Graduate Thesis Or Dissertation
The Role of Defects on the Optical, Electronic, and Magnetic Properties of Iron-Sulfur Solids Public Deposited
Solids composed of iron and sulfur are earth abundant and nontoxic, and can exhibit interesting and technologically important optical, electronic, and magnetic phenomena. Troilite (hexagonal FeS) is predicted to exhibit large room temperature magnetoelectricity and may exhibit high temperature superconducting properties, and pyrite (cubic FeS2) is a promising candidate for photovoltaic applications. However, defects naturally occur within both of these solids. Troilite contains Fe deficiencies with atomic formula Fe1-xS, while pyrite contains sulfur deficiencies with the atomic formula FeS(2-x). The Fe-S phase diagram is congested in these regions of slight non-stoichiometries, and even when the atomic composition changes by just a few percent at standard temperature and pressure, there are a myriad of stable crystal phases that form with qualitatively different electronic properties. This dissertation describes the computational efforts to understand and characterize the nature of defects within these two materials.
The first part of this thesis looks at the effects of iron vacancies in Fe-S solids using the stoichiometric end-member, troilite, as a reference system. Density functional theory (DFT) calculations show that iron vacancies in troilite shift the structure from hexagonal FeS to a monoclinic structure, similar to crystal structures of pyrrhotites, and suggest that this iron deficient troilite may be a stable intermediate between these crystal structures. The calculations also predict that the defects close the band gap of the slightly insulating material.
The second part of this thesis focuses on sulfur defects in pyrite. DFT calculations predict a narrowing of the band gap of the semiconductor and reveal strong electron-nuclear coupling in the material due to the extreme sensitivity of the energies of the spatially localized defect states to the nuclear coordinates. The defect states of sulfur deficient pyrite are further characterized by transforming the delocalized Bloch states obtained with DFT into maximally localized Wannier functions. In this now diabatic representation, the crystal field splitting of pyrite decreases by a factor of two in the presence of the defect. The degeneracies of the Fe d-orbitals orbitals lift with a sulfur defect, which may cause a spin crossover and an emergent magnetic state.
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