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First-principles investigation on electronic and thermoelectric properties of chalcogenide compounds and the effect of rare earth and oxygen substitutions
- Author(s)
- Tran Van Quang
- Advisor
- Kihong Kim, Miyoung Kim
- Department
- 일반대학원 물리학과
- Publisher
- The Graduate School, Ajou University
- Publication Year
- 2014-06
- Language
- eng
- Alternative Abstract
- Employing all-electron full-potential linearized augmented plane wave (FLAPW) method to solve the Kohn-Sham equation within the first-principles calculation in frame work of the density functional theory (DFT), and utilizing the solution of Boltzmann Transport Equation (BTE) in the constant relaxation-time approximation, this dissertation is dealing with the electronic structure and thermoelectric property of bismuth chalcogenides impacted by element substitutions, and strain effects. Some parts of it devote to study the magnetic property as well. The work will focus on searching for the effects to improve the thermoelectric efficiency of bismuth chalcogenides, in which the effective effects will be pointed out to stem mainly from the substitutions of oxygen and the lattice distortion or strain. Accordingly, bismuth chalcogenides with oxygen substitutions, Bi2O2X (X=S, Se, Te), are all found to be narrow band gap semiconductors, in which Bi2O2S has largest band gap, thereby it gives the largest Seebeck coefficient, but lowest electrical conductivity, contrarily; Bi2O2Se and Bi2O2Te yield large Seebeck coefficient and electrical conductivity as well, which promote them to be potential thermoelectric (TE) candidates. The calculation results are found to be consistent well with experimental data. The TE efficiency can be even increased by optimizing concentration. The oxygen vacancy in Bi2O2Se found to be the fact that improves the conductivities but drastically depress the Seebeck coefficient. Nevertheless, that the detrimental effect can be resolved by filling the vacancy by Se or by making oxygen redundant defects. When bismuth is substituted by Pb or Ge to form ABi4Te7 (A=Pb, Ge), the compounds are found to be semiconducting with narrow band gaps and complex band topologies at the band edges. Possessing low thermal conductivities, they also yield the high Seebeck coefficients. Our calculations show a good agreement with experiment. In addition, we found that their TE efficiencies can be improved markedly by optimizing the concentrations, and by elevating the operating temperature. By examining the effect of hydrostatic pressure on the electronic structure and TE property of Sb2-xBixTe3 with two compositions, i.e. x=1, and x=1/6, we found that the pressure tunes VBM and CBM to yield a tendency of the band convergence. The band convergence significantly increases both the Seebeck coefficient and the electrical conductivity, therefore to be beneficial for the TE efficiency. We found that the Seebeck coefficient is maximized at pressure around 1.0-4.0 GPa, while to maximize the power factor, pressure needs to be slightly larger, about 2.0-5.0 GPa. The results are discussed and compared with experimental data to show a good agreement. The atomic relaxation is found only to impact on the value of optimal pressures at which the Seebeck coefficient and the power factor are maximum, i.e. it slightly increases the optimal value. Therefore, it is suggested that to improve the TE efficiency, pressure and strain are the effective techniques. Using them to tune for the band convergence is beneficial for the TE performance of Sb2-xBixTe3 superlattices and is desirable for further studies in applying on the other materials as well. Substitutions of a rare earth f-elements, Gd and Ce, into Bi significantly reduce the band gap of Bi2Te¬3. The band gap is negligible or even negative. Therefore, the Seebeck coefficients are depressed, significantly. However, GdBiTe3 is found to suffer the metal-insulator transition under the cross-plane axial strain. Thus, the Seebeck coefficient is retrieved. The magnetic property is found to be independent with the strain. LaBiTe3 is found to be a narrow band gap semiconductor with band gap of 0.13 eV being beneficial for the TE property. Searching for the optimal Seebeck coefficient that maximize the power factor in RExBi2-xTe3 (RE=La, Ce, Gd) with (x=0, 1, 1/3), we found that the optimal Seebeck coefficient is in the range from 120-170 μVK-1 which aids the argument about the existence of the general optimal Seebeck coefficient to maximize the TE efficiency.
- Fulltext
- Appears in Collections:
- Graduate School of Ajou University > Department of Physics > 4. Theses(Ph.D)
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