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Fluorides and Oxyfluorides: Synthesis, Structural Characteristics, and Properties

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Noncentrosymmetry (NCS) is a requirement for many properties such as piezoelectricity, pyroelectricity, and nonlinear optical activity like second harmonic generation which are desirable for a variety of commercial applications. One method which has been employed to successfully synthesize NCS compounds utilizes acentric anionic groups, such as oxyfluoride metal complexes, as basic building units (BBUs) for a NCS structure. However, the presence of acentric BBUs in a structure is not always enough to ensure overall asymmetry; NCS packing is also required to achieve the desired properties. The role of hydrogen bonding in the NCS packing of lambda-shaped dimers in CuVOF4(H2O)7 is explored. The difficulty of determining the hydrogen positions by x-ray diffraction has left many questions about the strengths and roles of hydrogen bonds in this structure. Previous work had emphasized the intra-dimer hydrogen bonding. A co-refinement of single crystal x-ray and neutron diffraction provided accurate hydrogen atom positions and hydrogen bonding distances, leading to a better understanding of how the hydrogen bonding interactions affect the structure. CuMnF5(H2O)7 is a centrosymmetric structure composed of two different BBUs. A structural solution for this compound is presented for the first time. While it has similar BBUs to the centrosymmetric heterotypes of CuVOF4(H2O)7, the BBUs follow a different arrangement. The ionic octahedra are connected by a network of hydrogen bonding, but it is not enough to create NCS packing. CuMnF5(H2O)7 exhibits strong pleochroism, changing from red to green when the direction of polarized light is changed. This property is also exhibited by KNaMnF5, a novel compound whose structure and properties are described, which changes from orange to red. The NCS phase of KNaNbOF5 is examined in a series of single crystal x-ray diffraction studies over a range of temperatures. Though the structure as a whole has positive thermal expansion, that is not necessarily true of all its constituent parts. The K−ligand bond lengths increase with rising temperature, as expected, but the Nb−ligand bond lengths show a slight decrease. These results are not conclusive but indicate a promising direction for future studies in this area.

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  • 04/03/2019
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