| There are numerous examples of nanocrystalline compounds that exhibit unique properties such as enhanced reactivity or unusual catalytic, magnetic, or optical behavior relative to their bulk counterparts. These interesting properties of nanosized materials, typically less than ∼20 nm, are inextricably related to their size, shape, composition, and atomic structure. Developing a comprehensive understanding of these fundamental attributes of nanoparticles is a critical step towards understanding their size/shape/composition/structure-property relationships. These properties strongly influence their roles in geochemical and biological systems, and are crucial for unlocking their functional potential.;Experimental studies of nanoparticles are inherently challenging due to extremely small particle sizes, instability, and highly reactive surfaces. These qualities necessitated the development of specific procedures for the synthesis, processing, and handling of these compounds, some of which are air-sensitive. The protocols utilized are described in detail and represent an important contribution to future studies involving these and similar nanosized compounds. In the conclusion of this dissertation, the implications of the major findings are summarized, important unanswered questions are posed, and an outlook for future experiments is presented.;The principal goal of this dissertation was to quantitatively determine and evaluate the structures, i.e., the three-dimensional atomic arrangements, of a number of important nanocrystalline compounds. Included in this dissertation are detailed studies of the ferric oxyhydroxide mineral known as ferrihydrite (Fe(OH)3), the initial iron sulfide precipitate (FeS), and several manganese sulfide (MnS) polymorphs. While the majority of the samples were synthetically-derived, a sample formed naturally through biomineralization was also investigated. A quantitative evaluation of their structures was accomplished primarily though the application of high-energy X-ray total scattering coupled with pair distribution function analysis. The application of this method to these particular compounds has resulted in substantial advancements over prior attempts that relied on conventional methods for structure determination, specifically X-ray diffraction and X-ray absorption spectroscopy. The major findings of this dissertation resolve several long-standing questions, particularly regarding the structures of these compounds. |
