First-principles studies of electronic structure and electrical transport in thermoelectrics and transition-metal oxides

Abstract

First-principles material modeling is a practical approach in materials science to explore the underlying mechanisms of unexpected behaviors and predict changes in material performance. This dissertation studies the electronic structure and electrical transport property in thermoelectrics and transition-metal oxides via density functional theory calculations. Thermoelectrics are characterized by the mutual conversion between heat energy and electrical energy, indicating specific potential applications; thus, some promising thermoelectric materials are studied in detail. First, it is proven in n-type Mg3Sb2 that the apparent change in the electronic structure near the conduction band edge is introduced by doping Y or Sc atoms, achieving better electrical transport performance. Second, pressure-induced enhancements of the power factor are observed in both n-type and p-type Mg3Sb2, but distinctly different mechanisms dominate these enhancements; that is, the convergence of conduction bands for n-type Mg3Sb2 and the suppressed electron-phonon interaction for p-type Mg3Sb2. Third, the simultaneous optimization of the intrinsic electrical and thermal transport properties is achieved in Sb2Si2Te6 under out-of-plane tensile strains. Fourth, we use Mg2Si to show that long-range interaction can significantly amend the calculated electrical conductivity and carrier mobility, resulting in a better agreement between the theoretical electrical conductivity and carrier mobility of Mg2Si with the experimental data. Long-range interaction is also critical to understanding the effect of band convergence on Mg2Si. Fifth, the temperature-dependent band convergence on PbX (X = S, Se, Te) is shown by considering the effect of temperature on electron-phonon interactions. Transition-metal oxides in perovskite structures are important functional materials. We specialize in doped ferroelectrics and infinite layer nickelates. An approximate polar metallic phase can be achieved in electron-doped LiNbO3, although the ferroelectric displacements of LiNbO3 are suppressed with electron doping. We also find that hydrostatic pressure may cause an unexpected metal-insulator phase transition based on the rare polar metallic oxygen-deficient LiNbO3−δ where substantially increased polar displacements are observed under hydrostatic pressure. In contrast, electron doping and hydrostatic pressure both suppress the ferroelectric displacements of BaTiO3. In addition, superconductivity is observed in P4/mmm RNiO2 (R=La, Pr, Nd). We find that across the lanthanide series of RNiO2, the P4/mmm structure is dynamically structurally unstable when R is a late lanthanide element (Eu-Lu), tending to undergo out-of-phase “NiO4 square" rotations about the z-axis; therefore, a new I4/mcm crystal structure is preferred.