MATERIALS SCIENCE SEMINAR

ABSTRACT: Powder diffraction is a useful tool for examining a number of compounds that do not form single crystals for a variety of reasons. Unlike with single crystals, structure determination with powders is not a routine task. In this talk, I will present two families of compounds, coordination polymers containing pyrazine and HF₂^- and Prussian Blue analogs, that were investigated with powder diffraction.

The bifluoride ion, HF₂^-, contains a two-coordinate H-atom exhibiting the strongest known hydrogen bond. This was used to form compounds of the form, Ni(HF₂)(pyz)₂]X (X = PF₆^–, SbF₆^–). These compounds are quasi-1D magnets, with magnetic pathways along the biflouride ion. Two polymorphs of the PF₆^- version were found and have different magnetic behavior, directly related to the structure.

 Cs₂Mn^II[Mn^II(CN)₆] 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, NMe₄⁺ and NEt₄⁺ drive a rearrangement of the Mn-CN-Mn network and produce several previously unobserved Mn(II) coordination geometries.

This work was done in collaboration with the Joel Miller group at University of Utah (Prussian Blue compounds) and the Jamie Manson group at Eastern Washington University (pyrazine compounds).