Powder Diffraction Tells You What Your Material Really Is: Case Studies

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Lapidus, Saul Henri
The Graduate School, Stony Brook University: Stony Brook, NY.
Powder diffraction is a useful tool for examining a number of materials that do not form single crystal for a variety of reasons. Unlike with single crystals, structure determination with powders is not a routine task. In order to demonstrate the ability and efficacy of powder diffraction and its contribution to the understanding the correlation of structures and properties, a series of compound will be presented from a wide range of types of materials: polymorphic materials, cocrystals, metal organic frameworks and magnetic materials. Each of these represent the forefront of the ability of powder diffraction, as they all introduce complications through large numbers of independent molecules and/or disorder. Two families of materials will be presented in detail, coordination polymers containing pyrazine and HF<sub>2</sub><super>-</super> and Prussian Blue analogs, that were investigated with powder diffraction. The bifluoride ion, HF<sub>2</sub><super>-</super>, contains a two-coordinate H-atom exhibiting the strongest known hydrogen bond. This was used to form materials of the form, Ni(HF<sub>2</sub>)(pyz)<sub>2</sub>]X (X = PF<sub>6</sub><super>-</super>, SbF<sub>6</sub><super>-</super>). These materials are quasi-1D magnets, with magnetic pathways along the biflouride ion. Two polymorphs of the PF<sub>6</sub><super>-</super> version were found and have different magnetic behavior, directly related to the structure. Additionally a number of compounds of novel and unknown composition were found and determined, Ni<sub>2</sub>F<sub>2</sub>(pyz)<sub>3</sub>(H<sub>2</sub>O)<sub>4</sub>(BF<sub>4</sub>)<sub>2</sub> and NiF(pyz)<sub>1.5</sub>(H<sub>2</sub>O)<sub>2</sub> TaF<sub>6</sub>. Cs<sub>2</sub>Mn<super>II</super>[Mn<super>II</super>(CN)<sub>6</sub>] has the archetypal fcc Prussian blue structure, with the cations in the cubic voids. Substitution with smaller alkali ions lead to structural distortions and a marked increase in ordering temperatures. On the other hand, substitution of larger cations, NMe<sub>4</sub><super>+</super> drives a rearrangement of the Mn-CN-Mn network and produce several previously unobserved Mn<super>II</super> coordination geometries and a unexpected structure.
157 pg.