Graduate Thesis Or Dissertation
 

Optical, electronic, and thermodynamic properties of layered mixed anion compounds

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  • Mixed anion compounds that form ZrCuSiAs-type crystal structures exhibit remarkable properties such as superconductivity at low temperatures, hole conduction with optical transparency in the visible region, and exciton absorption at room temperature. In this thesis, properties of a selected group of materials out of the very large set of mixed anion compounds with the ZrCuSiAs-type structure are investigated by computational first principles methods. The discovery of superconductivity in LaFePO (T[subscript C] ~ 4 K) and LaFeAsO[subscript 1-x]F[subscript x] (T[subscript C] ~ 26 K) naturally led to the speculation that LaFeSbO is possibly a superconductor with an even higher transition temperature. However, all attempts to synthesize LaFeSbO failed. We performed a theoretical chemical stability analysis based on ab initio density functional theory (DFT) and determined that LaFeSbO is not stable with respect to all synthesis conditions. Instead La₂SbO₂ is found with most synthesis conditions and indeed this was found experimentally. A local stability analysis for LaFeSbO based on the calculation of the phonon dispersion curves from DFT supercells calculations reveals that LaFeSbO is potentially metastable, which opens up the possible synthesis by a non-equilibrium process. Layered oxychalcogenides LaCuChO (Ch = {S,Se,Te}) and isostructural layered fluorochalcogenides BaCuChF have drawn much interest in recent years as p-type wide bandgap semiconductors with applications in transparent electronics and photovoltaics. Previous experimental and computational studies concluded for both LaCuChO, with bandgaps between 2.4-3.1 eV, and BaCuChF, with optical bandgaps between 2.8-3.5 eV, that p-type conductivity is primarily due to copper vacancies. But, until now no quantitative defect analysis has been performed for LaCuChO. Furthermore, members of both materials show strong excitonic absorption features, which are observable at room and higher temperatures. However, disagreement exists in the literature on the origin and nature of the room temperature excitons in the respective families of materials, attributing the room temperature excitonic absorption in LaCuChO to 2 dimensional quantum confinement of hole carriers in the layered structure, and room temperature excitonic absorption in BaCuChF is described by a 3 dimensional Wannier-Mott excitonic model. A comparative ab initio computational study of the defect physics and optical properties including the excitonic effects for both families of materials has been conducted. Point defects and defect complexes are taken into account and previously omitted corrections have been included, while accurate chemical potential stability diagrams and formation energies are calculated using recently improved methods. The optical properties are calculated using the Bethe-Salpeter equation in the GW approximation.
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