DENSITY FUNCTIONAL THEORY STUDY OF STRUCTURAL, MECHANICAL, ELECTRONIC, OPTICAL, THERMAL AND THERMOELECTRIC PROPERTIES OF BaGe2P2 AND BaGe2As2

Thesis Title: DENSITY FUNCTIONAL THEORY STUDY OF STRUCTURAL, MECHANICAL, ELECTRONIC, OPTICAL, THERMAL AND THERMOELECTRIC PROPERTIES OF BaGe₂P₂ AND BaGe₂As₂

Student’s name: Nameme Debora

Supervisors’ names:

1. Michael Nakitare Waswa
2. George S. Manyali

ABSTRACT

Thermoelectric generators (TEG) offer promising opportunity of converting heat into electrical energy. There is the need to explore highly efficient thermoelectric (TE) materials with aim of improving the performance of TEG. Among the TE materials investigated, are Zintl compounds which have recently gained a lot of interest because of their high figure of merit. The structural, elastic, electronic, and optical properties of BaGe₂As₂ and BaGe₂P₂ have been studied theoretically, however their thermoelectric properties have yet to be documented. The current study analyzed the structural, elastic, electrical, optical, thermal and thermoelectric characteristics of BaGe₂As₂ and BaGe₂P₂ zintl phase compounds. This work used density functional theory in Quantum ESPRESSO code, as well as additional processing codes as Thermo-pw and BoltzTrap. The estimated lattice constants were consistent with other theoretical and experimental results, indicating the study’s validity. Both compounds are brittle, elastically anisotropic, mechanically and thermodynamically stable. They also possess indirect band gaps of 0.73 eV and 1.14 eV for BaGe₂As₂ and BaGe₂P₂, respectively. The PDOS analysis shows that P and As in BaGe₂P₂ and BaGe₂As₂ dominate the conduction band, whereas Ge dominates the valence band in both instances. The analysis of frequency-dependent optical properties reveals that BaGe₂P₂ has significantly higher absorption coefficients compared to BaGe₂As₂, indicating that BaGe₂P₂ is a more effective optical absorber. Additionally, the figure of merit for BaGe₂P₂ increases with rising temperature, reaching 0.593 at 1000K. In contrast, the figure of merit for BaGe₂As₂ increases with temperature up to 800K but then begins to decline. This pattern suggests that BaGe₂P₂ is a superior thermoelectric material compared to BaGe₂As₂.