First-Principles Study Of Mechanical, Electronic And Optical Properties Of BiMO₃, Nb₄SiC₃ And Doped Cr₂Nb

The structure, chemistry and physics properties of materials were simulated by the first-principles method based on the density functional theory to deeply understand our observe phenomenon and characteristics of materials on the scales from macrostructure to microstructure and to theoretically predict a new material and its properties, which provided theoretical direction and evidence for materials design and preparation. In this thesis, the structures and properties of four compounds BiMO3 (M = Al, Ga, In and Sc) were investigated by using the first-principles method. Furthermore, a new material Nb4SiC3 and its properties were also predicted. Finally, the effect of the additive elements X (V, W, Mo, Zr, Hf and Ta) on the ideal cleavage fracture of Cr2Nb was investigated.First, the geometry structures, electronic structures, elastic constants and optical properties of four compounds BiMO3 (M = Al, Ga, In and Sc) were studied. The mechanical stability of BiMO3 (M = Al, Ga and Sc) compounds has been confirmed by calculations of the elastic constants. The electronic structures of four compounds BiMO3 (M = Al, Ga, In and Sc) demonstrate that these compounds are insulators, and the band gaps are varied between 2.03 eV and 3.36 eV. Their band gaps are decided by O 2p states and Bi 6p states, and four compounds appear to have some covalent features. Moreover, the M-O bond possessed a stronger covalent bonding strength than the Bi-O bond. The electronic properties of four compounds BiMO3 are determined mainly by M atom, and therefore it is possible to design to possess desirable electronic properties through a proper choice of the M atoms. The optical properties of BiMO3 (M = Al, Ga, In and Sc) were also investigated by calculating and analyzing in details their dielectric functions, absorption spectra, refractive index, extinction coefficient, reflectivity and energy-loss spectra. The results show that the four compounds are promising dielectric materials. By studying these properties of the four compounds BiMO3 (M = Al, Ga, In and Sc), it is an important significant to extend their field of application.And then a new ceramic-metal composite Nb4SiC3 was predicted, and its structural stability, mechanical, electronic, theoretical hardness and optical properties were also studied. The results show that a stable Nb4SiC3 phase appears in theα-type crystal structure. The mechanical stability ofα- Nb4SiC3 has been confirmed by calculations of the elastic constants, and it also exhibits slight anisotropic elasticity. The electronic structures ofα- Nb4SiC3 reveal that it is a metal and exhibits covalent feature. Moreover, the C-Nb bonds possessed stronger covalent bonding than the Si-Nb bonds. The calculation of theoretical hardness reveals that the strong bonding between C and Nb is responsible for the high hardness ofα- Nb4SiC3, and it can enhance its hardness by substituting Al atoms with Si atoms in Nb4AlC3. On the other hand,α- Nb4SiC3 is more ductile and has a higher hardness thanα- Nb4AlC3. Furthermore, the dielectric function, the absorption spectrum, the conductivity, the energy-loss spectrum and reflectivity ofα- Nb4SiC3 were obtained and discussed in detail. Its dielectric function indicates thatα- Nb4SiC3 crystal has a Drude-like behaviour. Moreover,α- Nb4SiC3 has a strongest absorption capacity when the energy of incident light is 8 eV. Finally, by comparing the reflectivity spectra ofα- Nb4SiC3, Ti4AlN3 and TiN, we concluded thatα- Nb4SiC3 might be a better candidate material as a coating to avoid solar heating than Ti4AlN3.Finally, the effects of the additive elements X (V, W, Mo, Hf, Ta and Zr) on the ideal cleavage fracture of Cr2Nb were investigated using the first-principles method. The principal objective of this work is to understand the electronic mechanism behind the effects of the additive elements on the ideal cleavage fracture in Cr2Nb. Our calculated lattice parameters and elastic constants of the pure Cr2Nb are in agreement with the experimental data. The crystal structures of Cr2Nb with the additive elements X were also optimized. The results show that the lattice constants of Cr2Nb with X on Cr sites are larger than that of the pure Cr2Nb, while those of Cr2Nb with X on Nb sites are smaller except for Zr and Hf atoms. The site occupancy behaviors of the elements X were investigated by calculating the site occupancy energies. The results show that V occupies the Cr sites, and W and Mo have a weak site preference for the Cr sites. Ta, Zr and Hf preferentially occupy the Nb sites. The brittle cleavage energy Gc and critical stressσc were calculated. The results show that V on Cr sites and X (W and Zr) on Nb sites can increase the cleavage strength of Cr2Nb, while X (W and Mo) on Cr sites and X (Mo, Ta and Hf) on Nb sites can reduce the cleavage strength of Cr2Nb. Our results are consistent with available experimental ones. We also find that the effect of the element W on the cleavage strength of Cr2Nb strongly depends on its site preference in Cr2Nb. The charge densities induced by the additive elements X were also calculated in order to reveal the origin of the effects of X on the ideal cleavage fracture of Cr2Nb. The strong hybridization between X and Cr/Nb atoms is a necessary condition to enhance the strength of Cr2Nb.