First-principles Study Of Low-compressibility Of CrB₄, TC,Its Nitrides TcN_x(x=1-4) And C₂₀,C₁₂B₈,C(12)N₈

Superhard materials are attracting more and more attention for their great importance in basic research and technology, with various industrial applications in cutting and polishing tools, coatings and abrasives, etc. Attempts to experimentally synthesize or theoretically predict new superhard and ultra-incompressible materials is one of the subjects of intensive current research activities, in which compounds formed by the light elements B, C, N, O, Si and compounds of transition metals with B, C, N, O are two kinds of the experimentally accessible and predicted single-phase materials got too much attention, for there are the impossible presence of directional covalent bonding to greatly improve their modulus and hardness. In this thesis, the elastic, electronic properties and the mechanic of hardness of CrB4, Tc, its nitrides TcN, TCN2, TcN3, TcN4 and C20, C12B8 and C12N8 are systematically and deeply studied using first-principles calculations. The significant results are listed as follows:1. By studying the structure, elastic and electronic properties of orthorhombic CrB4, we found that the presence of the strong covalent B-B bond cage play a key role for high mechanical strength of CrB4, and that the shear modulus of CrB4 is large than its bulk modulus, which has never been found in other synthesized transition metal borides and nitrides.2. We first discovered that the mechanism of effect of bulk, shear modulus and hardness is different with increasing N content by incorporation into transition-metal Tc. 3. With increasing N content by incorporation into transition-metal Tc, the stereoscopic structures of materials change into layer to importantly affect its properties. The structure of marerial plays an important role in its properties.4. We found that the bulk modulus depends mainly on electrons per atomic volume and mass density, while covalent structure of chemical bonding and N content make more contribution to theoretical hardness, and appropriate content N-N covalent bonding with same direction is helpful to increase the shear modulus of nitrides.5. In the studies of C20, C12B8 and C12N8, the electronegativity of B, C, N and the saturation of non-bonding orbital have the significant effect on elastic and electronic properties, respectively.