Enhancement of thermoelectric properties of Strontium Titanates doped with TA and HO using density functional theory technique

Thermoelectric (TE) technique is a promising approach to convert waste thermal energies into electricity without using harmful chemicals or moving parts. Strontium titanate (SrTiO₃) is one of the most studied perovskite oxides for TE and other technological applications. In this thesis, the structural, electronic and thermoelectric properties of doped Strontium titanate by Tantalum (Ta) and Holmium (Ho) were investigated using density functional theory (DFT) technique. The calculations were performed using full-potential linear augmented plane wave (FP-LAPW) with generalized gradient approximation (GGA) of Perdew-BurkeErnzerhof (PBE) as implemented in WIEN2k code. Supercell of 60 atoms were developed from a unit cell of 5 atoms, for the purpose of doping with Ho at A site and B site, and Ta atom only doped at B- site due to ionic radii difference, all by 8 % and 17 % doping level. These structures were optimized for crystal structure and optimized lattice constant determination. The self-consistent field (SCF) calculation were performed with RKmax = 6, lmax = 12, Gmax = 24 and XKGEN = 150 for electronic properties investigation. Electronic properties were studied via band structure and density of states analysis. The thermoelectric properties were calculated using BoltzTraP code with high dense k-mesh of 2000 at the temperature range of 250 – 1200 K. After observed underestimation of band gap result of 2.25 eV (Γ-Γ) and 1.88 eV (Γ-R) by standard DFT method in SrTiO₃, Hubbard U parameter were adopted for the improvement. For the supercell structure of 60 atoms, Hubbard U parameter of 0.8 were found to gives band gap value of 3.26 eV (Γ-Γ) similar to experimental data of 3.25 eV. The band structure analysis shows that all Ho doped SrTiO₃ at A site and B site compounds exhibits metallic behaviour with no apparent band gap. In case SrTiO3 doped by Ta, semi-metallic behaviour were observed with the band gap of 2.76 eV and 2.35 eV for SrTi₀.92Ta₀.₀8₀ and SrTi₀.₀8₃Ta₀.17O3 respectively. The total density of states plots for all Ho and Ta doped samples confirm the conducting behaviour as observed in band structure analysis. The partial density of state plots for all Ho and Ta doped samples shows the role of Ho 4f orbital and Ta 5d orbital in filling the gap near the Fermi level to change the SrTiO3 from insulator of wide band gap into semi metallic or metallic behaviour. The thermoelectric performances ZT were obtained at range of 0.33 to 0.66 for all the studied doped compounds. The calculated lattice thermal conductivity is inversely proportional to increase in temperature which shows that the phonon-phonon is the dominant of lattice contribution in thermal conductivity for the investigated compounds. Total thermal conductivities decrease with increase in temperature as a result of the fact that they are dominated by lattice thermal contribution. It was clearly shown that all four Ho doped SrTiO₃ gave higher ZT value compared to all Ta doped SrTiO₃ compounds. This finding can be attributed to large Seebeck coefficient and very lower thermal conductivity due to low carrier concentration and low lattice contribution in thermal conductivity respectively, recorded for Ho atom comparing to Ta at the two cation sites; Sr and Ti. Also, it was notice that the sharp rise in density of state by Ho-4f state or flat bands near Fermi level constitute the enhancement of Seebeck coefficient that lead to their high value of ZT.