First-principles Calculations Of Novel BiS₂-based Superconducting Materials And Ti-based Antiperovskite Materials

In this thesis, by means of first-principles calculations, we studied two systems, i.e., BiS₂-based superconductor Sr_1-xLa_xFBiS₂ and antiperovskite Ti-based AXTi₃, respectively. The thesis mainly includes the following two contents:First, by means of first-principles calculations, we systematically studied the La doping effect for the recently discovered layered BiS₂-based superconductor Sr_1-xLa_xFBiS₂（x=0-0.7）. For the electronic structure, we obtained the band structure,density of states（DOS）, and Fermi surface（FS） at different doping levels. The parent compound Sr FBiS₂ is a semiconductor with a gap of 0.88 eV. The substitution of Sr by La dopes electrons and turns the system from a semiconductor to a metal. The band structure and the FS topology also change obviously with the increasing of doping level. Good FS nesting at the wave vector（π, Ï€, 0） has been found at certain doping levels, suggesting a proximity to charge-density-wave instability. Particularly,the doping dependence of DOS at the Fermi level resembles the available experimental variation of Tc as the function of x, indicating that it may be a conventional phonon-mediated superconductor. This was further proved by electron-phonon coupling calculation. Therefore, similar to LaO_1-xF_xBiS₂, our results suggest another example of superconductor derived from doped semiconductor.Second, we systematically studied the mechanical and electronic properties of a series of antiperovskite Ti-based ternary carbides and nitrides AXTi₃（A=Ba, Ca, In,Sn, Sr, Zn, Cu, Al, Ga, Cd and La; X=C, N） from first-principle calculations. By calculating the formation energies, elastic constants, and other mechanical parameters, we predicted that 7 carbides ACTi₃ and 7 nitrides ANTi₃ compounds are stable among the 22 compounds. The predicted large Young’s modulus and high hardness imply a good mechanical application prospect of AXTi₃ Particularly,Sn NTi₃ was found to show ferromagnetic ground state. For the electronic structure,our results confirm that the compounds are metallic in nature, and the DOS near the Fermi energy is predominately contributed by Ti-3d states. The effect of A- and X-site atom doping on AXTi₃ can be evaluated by rigid band approximation. Finally,we predicted In NTi₃ is moderate coupling Bardeen-Cooper-Schrieffer（BCS）superconductor with Tc of 6.32 K. Our predictions will be useful for the experimental exploration of the new antiperovskite compounds.