First-principles study of mechanical and electronic properties of TiB compound under pressure

The effects of applied pressures on the structural, mechanical and electronic properties of TiB compound were studied using the first-principles method based on the density functional theory. The results showed the pressures have the significant effects on the mechanical properties and electronic properties of TiB phase. The calculated structural and mechanical parameters (i.e., bulk modulus, shear modulus, Young's modulus, Poisson's ratio and Debye temperature) were in good agreement both with the previously reported experimental and theoretical results at zero pressure. Additionally, all these parameters presented the linearly increasing dependences on the external pressure. The B/G ratios signified the TiB crystals should exhibit the brittle deformation behavior at 0-100 GPa. The universal anisotropic index indicated the TiB compound was elastically isotropic under zero pressure, and may become anisotropic at higher pressures. Further, the density of states and Mulliken charge of TiB were discussed. The bonding nature in TiB was a combination of metallic, ionic and covalent at zero pressure. The metallic component was derived from free-electron transfer from the Ti4s to Ti3d and Ti3p states. The ionic component was originated from the charge transfer from Ti to B atoms. The covalent component had two sources. One was from the B2s-B2p hybridization in the B atomic chains. The other one was from B2p-Ti3d bonding hybridization. Under higher pressures, the ionic and covalent bonds were both improved with the rising of pressures. This should be the fundamental reason for the enhanced mechanical properties in the TiB compound. At the same time, the metallic bond kept leveled to ensure the electric conductivity.