing novel superhard materials theoretically are very necessary. We design a series of B-C-N superhard materials using a recently developed global optimization method (CALYPSO) in conjunction with density functionalj theory. The main results of the thesis are as follows:1. Boron doped diamond turns into a hole-doping metal and even a superconductor when the dopant concentration is higher than 2%. These intriguing properties have stimulated great interest in searching for boron carbides in the diamond structure with a high boron content. Recently, the diamondlike superhard materials were successfully synthesized, such as BC2N, BC5 and BC3, and several structural models have been proposed for these cubic phases. However, they all suffer from incorrect crystal symmetry and even the wrong bonding character compared to the experimentally observed diamondlike cubic structure with all the atoms in the sp3 bonding state. This lack of an accurate structural determination impedes further understanding and exploration of these novel B-C compounds, and it calls for an innovative approach to solving such complex crystal structures. In this work, We solve the crystal structure of recently synthesized cubic BC3 using an unbiased swarm structure search, which identifies a highly symmetric BC3 phase in the cubic diamond structure (d-BC3) that contains a distinct B-B bonding network along the body diagonals of a large 64-atom unit cell. This distribution of B-B bonds in the cubic cell enables the maintaining of the diamond lattice and helps its dense packing and stability. Simulated x-ray diffraction and Raman peaks of d-BC3 are in excellent agreement with experimental data. We provide a systematic assessment of the influence of the vibrational contribution and the configurational entropy contribution to the finite-temperature Gibbs free energy of d-BC3 and the quasirandom BC3 structure, and the results show that a temperature-induced disorder-order structural transition toward the d-BC3 structure as the specimen is quenched to ambient temperature where it is investigated. Calculated stress-strain relations of d-BC3 demonstrate its intrinsic superhard nature and reveal intriguing sequential bond-breaking modes that produce superior ductility and extended elasticity, which are unique among superhard solids. The present results establish the first boron carbide in the cubic diamond structure with remarkable properties, and these new findings also provide insights for exploring other covalent solids with complex bonding configurations2. Elemental carbon has been the long-term subject of extensive theoretical and experimental studies because of its scientific and technological importance. Carbon is an unique element which adopts a wide range of structures, ranging from superhard insulating (diamond and lonsdaleite) to ultrasoft semimetallic (graphite) and even superconducting (doped diamond and fullerenes). As we know, the physical-chemical properties of materials are intimately related to their structures, and thus searching for new carbon allotropes is of considerable interest. Using a recently developed Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) algorithm on structural search, we predicted a novel sp3 carbon allotrope possessing an orthorhombic lattice with space group of Cmmm (oC32). The calculated elastic constants and simulated hardness revealed that oC32 simultaneously possesses ultra-incompressible and superhard properties with a high bulk modulus of 457 GPa and a high Vickers hardness of 96.2 GPa. This oC32 phase is dynamically stable and energetically more preferable than the experientially observed cold compressed carbon, thus oC32 is expected to be experimentally synthesizable under extreme conditions. These results further expand the list of meta-stable carbon allotropes and superhard materials under atmospheric and extreme conditions.3. Due to the decreasing resource of fossil fuel and demand of reducing the carbon emission during the energy production, exploration of new clean energy is of great interest and fundamental importance. Moreover, the shortcomings of the current silicon solar cells have called for the exploration of new materials, which can make full use of the solar spectrum and possess better optoelectronic properties. The search for new candidate semiconductors with direct band gaps of-1.4 eV has attracted significant attention, especially among the two-dimensional (2D) materials, which have become potential candidates for next-generation optoelectronics. Herein, we systematically studied 2D Bx/2Nx/2C1-x (0<x<1) compounds in particular focus on the four stoichiometric Bx/2Nx/2Ci-x (x= 2/3,1/2,2/5 and 1/3) using a recently developed global optimization method (CALYPSO) in conjunction with density functional theory. Furthermore, we examine more stoichiometries by the cluster expansion technique based on a hexagonal lattice. The results reveal that all monolayer Bx/2Nx/2Ci-xstoichiometries adopt a planar honeycomb character and are dynamically stable. Remarkably, electronic structural calculations show that most of Bx/2Nx/2Ci-x phases possess direct band gaps within the optical range, thereby they can potentially be used in high-efficiency conversion of solar energy to electric power, as well as in p-n junction photovoltaic modules. The present results also show that the band gaps of Bx/2Nx/2Ci-x can be widely tuned within the optical range by changing the concentration of carbon, thus allowing the fast development of band gap engineered materials in optoelectronics. These newfindings may enable new approaches to the design of microelectronic devices.