Utilizing Powder X-ray Diffraction to Understand Unknown Structural Motifs and Phase Transitions
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, especially when external knowledge about the composition and structural makeup of the material are limited. I will present two interesting families of cyanide-based compounds, which highlight interesting and unique structural features and properties.
Cs2MnII[MnII(CN)6] has the archetypal Prussian blue structure with cations in the cubic voids. Substitution with smaller alkali ions lead to structural distortions and a marked increase in ordering temperatures with increasing distortions. 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 and very different structural motifs.
Zn(CN)2 forms an interpenetrated diamondoid structure and undergoes a number of transitions upon the elevation of pressure. The structures of the four new crystalline phases have been resolved through ab-initio structural determination by synchrotron powder diffraction. The specific transition depends on the hydrostatic fluid used, and surprisingly three of these new phases involve a close to 2-fold expansion of volume. This counter-intuitive expansion is due to minimization of the solid and fluid volume, rather than just the solid volume.